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NBS SPECIAL PUBLICATION 559

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Time and Frequency Users' Manual
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Mechanical

Engineering and Process Technology2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government. The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234.

: Some divisions within the center are located at Boulder, CO 80303.
Banroai mravm v. 

JAN 1 8 1980

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Time and Frequency Users' Manual

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Edited by:

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George Kamas

Sandra L. Howe

Time and Frequency Division

National Measurement Laboratory

National Bureau of Standards

Boulder, Colorado 80303

U.S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued November 1979
Library of Congress Catalog Card Number: 79-600169

National Bureau of Standards Special Publication 559

Nat. Bur. Stand. (U.S.), Spec. Pubi. 559, 256 pages (Nov. 1979)

CODEN. XNBSAV

(Supersedes NBS Technical Note 695, May 1977)

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

For sale by the Superintendent of Documents, U.S. Government Printing Office. Washington, D.C. 20402 Stock No. 003-003-02137-1 - Price $6. (Add 2u percent additional for other than U.S. mailing).
CONTENTS

PAGE

CHAPTER 1. INTRODUCTION: WHAT THIS BOOK IS ABOUT

1.1 WHO NEEDS TIME & FREQUENCY? 1 1.2 WHAT ARE TIME AND FREQUENCY ? 4 1.3 WHAT IS A STANDARD ? . . 5

1.3.1 CAN TIME REALLY BE A STANDARD? 6 1.3.2 THE NBS STANDARDS OF TIME AND FREQUENCY 6

1.4 HOW TIME AND FREQUENCY STANDARDS ARE DISTRIBUTED 7 1.5 THE NBS ROLE IN INSURING ACCURATE TIME AND FREQUENCY 7 1.6 WHEN DOES A MEASUREMENT BECOME A CALIBRATION ?

• . .8

1.7 TERMS USED . . 8

1.7.1 MEGA, MILLI, PARTS PER... AND PERCENTS ...... 8 1.7.2 FREQUENCY 10

1.8 DISTRIBUTING TIME AND FREQUENCY SIGNALS VIA CABLES AND TELEPHONE LINES . .11

1.9 TIME CODES 13

CHAPTER 2. THE EVOLUTION OF TIMEKEEPING

2.1 TIME SCALES 17

2.1.1 SOLAR TIME 17 2.1.2 ATOMIC TIME 18

A. Coordinated Universal Time ......... 19

B. The New UTC System 19

2.1.3 TIME ZONES 20

2.2 USES OF TIME SCALES 21

2.2.1 SYSTEMS SYNCHRONIZATION 21 2.2.2 NAVIGATION AND ASTRONOMY 22

2.3 INTERNATIONAL COORDINATION OF TIME AND FREQUENCY ACTIVITIES 23

2.3.1 THE ROLE OF THE INTERNATIONAL TIME BUREAU (BIH) 23 2.3.2 THE ROLE OF THE NATIONAL BUREAU OF STANDARDS (NBS) 23 2.3.3 THE ROLE OF THE U.S. NAVAL OBSERVATORY (USNO) AND THE DOD PTTI PROGRAM . 24

CHAPTER 3. THE ALGEBRA AND CONVENTIONS FOR TIME AND FREQUENCY MEASUREMENTS

3.1 EXPRESSING FREQUENCY DIFFERENCE (IN HERTZ, kHz, MHz , ETC.) 28 3.2 RELATIVE (FRACTIONAL) FREQUENCY, F (DIMENSIONLESS) 28 3.3 RELATIVE (FRACTIONAL) FREQUENCY DIFFERENCE, S (DIMENSIONLESS) . . . .28

3.4 EXAMPLE OF ALGEBRA AND SIGN CONVENTIONS IN TIME AND FREQUENCY MEASUREMENTS . . 29

3.4.1 Television Frequency Transfer Measurements (covered in Chapter 7) .29

3.4.2 Television Line-10 Time Transfer Measurements (covered in Chapter 8) . 29

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PAGE

3.5 USING TIME TO GET FREQUENCY 30 3.6 A MATHEMATICAL DERIVATION 31 3.7 DEFINITIONS 32

CHAPTER 4. USING TIME AND FREQUENCY IN THE LABORATORY

4.1 ELECTRONIC COUNTERS . . . .33

4.1.1 SOURCES OF COUNTER ERROR 34 4.1.2 FREQUENCY MEASUREMENTS 34

A. Direct Measurement . . . . . . . . . . .35

B. Prescaling 35

C. Heterodyne Converters .......... 35

D. Transfer Oscillators .......... 37

4.1.3 PERIOD MEASUREMENTS 39 4.1.4 TIME INTERVAL MEASUREMENTS 40

4.1.5 PHASE MEASUREMENTS 42 4.1.6 PULSE-WIDTH DETERMINATION MEASUREMENTS 43 4.1.7 COUNTER ACCURACY 43

A. Time Base Error ... . . . . . . . .43

B. Gate Error . 44

C. Trigger Errors . . . . . . . , .45

4.1.8 PRINTOUT AND RECORDING 48

4.2 OSCILLOSCOPE METHODS 48

4.2.1 CALIBRATING THE OSCILLOSCOPE TIME BASE ....... 49

4.2.2 DIRECT MEASUREMENT OF FREQUENCY 50 4.2.3 FREQUENCY COMPARISONS 50

A. Lissajous Patterns 50

B. Sweep Frequency Calibration: An Alternative Method . . . .55

4.2.4 TIME INTERVAL MEASUREMENTS 56

4.3 WAVEMETERS 56 4.4 HETERODYNE FREQUENCY METERS 57 4.5 DIRECT-READING ANALOG FREQUENCY METERS 58

4.5.1 ELECTRONIC AUDIO FREQUENCY METER 58 4.5.2 RADIO FREQUENCY METER 59

4.6 FREQUENCY COMPARATORS 60 4.7 AUXILIARY EQUIPMENT 60

4.7.1 FREQUENCY SYNTHESIZERS 60

4.7.2 PHASE ERROR MULTIPLIERS 61 4.7.3 PHASE DETECTORS 62 4.7.4 FREQUENCY DIVIDERS 63

A. Analog or Regenerative Dividers ........ 63

B. Digital Dividers ........... 63

4.7.5 ADJUSTABLE RATE DIVIDERS 64 4.7.6 SIGNAL AVERAGERS 65

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PAGE

4.8 PHASE LOCK TECHNIQUES 66 4.9 SUMMARY '.67

CHAPTER 5. THE USE OF HIGH-FREQUENCY RADIO BROADCASTS FOR TIME AND FREQUENCY

CALIBRATIONS

5.1 BROADCAST FORMATS 69 5.1.1 WWV/WWVH 69 5.1.2 CHU 70 5.1.3 ATA 71 5.1.4 IAM 72 5.1.5 IBF 72 5.1.6 JJY 72 5.1.7 OMA . . .74

5.1.8 VNG 74

A. Time Signals 74

B. Time Code 74

C. Announcements ............ 74

D. DUT1 Code 75

E. Accuracy ............. 75

F. Frequency and Time Generating Equipment ...... 75

5.1.9 ZUO 75

A. Carrier Frequencies and Times of Transmission . . . . .76

B. Standard Time Intervals and Time Signals ...... 76

C. Standard Audio Frequencies ......... 76

D. Accuracy 76

E. DUT1 Code 76

5.2 RECEIVER SELECTION 81 5.3 CHOICE OF ANTENNAS AND SIGNAL CHARACTERISTICS 82 5.4 USE OF HF BROADCASTS FOR TIME CALIBRATIONS 86

5.4.1 TIME-OF-DAY ANNOUNCEMENTS 86

5.4.2 USING THE SECONDS TICKS 86

A. Receiver Time Delay Measurements ....... 86

B. Time Delay Over the Radio Path 87

C. Using an Adjustable Clock to Trigger the Oscilloscope . . .88

D. Delayed Triggering: An Alternate Method that Doesn't Change the

Clock Output 90

E. Using Oscilloscope Photography for Greater Measurement Accuracy . 92

5.4.3 USING THE WWV/WWVH TIME CODE 92

A. Code Format . . 93

B. Recovering the Code . .94

5.5 USE OF HF BROADCASTS FOR FREQUENCY CALIBRATIONS 96

5.5.1 BEAT FREQUENCY METHOD 97 5.5.2 OSCILLOSCOPE LISSAJOUS PATTERN METHOD 98 5.5.3 OSCILLOSCOPE PATTERN DRIFT METHOD 100

5.5.4 FREQUENCY CALIBRATIONS BY TIME COMPARISON OF CLOCKS 100

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PAGE

5.6 FINDING THE PROPAGATION PATH DELAY 101

5.6.1 GREAT CIRCLE DISTANCE CALCULATIONS 101 5.6.2 PROPAGATION DELAYS ' . . . 102

5.7 THE NBS TELEPHONE TIME-OF-DAY SERVICE 105

5.8 SUMMARY. 105

CHAPTER 6. CALIBRATIONS USING LF AND VLF RADIO TRANSMISSIONS

6.1 ANTENNAS FOR USE AT VLF-LF 107 6.2 SIGNAL FORMATS 107 6.3 PROPAGATION CHARACTERISTICS AND OTHER PHASE CHANGES 108

6.4 FIELD STRENGTHS OF VLF-LF STATIONS Ill 6.5 INTERFERENCE .113

6.6 USING WWVB FOR FREQUENCY CALIBRATIONS 113

6.6.1 PHASE-SHIFT IDENTIFICATION 114 6.6.2 METHODS OF FREQUENCY COMPARISON 115

6.7 USING OTHER LF AND VLF STATIONS FOR TIME AND FREQUENCY CALIBRATIONS . . .115

6.7.1 THE OMEGA NAVIGATION SYSTEM 115

A. Operating Characteristics of Omega ....... 115

B. Synchronization Control .......... 117

C. Propagation Characteristics ......... 117

D. Omega Notices and Navigational Warnings ...... 118

6.7.2 DCF 77, WEST GERMANY (77.5 kHz) 118

A. Time Signals . 119

B. Time Code 119

6.7.3 HBG, SWITZERLAND (75 kHz) 120

A. Signal Format 12,0 B. Time Code . ... .

' 120

6.7.4 JG2AS/JJF-2, JAPAN (40 kHz) . . . . - 120 6.7.5 MSF, ENGLAND (60 kHz) 121

A. Fast Code 121

B. Slow Code 121

6.7.6 U.S. NAVY COMMUNICATION STATIONS 121 6.7.7 OMA, CZECHOSLOVAKIA (50 kHz) 123

A. Time Code 123

6.7.8 RBU (66-2/3 kHz) AND RTZ (50 kHz), USSR 124 6.7.9 VGC3 AND VTR3, USSR . . 124

6.8 MONITORING DATA AVAILABILITY 127

vi
PAGE

6.9 USING THE WWVB TIME CODE 127

6.9.1 TIME CODE FORMAT 127 6.9.2 TIME TRANSFER USING THE TRANSMITTED ENVELOPE 128

6.10 SUMMARY 130

CHAPTER 7. FREQUENCY CALIBRATIONS USING TELEVISION SIGNALS

7.1 HOW THIS FITS INTO OTHER NBS SERVICES 131

7.2 BASIC PRINCIPLES OF THE TV FREQUENCY CALIBRATION SERVICE 131

7.2.1 PHASE INSTABILITIES OF THE TV SIGNALS 133

7.2.2 TYPICAL VALUES FOR THE U.S. NETWORKS 134

7.3 HOW RELATIVE FREQUENCY IS MEASURED 135

7.3.1 COLOR BAR COMPARATOR 136

7.3.2 THE NBS SYSTEM 358 FREQUENCY MEASUREMENT COMPUTER 138

A. Block Diagram Overview .......... 138

B. Use of FMC with Unstable Crystal Oscillators 141

7.4 GETTING GOOD TV CALIBRATIONS 141 7.5 THE DIGITAL FRAME SYNCHRONIZER 142 7.6 USNO TV FREQUENCY CALIBRATION SERVICE 142

CHAPTER 8. FREQUENCY AND TIME CALIBRATIONS USING TV LINE-10

8.1 HOW THE SERVICE WORKS 145

8.1.1 USING LINE-10 ON A LOCAL BASIS 145

8.1.2 USING LINE-10 FOR NBS TRACEABILITY 146

8.2 EQUIPMENT NEEDED 147 8.3 TV LINE-10 DATA, WHAT DO THE NUMBERS MEAN ? 148

8.4 WHAT DO YOU DO WITH THE DATA ? 149

8.5 RESOLUTION OF THE SYSTEM 149

8.6 GETTING TIME OF DAY FROM LINE-10 151 8.7 THE DIGITAL FRAME SYNCHRONIZER 151 8.8 MEASUREMENTS COMPARED TO THE USNO 151 8.9 LINE-10 EQUIPMENT AVAILABILITY 151

CHAPTER 9. LORAN-C TIME AND FREQUENCY METHODS

9.1 BASIC PRINCIPLES OF THE LORAN-C NAVIGATION SYSTEM 153 9.2 BASIC LORAN-C FORMAT 160

9.2.1 CARRIER FREQUENCY 161 9.2.2 TRANSMITTED PULSE SHAPE 161 9.2.3 TRANSMITTED PULSE 161

vii
PAGE

9.2.4 GRAPHICAL REPRESENTATION 163

A. Envelope-to Cycle Difference . 163

B. ECD Determination 164

C. Pulse Group . 164

D. Existing Format 164

E. GRI 165

F. Blink Codes .165

G. Spectrum 166

H. Fine Spectra 166

I. Harmonics 166

9.3 WHAT IS THE EXTENT OF LORAN-C COVERAGE? ... 166

9.3.1 GROUNDWAVE SIGNAL RANGE 166

9.3.2 SKYWAVE SIGNAL RANGE 167

9.4 WHAT DO WE GET FROM LORAN-C ? 167

9.4.1 SIGNAL CHARACTERISTICS 167 9.4.2 TIME SETTING 168

9.4.3 FREQUENCY CALIBRATIONS USING LORAN-C 170

9.5 HOW GOOD IS LORAN-C ? 171

9.5.1 GROUNDWAVE ACCURACY 172 9.5.2 SKYWAVE ACCURACY . 172

9.6 ARE THE DATA VALID ? 172 9.7 SUMMARY 17>3

CHAPTER 10. SATELLITE METHODS OF TINE AND FREQUENCY DISSEMINATION

10.1 THE GOES SATELLITE TIME CODE 175

10.1.1 BACKGROUND 175

10.1.2 COVERAGE 176

10.1.3 SIGNAL CHARACTERISTICS 177 10.1.4 TIME CODE FORMAT 177 10.1.5 ANTENNA POINTING 180 10.1.6 PERFORMANCE 180

A. Uncorrected ............. 180

B. Corrected for Mean Path Delay 180

C. Fully Corrected 180

10.1.7 EQUIPMENT NEEDED 180

A. Antennas 180

B. Receivers ............ 181

10.1.8 SATELLITE-CONTROLLED CLOCK 181

A. Delay Corrections . 182

B. Programs for Computing Free-Space Path Delay 182

10.1.9 SMART CLOCK ..... 182

10.1.10 CLOCK CALIBRATION 183 10.1.11 RESULTS 184

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PAGE

10.1.12 Precautions . . . 186

A. Interference ............ 186

B. Outages 186

C. Continuity 187

10.2 THE TRANSIT NAVIGATION SYSTEM 187

10.2.1 GENERAL 187

10.2.2 EQUIPMENT NEEDED . 189

CHAPTER 11. AN INTRODUCTION TO FREQUENCY SOURCES

11.1 FREQUENCY SOURCES AND CLOCKS 191

11.2 THE PERFORMANCE OF FREQUENCY SOURCES . 193

11.3 USING RELATIVE FREQUENCY STABILITY DATA 194

11.4 RESONATORS . 196

11.5 PRIMARY AND SECONDARY STANDARDS 197 11.6 QUARTZ CRYSTAL OSCILLATORS 199

11.6.1 TEMPERATURE AND AGING OF CRYSTALS 201

11.6.2 QUARTZ CRYSTAL OSCILLATOR PERFORMANCE 201

11.7 ATOMIC RESONANCE DEVICES ' . . . . 202

11.7.1 STATE SELECTION 203

11.7.2 HOW TO DETECT RESONANCE 204

11.7.3 ATOMIC OSCILLATORS 205

11.7.4 ATOMIC RESONATOR FREQUENCY STABILITY AND ACCURACY 206

11.8 AVAILABLE ATOMIC FREQUENCY DEVICES 207

11.8.1 CESIUM BEAM FREQUENCY OSCILLATORS 207

11.8.2 RUBIDIUM GAS CELL FREQUENCY OSCILLATORS 208

11.8.3 ATOMIC HYDROGEN MASERS 209

11.9 TRENDS 210

CHAPTER 12. SUMMARY OF AVAILABLE SERVICES 213

BIBLIOGRAPHY 217

GLOSSARY 219

INDEX 225

ix
TABLES

Table 1 1 PREFIX CONVERSION CHART 9 Table 1 2 CONVERSIONS FROM PARTS PER . . . TO PERCENTS 9 Table 1 3 CONVERSIONS TO HERTZ 10 Table 1 4 RADIO FREQUENCY BANDS 10 Table 4 1 READINGS OBTAINED WITH VARIOUS SETTINGS

OF TIME BASE AND PERIOD MULTIPLIER CONTROLS .... 40

Table 5. 1 STANDARD FREQUENCY AND TIME SIGNAL BROADCASTS

IN THE HIGH FREQUENCY BAND 77 Table 5 2 IMAGE FREQUENCIES FOR WWV/WWVH 82 Table 6 1 CHARACTERISTICS OF THE OMEGA NAVIGATION SYSTEM STATIONS . 116

Table 6 2 STANDARD FREQUENCY AND TIME SIGNAL

BROADCASTS IN THE LOW FREQUENCY BAND . 125

Table 9 1 LORAN-C CHAINS . 154

Table 9 2 ECD TABLE: HALF CYCLE PEAK AMPLITUDE vs ECD (LORAN-C) . . 163

Table 9. 3 GROUP REPETITION INTERVALS (LORAN-C) . 164

Table 10 1 SPECIFICATIONS FOR THE GOES SATELLITES . 175

Table 11. 1 COMPARISON OF FREQUENCY SOURCES . 211

Table 12 1 CHARACTERISTICS OF THE MAJOR T&F DISSEMINATION SYSTEMS . . . 213

Table 12. 2 HOW FREQUENCY IS CALIBRATED . 214

Table 12. 3 HOW TIME IS CALIBRATED . . 214

FIGURES

Figure 1. 1 ORGANIZATION OF THE NBS FREQUENCY AND TIME STANDARD 6 Figure 1. 2 IRIG TIME CODE, FORMAT H 13 Figure 1. 3 IRIG TIME CODE FORMATS 14 Figure 2. 1 UNIVERSAL TIME FAMILY RELATIONSHIPS 18 Figure 2. 2 FIRST ATOMIC CLOCK 18 Figure 2. 3 CLASSES OF TIME SCALES & ACCURACIES 19 Figure 2. 4 DATING OF EVENTS IN THE VICINITY OF A LEAP SECOND .... 19 Figure 2. 5 STANDARD TIME ZONES OF THE WORLD REFERENCED TO UTC ... 20 Figure 2. 6 NBS, USNO, AND BIH INTERACTIONS 23 Figure 4. 1 ELECTRONIC COUNTERS 33 Figure 4 2 SIMPLIFIED BLOCK DIAGRAM OF AN ELECTRONIC COUNTER .... 34 Figure 4. 3 DIAGRAM OF A COUNTER IN THE FREQUENCY MEASUREMENT MODE . 35 Figure 4 4 PRESCALER-COUNTER COMBINATION . . . 36 Figure 4. 5 TYPICAL MANUALLY-TUNED HETERODYNE CONVERTER 36 Figure 4 6 TYPICAL MANUALLY-TUNED TRANSFER OSCILLATOR 37 Figure 4 7 AUTOMATIC TRANSFER OSCILLATORS 38 Figure 4 8 DIAGRAM OF COUNTER IN THE PERIOD-MEASUREMENT MODE .... 39 Figure 4 9 DIAGRAM OF COUNTER IN THE MULTIPLE-PERIOD MODE .... 39

X
Figure 4. 10 EFFECT OF NOISE ON TRIGGER POINT IN PERIOD MEASUREMENTS 41

Figure 4. 11 DIAGRAM OF COUNTER IN THE TIME INTERVAL MODE 41

Figure 4. 12 OPTIMUM TRIGGER POINTS FOR START-STOP PULSES 42 Figure 4. 13 USE OF TIME-INTERVAL UNIT FOR PHASE MEASUREMENTS .... 42 Figure 4. 14 TIME- INTERVAL COUNTER WITH START-STOP CHANNELS CONNECTED

TO COMMON SOURCE FOR PULSE-WIDTH OR PERIOD MEASUREMENTS 43 Figure 4. 15 TYPICAL TIME BASE STABILITY CURVE 44 Figure 4. 16 CONSTANT GATING INTERVAL WITH AMBIGUITY OF ± 1 COUNT 45

Figure 4. 17 ACCURACY CHART FOR PERIOD AND FREQUENCY MEASUREMENTS 45

Figure 4. 18 UNDIFFERENTIATED SCHMITT-TRIGGER WAVEFORMS 46

Figure 4. 19 TIME ERROR PRODUCED BY IMPROPER CALIBRATION OF TRIGGER LEVEL CONTROL 47 Figure 4. 20 SINE WAVE METHOD OF CHECKING TRIGGER LEVEL CALIBRATION . . 47 Figure 4. 21 DIGITAL-TO-ANALOG ARRANGEMENT FOR CHART RECORDING .... .48

Figure 4. 22 SIMPLIFIED BLOCK DIAGRAM OF A CATHODE-RAY OSCILLOSCOPE . 48

Figure 4. 23 WAVEFORM OF HORIZONTAL DEFLECTION VOLTAGE FROM

TIME-BASE GENERATOR . 49

Figure 4. 24 TIME BASE CALIBRATION HOOKUP USING AN EXTERNAL FREQUENCY SOURCE .49

Figure 4. 25 SINE WAVE DISPLAY AS VIEWED ON OSCILLOSCOPE 50 Figure 4. 26 EQUIPMENT HOOKUP FOR A 1:1 LISSAJOUS PATTERN 50 Figure 4. 27 ELLIPTICAL LISSAJOUS PATTERNS FOR TWO

IDENTICAL FREQUENCIES OF DIFFERENT PHASE 51 Figure 4. 28 DEVELOPMENT OF A 3:1 LISSAJOUS PATTERN 51 Figure 4. 29 LISSAJOUS PATTERNS 52 Figure 4. 30 ARRANGEMENT FOR MEASURING SMALL FREQUENCY DIFFERENCES

WITH OSCILLOSCOPE AND FREQUENCY MULTIPLIERS .... 53 Figure 4. 31 DOUBLE-BALANCED MIXER 54

Figure 4. 32 ARRANGEMENT FOR FREQUENCY COMPARISON USING OSCILLOSCOPE

AND PHASE-ERROR MULTIPLIER 54 Figure 4. 33 ARRANGEMENT FOR CALIBRATING A SIGNAL GENERATOR .... 54 Figure 4. 34 VIEWING FREQUENCY DRIFT 55 Figure 4. 35 WAVEMETER RESONANCE INDICATORS 56 Figure 4. 36 BLOCK DIAGRAM OF HETERODYNE FREQUENCY METER 57 Figure 4. 37 ARRANGEMENT FOR CHECKING TRANSMITTING FREQUENCY

WITH HETERODYNE FREQUENCY METER AND RECEIVER .... 58 Figure 4. 38 ELECTRONIC AUDIO FREQUENCY METER 59 Figure 4 39 DIRECT-READING RADIO FREQUENCY METER 59 Figure 4 40 BASIC FREQUENCY COMPARATOR 60

Figure 4 41 PHASE ERROR MULTIPLIER 61

Figure 4 42 LINEAR PHASE DETECTOR 62 Figure 4 43 SCHEMATIC DIAGRAM OF NONLINEAR PHASE DETECTOR .... 63 Figure 4 44 ANALOG OR REGENERATIVE TYPE OF DECADE FREQUENCY DIVIDER 63
Figure 4 45 BLOCK DIAGRAM OF A FOUR-STAGE FLIP-FLOP DIVIDER .... . Figure 4 46 FOUR-STAGE FLIP-FLOP DIVIDER WAVEFORMS 64

Figure 4 47 SIMPLIFIED DIAGRAM OF A SIGNAL AVERAGER 65 Figure 4 48 SIMPLE PHASE-TRACKING RECEIVER 66 Figure 5 1 FORMAT OF WWV AND WWVH SECONDS PULSES 69 Figure 5 2 CHU BROADCAST FORMAT . . 70 Figure 5 3 TRANSMISSION SCHEDULE OF ATA OVER A DAY, HOUR, AND MINUTE 71 Figure 5 4 TIME FORMAT OF ATA SIGNAL . 71 Figure 5 5 TRANSMISSION SCHEDULE OF IAM 72 Figure 5 6 JJY EMISSION SCHEDULE . 73 Figure 5 7 METHOD OF INDICATING DUT1 ON JJY . 73 Figure 5 8 VNG TIME CODE . 75 Figure 5 9 DUT1 CODE ON ZUO . 76 Figure 5 10 BLOCK DIAGRAM OF A TYPICAL HIGH-PERFORMANCE HF RECEIVER . 81

Figure 5. 11 QUARTER-WAVELENGTH VERTICAL ANTENNA AND HORIZONTAL

HALF-WAVELENGTH ANTENNA 84

Fi gure 5. 12 MODIFIED HALF-WAVELENGTH VERTICAL ANTENNA FOR USE AT 15, 20, AND 25 MHz . 85

Figure 5. 13 EQUIPMENT SETUP FOR RECEIVER TIME DELAY MEASUREMENTS . 87

Figure 5. 14 OSCILLOGRAM OF DELAYED AND UNDELAYED 1 kHz SIGNAL .... . 87

Figure 5. 15 BLOCK DIAGRAM OF EQUIPMENT CONNECTION FOR DIRECT

TRIGGER METHOD OF TIME SYNCHRONIZATION . 88

Figure 5. 16 WWV TICK AT A SWEEP RATE OF 0.2 SECOND/DIVISION .... 89 Figure 5. 17 CHU TIME TICK AT A SWEEP RATE OF 0.2 SECOND/DIVISION . 89 Figure 5. 18 WWV TICK AT A SWEEP RATE OF 0.1 SECOND/DIVISION .... . 89

Figure 5. 19 SECOND ZERO CROSSOVERS OF SECONDS PULSE AT A SWEEP RATE

OF 1 MILLISECOND/DIVISION (WWV & WWVH) 89

Figure 5. 20 EQUIPMENT SETUP FOR DELAYED TRIGGER METHOD OF TIME SYNCHRONIZATION 91 Figure c 21 SECOND ZERO CROSSOVER OF WWV TICK AT A SWEEP RATE

OF 1 MILLISECOND/DIVISION 91 Figure 5. 22 SECOND ZERO CROSSOVER OF WWV TICK AT A SWEEP RATE

OF 100 MICROSECONDS/DIVISION (4 SWEEPS) . 91

Figure 5. 23 WWV TICK AT A SWEEP RATE OF 1 MILLISECOND/DIVISION

(5 OVERLAPPING EXPOSURES) . 92 Figure 5. 24 WWV TICK AT A SWEEP RATE OF 100 MICROSECONDS/DIVISION

(OVERLAPPING EXPOSURES) 92

F igure 5. 25 THE WWV SPECTRUM AT 10 MHz . 93 Figure 5. 26 WWV/WWVH TIME CODE FORMAT . . . .' . . 95 Figure 5 27 EQUIPMENT SETUP FOR BEAT FREQUENCY METHOD OF CALIBRATION 96

Fi gure 5. 28 DIODE CLIPPING CIRCUIT TO PRODUCE HARMONICS FROM

A PURE SINE WAVE SIGNAL . 97 Figure 5 29 OSCILLOSCOPE DISPLAY SHOWING LISSAJOUS PATTERN .... 99
Figure 5 30 OSCILLOSCOPE DISPLAY USING PATTERN DRIFT METHOD . 100

Figure 5 31 DIAGRAM FOR GREAT CIRCLE DISTANCE CALCULATIONS . 101

Figure 5 32 REFLECTION OF RADIO WAVES AT DIFFERENT IONIZED LAYERS . . . . . 103

Figure 5 33 SINGLE-HOP REFLECTIONS FROM F £ LAYER AT DIFFERENT WAVE ANGLES . 103

Figure 6 1 PHASE OF WWVB AS RECEIVED IN MARYLAND . 108

Figure 6 2 NBS DATA PUBLISHED IN NBS TIME AND FREQUENCY BULLETIN . . 109

Figure 6 3 USNO PHASE DATA, PUBLISHED WEEKLY . 110

Figure 6 4 MEASURED FIELD INTENSITY CONTOURS: WWVB @ 13 kW ERP . Ill

Figure 6 5 MEASURED FIELD INTENSITY, NANTUCKET, MASSACHUSETTS

RADIAL, WWVB 60 kHz . . . 112

Figure 6. 6 MEASURED FIELD INTENSITY, BROWNSVILLE, TEXAS RADIAL, WWVB 60 kHz . . 112

Figure 6. 7 WWVB RECORDING USING A HIGH-QUALITY OSCILLATOR . 113

Figure 6. 8 TYPICAL WWVB PHASE RECORDING . 113

Figure 6. 9 WWVB PHASE PLOT SHOWING PHASE-SHIFT IDENTIFICATION . . 114

Figure 6. 10 OMEGA SIGNAL TRANSMISSION FORMAT . . 117

Figure 6. 11 DCF 77 TIME CODE FORMAT . 119

Figure 6. 12 JG2AS EMISSION SCHEDULE . 120

Figure 6. 13 MSF 60 kHz FAST CODE FORMAT . 122

Figure 6. 14 MSF 60 kHz SLOW CODE FORMAT . 122

Figure 6. 15 TIME CODE FORMAT OF OMA-50 kHz . 123

Figure 6. 16 WWVB TIME CODE FORMAT . 128

Figure 6. 17 WWVB ENVELOPES AS TRANSMITTED . . 129

Figure 7. 1 EQUIPMENT SETUP FOR COMPARING A 5 MHz OSCILLATOR

WITH NETWORK 5 MHz OSCILLATOR . 132

Figure 7. 2 ILLUSTRATION OF PHASE INSTABILITIES INTRODUCED BY NETWORK PATH . . 133

Figure 7. 3 BLOCK DIAGRAM OF COLOR BAR COMPARATOR . 136

Figure 7. 4 SCHEMATIC, COLOR BAR COMPARATOR . 137

Figure 7. 5 THE NBS PROTOTYPE, SYSTEM 358 FREQUENCY MEASUREMENT COMPUTER . 139

Figure 7. 6 BLOCK DIAGRAM, SYSTEM 358 FREQUENCY MEASUREMENT COMPUTER . 139

Figure 7. 7 SAMPLE WORKSHEETS FOR USING TELEVISION SUBCARRIER FOR FREQUENCY TRANSFER . 143

Figure 8. 1 DIFFERENTIAL TIME DELAY, Tg - T^ . 146

Figure 8. 2 TYPICAL ROUTING OF TV SIGNALS FROM NEW YORK CITY ORIGINATING

STUDIOS TO DISTANT RECEIVERS . 146

Figure 8. 3 NBS LINE-10 DATA FROM NBS TIME & FREQUENCY BULLETIN . . 147

Figure 8 4 EQUIPMENT NEEDED FOR TV LINE-10 METHOD OF CLOCK SYNCHRONIZATION . . 148

Figure 8 5 PRINTOUT OF NBC LINE-10 DATA . 148

Figure 8 6 TELEVISION NETWORK FREQUENCIES RELATIVE TO THE NBS FREQUENCY

STANDARD AS PUBLISHED IN THE NBS TIME & FREQUENCY BULLETIN . 149

Figure 8 7 COMPUTATION OF OSCILLATOR FREQUENCY USING TV LINE-10 DATA . . 150

Figure 8 8 SCHEMATIC OF TV LINE-10 EQUIPMENT DEVELOPED BY NBS . . 152

Figure 9 1 LORAN-C PULSE GROUP AND PHASE-CODE FORMAT . 153

Figure 9 2 LORAN-C SIGNAL FORMAT (CHAIN OF SIX STATIONS AT GRI CODE 9930) . 160

I
Figure 9 3 STANDARD LORAN-C PULSE (ECD = 0, POSITIVELY PHASE-CODED) . 162

Figure 9 4 LORAN-C PULSE WITH ECD = +3.0 MICROSECONDS . 162

Figure 9 5 LORAN-C PHASE CODES . 165

Figure 9 6 LORAN-C BLINK CODE . 165

Figure 9 7 THE 100 kHz LORAN-C PULSE . 167

Fgiure 9 8 INSTRUMENTATION FOR UTILIZING PULSES WITHIN THE LORAN-C

PULSE GROUPS ... . 168

Figure 9 9 TIMING OF LORAN-C SIGNALS . 168

Figure 9 10 STEPS IN LOCKING ONTO A LORAN-C SIGNAL . 169

Figure 9 11 SAMPLE RECORD OF RECEIVED LORAN-C SIGNAL . 171

Figure 9 12 PLOT OF THE RECEIVED LORAN-C SIGNAL . 172

Figure 10 1 COVERAGE OF THE GOES SATELLITES • 176

Figure 10 2 INTERROGATION CHANNEL FORMAT . 177 Figure 10 3 TIME CODE FORMAT . 177 Figure 10 4 WESTERN SATELLITE POINTING ANGLES . 178

Figure 10 5 EASTERN SATELLITE POINTING ANGLES . 178

Figure 10 6 WESTERN SATELLITE MEAN DELAYS . 179

Figure 10 7 EASTERN SATELLITE MEAN DELAYS . 179

Figure 10 8 TYPICAL DELAY VARIATIONS FOR THE EASTERN SATELLITE . 180

Figure 10 9 RIGHT-HAND CIRCULARLY POLARIZED HELIX ANTENNA USED

TO RECEIVE GOES SIGNALS . 181

Figure 10 10 MICROSTRIP ANTENNA USED TO RECEIVE GOES TIME CODE .... , 18a' Figure 10 11 GOES TIME CODE RECEIVER . 181

Figure 10. 12 SMART CLOCK . 182

Figure 10. 13 SMART CLOCK BLOCK DIAGRAM . 183

Figure 10. 14 MEASUREMENT OF GOES SIGNALS . 185

Figure 10. 15 SATELLITE SIGNALS CORRECTED AND UNCORRECTED . 185

Figure 10. 16 TYPICAL RESULTS . 186

Figure 10. 17 FREQUENCY USE . 186

Figure 10. 18 SOLAR ECLIPSE TIME . 187 Figure 10. 19 TRANSIT SATELLITE CLOCK CONTROL SYSTEM . 188

Figure 10. 20 TRANSIT TIMING RECEIVER BLOCK DIAGRAM . 189

Figure 11. 1 EXAMPLE OF A CLOCK SYSTEM . 191

Figure 11. 2 DEFINITION OF TIME AND FREQUENCY . 192

Figure 11. 3 FREQUENCY SOURCE AND CLOCK . 193

Figure 11. 4 RELATIONSHIPS BETWEEN CLOCK ACCURACY, FREQUENCY

STABILITY, AND FREQUENCY OFFSET . 195

Figure 11. 5 EXAMPLES OF RESONATORS . 196

Figure 11. 6 DECAY TIME, LINEWIDTH , AND Q- VALUE OF A RESONATOR .... . 196

Figure 11. 7 THE U.S. PRIMARY FREQUENCY STANDARD, NBS-6 . 198

Figure 11. 8 HIERARCHY OF FREQUENCY STANDARDS . 198

Figure 11. 9 THE PIEZOELECTRIC EFFECT . 199

Figure 11 10 PRINCIPAL VIBRATIONAL MODES OF QUARTZ CRYSTALS .... . 200
/ Figure 11 11

T\/r\T O A 1 AIIAHT7 Pn\/fTA 1 MAI HIT TYPICAL QUARTZ CRYSTAL MOUNT ........ AAA ^UU

Figure 11 12 QUARTZ CRYSTAL OSCILLATOR ........ AAA 200

Figure 11 13 FUNDAMENTAL AND OVERTONE RESONANCE FREQUENCIES .... 2UU

Figure 11 14 rnrni irnp\/ a~ ~r a n t i tt\/ r\ r~ Till- nrTTrn ni ia nT7 /^n\/rT A l at ati i a ~r A n (~ FREQUENCY STABILITY OF THE BETTER QUARTZ CRYSTAL OSCILLATORS AAA 202

Figure 11 15

/-> r\ n ~r t a i /—r a t r" rn r at t ah SPATIAL STATE SELECTION ......... A A *> 203

Figure 11 16 OPTICAL STATE SELECTION ......... 203

Figure 11 17

A » i n TTTATT Ail ATOM DETECTION ........... 204

Figure 11 18 OPTICAL DETECTION .......... 205

Figure 11 19 UT Afirtl 1 A I IT~ nTTTATT All MICROWAVE DETECTION . . . - . AAA 206

Figure 11 20 ATOMIC FREQUENCY OSCILLATOR ........ 206

Figure 11 21 Arr ti iki n r a kt rnrAi irki^v/ aaat i i a Trv n CESIUM BEAM FREQUENCY OSCILLATOR ....... 208

Fi gure 11 A

22 rnrAi itmau atadti t t\/ a r a AMimr nAT ai att tiiu n r a m FREQUENCY STABILITY OF COMMERCIAL CESIUM BEAM mrAi irn a\/ aa ati i atapit FREQUENCY OSCILLATORS ......... 208

Figure 11 23 ni in t at i iiji a a r r* r i i rnrAi irkiA\/ a r a t i i ataa RUBIDIUM GAS CELL FREQUENCY OSCILLATOR ...... . 208

Figure 11. 24 rnrAi irkiA\/ t a n t i t t\/ Ar AAkuirnAT a i ni in t n ti im FREQUENCY STABILITY OF COMMERCIAL RUBIDIUM

<-> a /- f> r— i i r— n r a i ii— ha\/ /"> /- i~> -r i i a t/"\ /~ GAS CELL FREQUENCY OSCILLATORS ....... . 209

Fi gure 11. 25 i lUnnAArki k* a a rn a r a t i i ataa HYDROGEN MASER OSCILLATOR ........ . 209

Figure 11 26 FREQUENCY STABILITY OF A HYDROGEN MASER OSCILLATOR . 210

Figure 12. 1 WWV/WWVH HOURLY BROADCAST FORMAT . 215

XV
ABSTRACT

This manual has been written for the

person who needs information on making time

and frequency measurements. It has been

written at a level that will satisfy those

with a casual interest as well as laboratory

engineers and technicians who use time and

frequency every day. It gives a brief history of time and frequency, discusses the

roles of the National Bureau of Standards,

the U. S. Naval Observatory, and the International Time Bureau, and explains how time

and frequency are internationally coordinated.

It also explains what time and frequency services are available and how to use them. It discusses the accuracies that can be achieved

using the different services as well as the

pros and cons of using various calibration

methods.

Key Words: Frequency calibration; high fre- quency; Loran-C; low frequency; radio broadcasts; satellite broadcasts; standard fre- quencies; television color subcarrier; time

and frequency calibration methods; time calibration; time signals.

PREFACE

This manual was written to assist users

of time and frequency calibration services

that are available in the U.S. and throughout

the world. An attempt has been made to avoid

complex derivations or mathematical analysis.

Instead, simpler explanations have been given

in the hope that more people will find the

material useful.

Much of the information contained in this book has been made available in NBS Technical Notes. These were published in the

last few years but have been edited and re- vised for this book. Many people and organizations have contributed material for this

book, including other government agencies,

international standards laboratories, and

equipment manufacturers.

Since each topic could not be covered in great depth, readers are encouraged to request

additional information from NBS or the respon- sible agency that operates the system or ser- vice of interest.

xvi
CHAPTER 1. INTRODUCTION: WHAT THIS BOOK IS ABOUT

Time and frequency are all around us. We

hear the time announced on radio and television, and we see it on the time and temperature sign at the local bank. The frequency

markings on our car radio dials help us find our favorite stations. Time and frequency are

so commonly available that we often take them

for granted; we seldom stop to think about

where they come from or how they are measured.

Yet among the many thousands of things

that man has been able to measure, time and

time interval are unique. Of all the stan- dards—especially the basic standards of

length, mass, time, and temperature—time (or more properly time interval) can be measured

with a greater resolution and accuracy than

any other. Even in the practical world away

from the scientific laboratory, routine time

and frequency measurements are made to resolutions of parts in a thousand billion. This is in sharp contrast to length measurements, for

instance, where an accuracy of one part in ten thousand represents a tremendous achievement.

1.1 WHO NEEDS TIME & FREQUENCY?

Without accurate time, our daily lives

could not function in an orderly manner. If you don't know what time it is, how can you

meet a friend for lunch? Or get to school or work on time? It's all right to get to church

early, but it's embarrassing to walk in during

the sermon. And you would probably be more

than a little disappointed if you missed your

airplane after months of planning a Hawaiian

vacation.

In these instances, knowing the correct

time to within a few minutes is usually sufficient. But even a few minutes can sometimes

be quite important. For instance, every day

hundreds of people drop nickels, dimes, and

quarters into parking meters, coin-operated

washers and dryers, and "fun" machines that

give children a ride on a galloping horse or a miniature rocket. Housewives trust their

cakes and roasts, their clothing and fine

china to timers on ovens, washing machines,

and dishwashers. Businesses pay thousands of

dollars for the use of a computer's time. We

all pay telephone bills based on the number of

minutes and parts of minutes we spend talking to relatives or friends halfway across the

nation.

All of these activities require accurate

time. Fifteen minutes on a parking meter

should really be 15 minutes and not 14. An error in the meter's timer could mean a parking ticket. And if we only talk on the phone

for 7 minutes, we don't want to be billed for

9 or 10.

Frequency is just as important as time.

Accurate frequency control at TV stations means programs are transmitted on the exact frequencies assigned by the Federal Communications Commission. Controlling electric

power flow in homes and offices at 60 hertz

keeps electric clocks from running too fast or too slow. It means that a hi-fi will play a record at 33-1/3 rpm (revolutions per minute)

instead of 32 or 34 rpm, thus reproducing the

recorded music accurately.

Our voices range in frequency from about

87- hertz (bass) to 1175 hertz (soprano), and our ears can detect sounds ranging from about

16 to 16,000 hertz. For example, you hear a telephone dial tone at about 400 hertz, a smoke alarm at 600 hertz, and a dentist's

ultrasonic drill at 10,000 hertz. A piano

1
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£ X ^

CC LU

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CYCLES PER SECOND BASED ON CURRENT MUSICAL PITCH A=440; PHYSICAL PITCH A=426.667; INTERNATIONAL PITCH A=435

LIMITS ON HUMAN EAR SENSITIVITY

ranges in frequency from 27.5 hertz (A4 ) to 4186 hertz (E 4

). "A" above middle "C" in the musical scale is 440 hertz--the

frequency reference most often used to tune musical instruments.

There are many industries and professions that need accurate time and frequency, and there are many services that

provide this information. Many of these will be discussed in this book.

But, you ask, who needs time and frequency to the accu- racy provided by these NBS stations? The truth is, for many

applications, just knowing the exact minute is often not

enough. Sometimes, it is essential to know the exact second

or even millionth of a second (microsecond). Let's take a look at some of the more sophisticated users of time and

frequency:

CELESTIAL NAVIGATORS need time to determine their exact

location. An error of 2 seconds could cause a ship to miss its destination by about 1 kilometer. Other

SHIPPERS and BOATERS need time even more accurately.

When using sophisticated electronic navigation systems,

an error of only 3 microseconds could cause the same 1 ki lometer error.

POWER COMPANIES use frequency to control electric power flow

at 60 hertz. If they didn't, clocks could not maintain

the correct time. They need time to monitor the power

grid to help alleviate "brownouts" and massive power

failures. They use it to help locate power outages and

trouble on the lines. To minimize down time, they need

to know the exact second when outages occur. Time is also important for keeping track of power flow among the

various companies in the interconnected network for

bi 1 1 ing purposes.

RADIO & TV STATIONS need accurate frequency to send signals

at exactly their assigned frequencies. They need accu- rate time to set station clocks so they can join the

network at the right instant.

The MEDICAL PROFESSION uses time and frequency for medical test equipment calibration, for date and time printouts

in coronary care units, and for timing therapy and

observational procedures for daily health care.

2
The OIL INDUSTRY needs accurate time to help automate oil

well drilling, especially offshore.

JEWELERS & CLOCK/WATCH MANUFACTURERS need to set digital

watches and clocks to the correct time before they leave

the factory.

RAILROADS use time to set watches and clock systems. E.g., AMTRAK gets accurate time three times a day to set clocks in the AMTRAK system. This insures that trains

arrive and depart on schedule.

The COMPUTER INDUSTRY needs accurate time for billing pur- poses, for timing the beginning and end of events for

data processing, and for synchronizing communication

between systems many miles apart.

POLICEMEN need time to check stopwatches used to clock

speeders and they use frequency to calibrate radar

"speed guns" used for traffic control.

SURVEYORS need time to measure distance and location. As already stated, 3 microseconds translates into 1 kilo- meter in distance when modern electronic instrumentation

is used.

The COMMUNICATIONS INDUSTRY depends on accurate frequency

control for its ability to deliver messages to its users. Time is needed for labeling the time of occur- rence of important messages. For example, radio station

WWV is used at the communications center at Yellowstone

National Park.

The MUSIC INDUSTRY uses frequency (the 440-hertz tone from

WWV, for example) to calibrate tuning forks which are

used to tune pianos, organs, and other musical instruments.

MANUFACTURERS need time and frequency to calibrate counters,

frequency meters, test equipment, and turn-on/turn-off

timers in electric appliances.

The TRANSPORTATION INDUSTRY needs accurate time to synchronize clocks used in bus and other public transportation

systems, and for vehicle location, dispatching, and

control

SPORTSMEN use time. Sports car rallies are timed to 1 /l 00th

of a second. Even carrier pigeon racers need accurate

time.

The TELEPHONE INDUSTRY needs accurate time for billing

purposes and telephone time-of-day services. Accurate frequency controls long-distance phone calls so that

messages don't become garbled during transmission.

NATURALISTS studying wildlife habits want accurate time and

frequency to help monitor animals they have fitted with

radio transmitters.

The TELECOMMUNICATIONS INDUSTRY needs time accurate to one microsecond to synchronize satellite & other communications terminals spread over wide geographical areas.

ASTRONOMERS use time for observing astronomical events, such

as lunar occultations and eclipses.

3
GEOPHYSICISTS/SEISMOLOGISTS studying lightning, earthquakes,

weather, and other geophysical disturbances need time to enable them to obtain data synchronously and automatically over wide geographical areas. They use it for

labeling geophysical events. Other SCIENTISTS use time

for controlling the duration of physical and chemical

processes.

Accurate time is required in MASTER CLOCK SYSTEMS in large

institutions, such as airports, hospitals, large factories, and office buildings so that all clocks in the

system read the same time.

The AVIATION/AEROSPACE INDUSTRY needs accurate time for aircraft traffic control systems and for synchronization at satellite and missile tracking stations. The FAA rec- ords accurate time on its audio tapes along with the

air-to-ground communications from airplanes. Having an accurate record of when particular events happened can

-T~ be an important factor in determining the cause of a IZ_ plane crash or equipment malfunction.

MILITARY organizations use accurate time to synchronize

clocks on aircraft, ships, submarines, and land vehicles. It is used to synchronize secure communications

between command posts and outposts. Stable frequency

will be necessary for navigation using a future satel1 ite system.

1.2 WHAT ARE TIME AND FREQUENCY?

We should pause a moment to consider what

is meant by the word "time" as we commonly use

it. Time of day or date is the most often

used meaning, and even that is usually pre- sented in a brief form of hours, minutes, and

seconds, whereas a complete statement of the

time of day would also include the day of the

week, month, and year. It could also extend

to units of time smaller than the second

going down through milliseconds, microseconds,

nanoseconds, and picoseconds.

TIME OF DAY: 9:00 A.M.

5 MINUTES

TIME INTERVAL

We also use the word time when we mean

the length of time between two events, called

time intervals. The word time almost always

needs additional terms to clarify its meaning;

for instance, time of day or time interval .

Today time is based on the definition of

a second. A second is a time interval and it

is defined in terms of the cesium atom. This

is explained in some detail in the chapter on atomic frequency sources (chapter 11). Let us say here that a second consists of counting

9,192,631,770 periods of the radiation associated with the cesium- 133 atom.

The definition of frequency is also based

on this definition. The term used to describe

frequency is the hertz which is defined as one cycle per second.

What does all this mean to users of time

and frequency? Where does the laboratory scientist, industrial engineer, or for that

matter, the man on the street go when he needs

information about frequency and time measurements or about performing those measurements

himself? That's what this book is about. It has been written for the person with a casual

interest who wants to set his watch and for

those with a specific need for frequency and

time services to adjust oscillators or perform

related scientific measurements.

This book has been deliberately written

at a level which will satisfy all of these

4
VOLUME !

users. It will explain what is available in our world today in the form of services that

provide time and frequency for many classes of

users. In addition to providing information

regarding these services, detailed explanations are given on how to use each service.

Many countries throughout the world provide

time and frequency services.

You can get time of day by listening to high frequency radio broadcasts, or you can

call the time-of-day telephone services. You

can decode time codes broadcast on various

high, low, and very low frequency radio stations. You can measure frequency by accessing

certain signals on radio and television broadcasts. You can obtain literature explaining

the various services. And if your needs are critical, you can carry a portable clock to NBS or the USNO for comparison. This book

describes many of these services and explains

how to use them for time and frequency calibrations.

1.3 WHAT IS A STANDARD?

Of course, before you can have a standard

frequency and time service , you have to have a standard , but what is it? The definition of a legal standard is, according to Webster,

"something set up and established by authority, custom, or general consent as a model or example." A standard is the ultimate unit used

for comparison. In the United States, the

National Bureau of Standards (NBS) is legally

responsible for maintaining and disseminating

all of the standards of physical measurement.

There are four independent standards or base units of measurement. These are length, mass, time, and temperature. By calling them

independent, we mean that all other measurements can be derived from them. It can be

shown mathematically that voltage and pressure

measurements can be obtained from measurements

of these four base units. It is also true

5
that frequency or its inverse, time interval,

can be controlled and measured with the smallest percentage error of any physical quantity. Since a clock is simply a machine that

counts frequency or time intervals, then time

is kept with equal accuracy.

1.3.1 CAN TIME REALLY BE A STANDARD?

Time is not a "standard" in the same sense as the meter stick or a standard set of

weights. The real quantity involved here is that of time interval (the length of time

between two events). You can make a time

interval calibration by using the ticks or tones on WWV, for example, to obtain second,

minute, or hour information, but you usually

need one more piece of information to make

that effort worthwhile. The information you

need is the time of day. All national laboratories, the National Bureau of Standards among

them, do keep the time of day; and even though

it is not a "standard" in the usual sense, extreme care is exercised in the maintenance

of the Bureau's clocks so that they will

always agree to within a few microseconds with

the clocks in other national laboratories and

those of the U. S. Naval Observatory. Also,

many manufacturing companies, universities,

and independent laboratories find it convenient to keep accurate time at their facilities.

1.3.2 THE NBS STANDARDS OF TIME AND FREQUENCY

As explained later in this chapter and

elsewhere in this book, the United States standards of frequency and time are part of a coordinated worldwide system. Almost the

entire world uses the second as a standard

unit of time, and any variation in time of day

from country to country is extremely small.

But unlike the other standards, time is always changing; so can you really have a time

standard? We often hear the term standard

time used in conjunction with time zones. But

is there a standard time kept by the National Bureau of Standards? Yes there is, but be- cause of its changing nature, it doesn't have

the same properties as the other physical

standards, such as length and mass. Although

NBS does operate a source of time, it is adjusted periodically to agree with clocks in other countries. In the next chapter we will

attempt to explain the basis for making such

changes and how they are managed and organized

throughout the world.

So even though NBS does not have a glassenclosed clock that is untouched or unadjusted, we do have a standard for frequency and

time interval. This standard is located in Boulder, Colorado. It carries the designation

NBS-6 because it is the sixth in a series of

NBS-6

(PRIMARY CLOCK)

(SECONDARY CLOCKS)

RADIO

BROADCASTS

SERVICES

TELEVISION

TIME & FREQUENCY

TRANSFER

TIME

SCALE SERVICES

COMPARISONS

WITH TIME

SCALES IN OTHER COUNTRIES TELEPHONE

TIME-OF-DAY

SERVICE

SATELLITE

FIGURE 1.1. ORGANIZATION OF THE NBS FREQUENCY AND TIME STANDARD.

6
atomic oscillators built and maintained by NBS

to provide the official reference for frequency and time interval in the United States. NBS-6 is referred to as the "master" or "primary" clock. It is used to calibrate other

oscillators ("secondary" clocks) which are used to operate the time scale. (A time scale

is a system of counting pendulum swings.) The

organization of the NBS frequency and time

standard is illustrated in figure 1.1. Shown

is the primary atomic standard NBS-6, which is used to calibrate an ensemble of other atomic

oscillators which operate the time scale,

which is in turn compared and adjusted to similar time scales in other countries. Periodic adjustments are made to keep them all in agreement.

The useful output of the NBS standard and

its associated time scale are the services

provided to users. These services can take

many forms and use several methods to get the

actual needed calibration information to the

user.

Notice that figure 1.1 looks a lot like any timepiece. We have a source of frequency,

a means of counting that frequency and keeping

time of day in a time scale, and as mentioned,

a way of getting a useful output. This is exactly what is contained in almost every

clock or watch in use today. The frequency

source can be a 60 Hz power line, a balance

wheel, tuning fork, or quartz crytal. The counter for totaling the cycles of frequency

into seconds, minutes, and hours can be gears

or electronics. The readout can be a face

clock or digits.

In early times, the sun was the only

timekeeping device available. It had certain

disadvantages--cloudy days, for one. And

also, you could not measure the sun's angle

very accurately. People started developing

clocks so they could have time indoors and at

night. Surely, though, the sun was their

"standard" instead of a clock. Outdoors, the

sundial was their counter and readout. It is reasonable to suppose that if you had a large

hour glass, you could hold it next to the

sundial and write the hour on it, turn it over, and then transport it indoors to keep

the time fairly accurately for the next hour.

A similar situation exists today in that

secondary clocks are brought to the master or primary clock for accurate setting and then

used to keep time.

1.4 HOW TIME AND FREQUENCY STANDARDS

ARE DISTRIBUTED

In addition to generating and distributing standard frequency and time interval, the

National Bureau of Standards also broadcasts

time of day via its radio stations WWV, WWVH,

and WWVB. Each of the fifty United States has

a state standards laboratory which deals in many kinds of standards, including those of

frequency and time interval. Few of these

state labs get involved with time of- day,

which is usually left to the individual user, but many of them can calibrate frequency

sources that are used to check timers such as those used in washing machines in laundromats, car washes, and parking meters.

STDS STDS STDS

LAB LAB LAB

In private industry and other government

agencies where standards labs are maintained,

a considerable amount of time and frequency

work is performed, depending on the end pro- duct of the company. As you might guess, electronics manufacturers who deal with

counters, oscillators, and signal generators

are very interested in having an available

source of accurate frequency. This will very

often take the form of an atomic oscillator

kept in the company's standards lab and

calibrated against NBS.

Although company practices differ, a typical arrangement might be for the company

standards lab to distribute a stable frequency

signal to the areas where engineers can use the signal for calibration. Alternatively,

test equipment can periodically be routed to the standards lab for checking and adjustment.

As mentioned elsewhere in this book, the

level of accuracy that can be achieved by a standards lab when making a frequency calibration ranges from a few parts per thousand

to one part in a thousand billion, or 1 part

in 10 12

. Precise calibrations do involve more

careful attention to details, but are not in fact that difficult to achieve.

1.5 THE NBS ROLE IN INSURING ACCURATE

TIME AND FREQUENCY

The National Bureau of Standards is responsible for generating, maintaining, and

7
NBS TO STDS STDS

LAB VIA WWV. LAB WATCH

distributing the standards of time and fre- quency. It is not a regulatory agency so it does not enforce any legislation. That is,

you cannot get a citation from NBS for having

the incorrect time or frequency.

Other government agencies, however, do issue citations. Most notable among these is the Federal Communications Commission. This

agency regulates all radio and television

broadcasts, and has the authority to issue

citations and/or fines to those stations that

do not stay within their allocated frequencies. The FCC uses NBS time and frequency

services to calibrate their instruments which,

in turn, are used to check broadcast transmitters.

trace the signal backward toward its source, you will find that it perhaps goes through a distribution amplifier system to a frequency

source on the manufacturing plant grounds. Let's say this source is an oscillator. Some means will have been provided to calibrate its

output by using one of several methods--let'

say NBS radio signals. With suitable records,

taken and maintained at regular intervals, the

oscillator (frequency source) can claim an accuracy of a certain level compared to NBS. This accuracy is transferred to your laboratory at perhaps a slightly reduced accuracy. Taking all these factors into account, the

signal in your laboratory (and therefore your

watch) is traceable to NBS at a certain level of accuracy.

The Bureau's role is simply to provide

access to the standards of frequency and time

interval to users and enforcement agencies

alike. When radio and television stations

make calibrations referenced to NBS, they can

be confident that they are on frequency and are not in violation of the broadcasting regulations.

1.6 WHEN DOES A MEASUREMENT BECOME

A CALIBRATION ?

Using an ordinary watch as an example,

you can either move the hands to set the time

of day, or you can change the rate or adjust

the frequency at which it runs. Is this measurement a calibration? It depends on what

reference you use to set the watch.

If the source you use for comparison is traceable at a suitable level of accuracy back

to the National Bureau of Standards, (or the

U.S. Naval Observatory in the case of DOD users), then you can say you have performed a calibration. It is very important to keep in mind that every calibration carries with it a measure of the accuracy with which the calibration was performed.

The idea of traceability is sometimes

difficult to explain, but here is an example.

Suppose you set your watch from a time signal

in your laboratory that comes to you via a company-operated distribution system. If you

What kind of accuracy is obtainable? As we said before, frequency and time can both be

measured to very high accuracies with very great measurement resolution. As this book

explains, there are many techniques available

to perform calibrations. Your accuracy de- pends on which technique you choose and what

errors you make in your measurements. Typically, frequency calibration accuracies range from parts per million by high frequency radio I signals to parts per hundred billion for

television or Loran-C methods. A great deal depends on how much effort you are willing to expend to get a good, accurate measurement.

1.7 TERMS USED

1.7.1 MEGA, MILLI, PARTS PER... AND PERCENTS

Throughout this book, we refer to such

things as kilohertz and Megahertz, milliseconds and microseconds. We further talk about ". accuracies of parts in 10 9 or 0.5%. What do

all of these terms mean? The following tables

explain the meanings and should serve as a convenient reference for the reader.

Table 1.2 gives the meaning of 1 x 10- 6 but what is 3 x 10- 6

? You can convert in the same way. Three parts per million or 3 x 10- 6 is .0003%. Manufacturers often quote percentage accuracies in their literature rather ; than "parts per. ..."

8
TABLE 1.1 PREFIX CONVERSION CHART

PREFIX DEFINITION EXAMPLE

LESS THAN 1

MILLI ONE THOUSANDTH MILLISECOND (ms) = ONE THOUSANDTH OF A SECOND

MICRO ONE MILLIONTH MICROSECOND (ys) = ONE MILLIONTH OF A SECOND

NANO ONE BILLIONTH NANOSECOND (ns) = ONE BILLIONTH OF A SECOND

PICO ONE TRI LLIONTH PICOSECOND (ps) = ONE TRI LLIONTH OF A SECOND

MORE THAN 1

Kl LO ONE THOUSAND KILOHERTZ (kHz) = ONE THOUSAND HERTZ (CYCLES PER SECOND)

MEGA ONE MILLION MEGAHERTZ (MHz) = ONE MILLION HERTZ

GIGA ONE BILLION GIGAHERTZ (GHz) = ONE BILLION HERTZ

TERA ONE TRILLION TERAHERTZ (THz) = ONE TRILLION HERTZ

TABLE 1.2 CONVERSIONS FROM PARTS PER... TO PERCENTS

PARTS PER PERCENT

1 PART PER HUNDRED 1 x

10"2 =

1 100 1.0*

1 PART PER THOUSAND I x 10"3 1

1 ,000 0. 1*

1 PART PER 10 THOUSAND 1 x 10

4 1 10,000 0.01?

1 PART PER 100 THOUSAND 1 x

10~ 5 1 100,000 0.001*

1 PART PER MILLION 1 x

10~ 6 1

1 ,000,000 0.0001*

1 PART PER 10 MILLION 1 x

10" 7 1 10,000,000 0.00001*

1 PART PER 100 MILLION 1 x 10" 8 1 100,000,000 0.000001*

1 PART PER BILLION 1 x

10" 9 1

1 ,000,000,000 0.0000001*

1 PART PER 10 BILLION 1 x l(f

10 1 10,000,000,000 0.00000001*

1 PART PER 100 BILLION - 1 x 10" 11 1 100,000,000,000 0. 000000001%

1 PART PER 1 ,000 BILLION

(1 PART PER TRILLION) = 1 x 10" 12 1

1 ,000,000,000,000 0.0000000001%

1 PART PER 10,000 BILLION 1 x 10" 13 1 10,000,000,000,000 0.00000000001*

9
1.7.2 FREQUENCY

The term used almost universally for

frequency is the hertz, which means one cycle

per second. With the advent of new integrated

circuits, it is possible to generate that

"cycle" in many different shapes. It can be a sine wave or a square, triangular, or sawtooth

wave. So the reader is cautioned as he pro- ceeds through this book to keep in mind that

the waveforms being considered may not in fact

be sinusoids (sine waves). In this book we

will not concern ourselves greatly with the

possibility that the frequency of interest can contain zero frequency or DC components.

Instead we will assume that the waveform

operates, on the average, near zero voltage.

Throughout this book, consideration is given to frequencies of all magnitudes--f rom the one hertz tick of the clock to many billions of hertz in the microwave region. Table

1.3 gives the prefixes used for frequencies in

different ranges and also the means of con- verting from one kind of unit to another.

Thus it is possible to refer to one thousand

on the AM radio band as either 1000 kHz or 1 MHz.

Table 1.4 lists the frequencies by bands. Most frequencies of interest are included in

this table--the radio frequency band contains

the often heard references to high frequency,

very high frequency, low frequency, etc.

The reader is cautioned that the difficulty in measuring frequency accurately is not directly related to the frequency range. Precise frequency measurements at audio frequencies are equally as difficult as those in the high frequency radio bands.

Scattered throughout this book are ref- erences to the wavelength rather than the

frequency being used. Wavelength is especially convenient when calculating antenna

TABLE 1.3 CONVERSIONS TO HERTZ

FREQUENCY EXAMPLES

1 Hz = 10° =1 CYCLE PER SECOND

2000 Hz = 2 kHz = 0.002 MHz

- 1 kHz = 10

3 =1 ,000 Hz

25 MHz = 25,000 kHz = 25 MILLION Hz

1 MHz = 10

6 =1 ,000,000 Hz

1 GHz = 10 9 = 1,000,000,000 Hz

10 GHz = 10,000 MHz = 10 MILLION kHz

1 THz = 10

12 = i 000 BILLION Hz

TABLE \.k RADIO FREQUENCY BANDS

RF BAND FREQUENCY RANGE WAVELENGTH (A)

h VLF (VERY LOW FREQUENCY) 3 - 30 kHz 10 5

- 1 0^ METERS

5 LF (LOW FREQUENCY) 30 - 300 kHz 10 - 10 5 "

6 MF (MEDIUM FREQUENCY) 300 kHz - 3 MHz

3 2 lcr - 10

7 HF (HIGH FREQUENCY) 3 - 30 MHz 10

2

- 10 "

8 VHF (VERY HIGH FREQUENCY) 30 - 300 MHz 10-1 "

9 UHF (ULTRA HIGH FREQUENCY) 300 MHz - 3 GHz 1 - 0.1 "

10
lengths. In fact, a glance at an antique

radio dial shows that the band was actually

marked in wavelengths. For example, radio amateurs still refer to their frequency allocations in terms of the 20, 10, or 2 meter

bands.

The conversion of wavelengths to frequency can be made for most purposes by using

the simple equation

. _ 300,000,000

where A. is the wavelength in meters,

300,000,000 meters per second is the speed of

light, and f is the frequency in hertz. So we

can see that the ten meter band is approximately 30 MHz and 1000 on the broadcast band

is 300 meters.

This equation can be converted to feet

and inches for ease in cutting antennas to exact length. Precise calculations of wavelength would have to take into account the

medium and allow for the reduced velocity

below that of light, for instance, inside

coaxial cables.

Many users of frequency generating de- vices tend to take frequency for granted,

especially in the case of crystals. A popular

feeling is that if the frequency of interest

has been generated from a quartz crystal, it cannot be in error by any significant amount. This is simply not true. Age affects the

frequency of all quartz oscillators. Although

frequency can be measured more precisely than

most phenomena, it is still the responsibility

of the calibration laboratory technician or general user to keep in mind the tolerances

needed; for example, musical notes are usually

measured to a tenth of a hertz or better. Power line frequency is controlled to a millihertz. If you dial the NBS time-of-day telephone service, you will hear audio frequency

tones that, although generated to an accuracy

of parts per one thousand billion, can be sent

over the telephone lines only to a few parts

in one thousand. So the responsibility is the

users to decide what he needs and whether, in fact, the measurement scheme he chooses will

satisfy those needs.

Throughout this book, mention is made of

the ease with which frequency standards can be

calibrated to high precision. Let us assume

that you have, in fact, just calibrated your

high quality crystal oscillator and set the

! associated clock right on time. What happens

j next? Probably nothing happens. The oscillators manufactured today are of excellent

quality and, assuming that a suitable battery

supply is available to prevent power outages,

the clock could keep very accurate time for

many weeks. The kicker in this statement is,

of course, the word "accurate." If you want

to maintain time with an error as small or smaller than a microsecond, your clock could

very easily have that amount of error in -a few

minutes. If you are less concerned with

microseconds and are worried about only milliseconds or greater, a month could elapse

before such an error would reveal itself.

The point to be made here is that nothing

can be taken for granted. If you come into

your laboratory on Monday morning hoping that

everything stayed put over the weekend, you

might be unpleasantly surprised. Digital

dividers used to drive electronic clocks do

jump occasionally, especially if the power

supplies are not designed to avoid some of the

glitches that can occur. It makes sense, therefore, to check both time and frequency

periodically to insure that the frequency rate

is right and the clock is on time.

Many users who depend heavily on their

frequency source for cal ibrations--for example, manufacturing plants--find it convenient

to maintain a continuous record of the frequency of their oscillators. This usually

takes the form of a chart recording that shows

the frequency variations in the oscillator

versus a received signal from either an NBS station or one of the many other transmissions

which have been stabilized. Among these are the Omega and Loran-C navigation signals.

If you refer to Chapter 11 of this book,

which deals with the characteristics of oscillators, you will notice that crystal oscillators and even rubidium oscillators will drift

in frequency so, depending on your application, periodic adjustments are required.

1.8 DISTRIBUTING TIME AND FREQUENCY SIGNALS

VIA CABLES AND TELEPHONE LINES

Users of time and frequency signals some- times want to distribute either a standard

frequency waveform or a time signal using

cables. This is often the case in a laboratory or manufacturing plant where the stan- dards laboratory provides signals for users throughout the plant area. Of course, the

solution to any given problem depends on good

engineering practices and consideration for

the kind of result desired. That is to say,

if you start with a cesium oscillator and want

to maintain its accuracy throughout a large

area, you must use the very best of equipment

and cables; and even then, the signal accuracy

will be somewhat deteriorated. On the other

hand, if all that is needed is a time-of-day

signal on the company telephone switchboard,

the specifications can be relaxed.

11
Without knowing the particular conditions

under which signals are to be distributed, it would be hard to specify the maximum accuracy

obtainable. A number of articles have been

published for distribution systems using both

coaxial cables and telephone lines. The

results that the individual writers reported

varied from parts per million to almost one part in 10 billion. In each case the results

were in direct ratio to the amount of effort

expended. Since the articles were written a number of years ago, it is expected that bet- ter results could be achieved today. Other

than the good engineering practices mentioned

above, there is no simple formula for successful transmission of standard frequency and

time signals.

Often the main problem is noise pickup in

the cables that causes the signal at the end

of the line to be less useful than desired.

Another problem often reported is the difficulty in certifying the received accuracy or

to establish NBS traceability over such a distribution system. A suggestion would be

that careful system management be observed and

that periodic evaluation of the system be

attempted. One method of evaluating a distribution system would be to route the signal on

a continuous loop and look at the signal going

in and the signal coming out for a particular

cable routing.

For commercial telephone lines (which are usually balanced systems), the highest practical frequency that can be distributed is 1000 Hz. Coaxial cables have been used suc- cessfully for frequencies up to 100,000 Hz.

Any attempt to send pulses over a voice-grade

telephone line would be unsuccessful due to the limited bandwidth.

At the NBS Boulder Laboratories, 5 MHz signals are transmitted within the building.

Commercial distribution amplifiers with failsafe provisions are used to drive the cables

at a nominal 50 ohms impedance. Careful cable

management assures good results. Even so, very often noise will appear on the cables and

degrade the signals. The noise on some cables

due to their routing was so bad they were

abandoned.

The NBS telephone time-of-day signal is received at Boulder via a leased commercial

telephone line that is about 100 miles in length. The measured delay for this line is approximately 3 to 5 milliseconds. This seems

to be the usual order of magnitude of delay

reported for commercial telephone circuits.

If you plan to use such circuits for distribution of signals, keep in mind that a variety

of leased lines of varying quality are available from telephone companies.

For signals that are being transmitted

over long distances on telephone lines, the error in the received frequency is usually

several hertz or more. This is a consequence

of the method used by the telephone companies

to combine several signals on a transmission

line using frequency division multiplexing

techniques. This would be the case, for

example, if you dialed the NBS time-of-day

service on 303-499-7111. The tone as received

could be in error by several parts in 103

.

12
For those users wanting to distribute

either very accurate frequency or precise

time, it is almost fair to say that local

cable and wire systems are not economically

practical. If your requirements are critical,

it is usually cheaper in the long run to simply reestablish time and/or frequency at the destination point. The burden of installing and maintaining a distribution system can

be very great for high accuracy systems.

1.9 TIME CODES

Whenever time is available from a digital

clock at one location and needed at another,

it is often transferred over wires or radio by means of a time code. A time code is a series

of pulses which can easily be a simple teletypewriter code, but is usually a more efficient binary code where a set of pulses represents one digit. Let's say a 4 is sent, meaning 4 hours, 4 minutes, or 4 seconds. The

location of a particular binary digit in the

code tells you its meaning; that is, whether

it is an hour, minute, or second. Depending

on the application, the code can be sent as

a direct current level shift or as modulated

pulses on a carrier or perhaps as tones where

one frequency of tone represents a binary "1" and an alternate tone represents a binary 0.

What has been described above is a serial

code where one digit is sent, then another,

then another. Interspersed among the time

bits are position locators which help the

electronic equipment to recognize what the

following bit is going to mean. It is also

possible to send time codes in parallel on many conductor cables. Each wire would then

carry its respective bit.

Time codes have evolved through the years

and have been strongly influenced by the

recommendations of the Inter Range Instrumen- tation Group, known as I RIG . Figure 1.2 shows

a typical I RIG time code format which differs

only slightly from the WWV time code shown

elsewhere in this book. Figure 1.3 shows the

major characteristics of a number of I RIG codes. For those readers who want more information on time codes, many manufacturers of

digital timekeeping equipment provide booklets

listing all of the popular codes being used

and detailed explanations of the individual

formats.

If you look in a book listing the available time codes, you will notice that they

vary in frame length and rate. The NBS time

codes were chosen as a compromise on speed and

length that matched the other radio station

characteristics. The codes on the NBS sta- tions are transmitted very slowly--so slowly

that they can be recorded directly on chart

TIME FRAME I MINUTE

20 30

nr

P0

-REFERENCE TIME — REFERENCE MARKER

_| I I 1 I |

I I I I |I I I I

|

I I I I

|

I I I I |l I I I )I I I I | 'ill

INDEX COUNT 1 SEC.

DAYS

IU JU1 11.JLU

« 0.8 SEC.

1 2 4 8 10 20 40 2 4 8 10 20 JTUimUJljlJLJT^ Dl Di DO Da O c

j —0.2 SEC. 8INARY '0 1 (TYPICAL) —0.5 SEC. BINARY T

(TYPICAL)

10 20 40 80 100 200 JIT

P4

- 1 SEC. (TYPICAL)

0.8 SEC. POSITION IDENTIFIER

(TYPICAL)

Time at this point equals 173 doys, 21 hours, 24 minutes, 57 seconds

TYPICAL MODULATED CARRIER

Recommended Frequency 100 Hz of 1000 H*

IRIG STANDARD TIME CODE

FORMAT H' (1PPS Code)

Reference IRIG Document 104-70

FIGURE 1.2. IRIG TIME CODE, FORMAT H.

13
FORMAT A

1000-PPS, 34-BIT BCD TIME-OF-YEAR/17-BIT BINARY TIME-OF-DAY CODE

TIME FRAME 0.1 SECOND

INDEX COUNT » L t-*-' '

1

I

'

TIME

40

1 I

' ' ' ' I

'

—| [— 001 SEC W—1|—

I T II""

"""I I I I" mS\ f "

"I L«JLsECON0S J L M|NutesJ LhoURS-I ' DAY 1 SECOND!

TIME - W

' I

' ' '

1

I

'

-INDEX MARKER . 1.01 SEcl -INDEX MARKER (.001 SEC)

POINT A-l

-CONTROl FUNCTIONS

Tl

-(TIME OF DAY)-

STRAKvHT BINARY SECONDS

TIME AT POINT A 21 18 42 8 - 07 005 21 MR. IB MIN 42 875 SEC ON DAY 173

FORMAT B

100-PPS, 30-BIT BCD TIME-OF-YEAR/17-BIT BINARY TIME-OF-DAY CODE

STRAIGHT BINARY SECONDS

FORMAT D

1-PPM. 16-BIT BCD TIME -OF -YEAR CODE

TIME AT POINT A Jl HR. • S7 MIN ON DAY 173

FIGURE 1-3. IRIG TIME CODE FORMATS.

recorders or read by ear if necessary. CHU in Canada and other time and frequency stations

also have time codes as part of their broad- cast format.

The time codes provided on the NBS radio

stations WWV, WWVH, and WWVB can be used for

clock setting. But because of fading and

noise on the radio path, these pulses cannot usually be used directly for operating a time

display. Instead, the information is received

and decoded and used to reset a clock that is already operating and keeping time. However,

the noise and fading often cause errors in the

decoding so any clock system that intends to use radio time codes must have provisions to flywheel with a crystal oscillator and maintain the required accuracy between the suc- cessful decodes.

Does this mean that the time codes as transmitted and received are of little practical value? No, on the contrary. There are

so few sources of accurate, distributed time

that these time codes are of immense value to people who must keep time at remote locations.

The description of how to use the time codes

would suggest that they are complicated and

cumbersome. This may have been true a few

years ago. But with the advent of modern

integrated circuit technology, both the cost

and complexity have been reduced to manageable

proportions. The real point to be noticed is that the code cannot be used directly but must

be used to reset a running clock with suitable

data rejection criteria.

Information obtained by the NBS from its

mail and direct contact with users suggests

that time codes will be used more and more. Several factors have strongly influenced this

increased usage. One of these is the availability of low-cost digital clocks. The other

14
factor, as previously mentioned, seems to be

the cost reductions in integrated circuits.

These circuits are so inexpensive and dependable that they have made possible new applications for time codes that were impractical

with vacuum tubes.

The accuracy of time codes as received

depends on a number of factors. First of all,

you have to account for the propagation path

delay. A user who is 1000 miles from the

transmitter experiences a delay of about 5 microseconds per mile. This works out to be 5 milliseconds time error. To this we must add

the delay through the receiver. The signal

does not instantly go from the receiving

antenna to the loudspeaker or the lighted

digit. A typical receiver delay might be

one-half millisecond. So a user can experience a total delay as large as 8 to 10 milliseconds, depending on his location. This amount of delay is insignificant for most

users. But for those who do require accurate synchronization or time of day, there is a method for removing the path delay. Manufacturers of time code receivers very often

include a simple switching arrangement to dial out the path delay. When operating time code

generators, the usual recommendations for

battery backup should be followed to avoid

errors in the generated code.

THE UTILITY OF TIME CODES . In addition

to using codes for setting remote clocks from,

say, a master clock, time codes are used on magnetic tape systems to search for information. The time code is recorded on a separate

tape track. Later, the operator can locate

and identify information by the time code

reading associated with that particular spot

on the tape. Many manufacturers provide time

code generation equipment expressly for the

purpose of tape search.

Another application of time codes is for

dating events. The GOES satellite time code

(chapter 10) is used to add time and a date to environmental data collected by the satellite.

There are many other uses. The electric

power industry automatically receives the time

code from WWVB, decodes the time of day, and

uses this information to steer the electric

power network. Many of the telephone companies use a decoded signal to drive their

machines which actually answer the phone when

you dial the time of day. Even bank signs

that give time and temperature are often

driven from the received time code. Why is this? The main reason a time code is used is to avoid errors. It also reduces the amount

of work that must be done to keep the particular clock accurate.

15

CHAPTER 2. THE EVOLUTION OF TIMEKEEPING

2.1 TIME SCALES

A time scale is simply a system of assigning dates to events or counting pendulum

swings. The apparent motion of the sun in the

sky is one type of time scale, called astronomical time. Today, we also have atomic

time, where an atomic oscillator is the "pendulum". To see how the various time scales

came about, let's take a brief look at the

evolution of time.

discrepancy was eventually identified as being

caused by a wobble in the earth's axis. The amount of wobble is about 15 meters. By careful measurements made at various observatories

throughout the world, this wobble was corrected for and a new time designation called UT1 was born. In our search for uniformity, we

have now taken care of the non-circular orbit

(UTO) and the axis wobble of the earth (UT1).

As science and technology improved pen- dulum and quartz crytal clocks, it was dis2.1.1 SOLAR TIME

To review what has happened

to timekeeping and the name given to the resulting time of day,

consider a time system that uses

the sun and the sundial. The sun

is the flywheel and has a period

of 24 hours. The sundial can

indicate the fractions of cycles

(time of day). As complete days

elapse, calendars can be used to

count the days and divide them

into weeks and years. But our

newly formed clock is not uniform in its timekeeping because

the earth's orbit around the sun

is not circular. It slows down

and speeds up depending on its

distance from the sun. The early astronomers and mathematicians understood these laws of

motion and were able to correct

the "apparent solar time" to obtain a more uniform time called

"mean solar time". This correction is called the Equation of

Time and is often found engraved

on sundials. Universal Time

(UTO) is equal to mean solar time

if you make the correction at the

Greenwich meridian in England.

We now have UTO, the first in

of designations for time that has evolved

through the past years.

series

If you use a star that is farther away

than our own sun, the fact that the earth is not in perfect circular orbit becomes unimportant. This is called "sidereal time" and

is similar to mean solar time since both are

based on the earth's spinning on its axis.

The rate is different by one day per year

since the earth circles the sun in a year.

As better clocks were developed, astronomers began to notice a discrepancy in Universal Time measured at different locations. The

covered that UT1 had periodic fluctuations

whose origin was unknown. Due to the availability of excellent clocks, these fluctuations

could be and were removed, resulting in an even more uniform UT2.

To review, UT1 is the true navigator's

scale related to the earth's angular position.

UT2 is a smooth time and does not reveal the

real variations in the earth's position. When

the world's timekeepers went to UT2, they

bypassed the navigators' real needs. A little

later we shall describe the present-day system

which in effect remedies the dilemma.

17
Up until now we have been talking about

the Universal Time family, and figure 2.1 shows the relationship between the different

universal times. Let us now examine the other

members of the time family. The first of

these is "ephemeris time". An ephemeris is simply a table that predicts the positions of

the sun, moon, and planets. It was soon noticed that these predicted positions on the

table did not agree with the observed positions. An analysis of the difficulty showed

that in fact the rotational rate of the earth

was not a constant, and this was later con- firmed with crystal and atomic clocks. In response to this problem, the astronomers

created what is called "ephemeris time". It

is determined by the orbit of the earth about

the sun, not by the rotation of the earth on its axi s.

UNIVERSAL TIME FAMILY

APPARENT SOLAR TIME

CORRECTED BY EQUATION OF TIME MEAN SOLAR TIME w

[UTO]

CORRECTED FOR MIGRATION OF POLES Ai 0 05sec m UT1

CORRECTED FOR KNOWN PERIODICITY Al 0 OSsec ^ UT2

FIGURE 2.1. UNIVERSAL TIME FAMILY RELATIONSHIPS.

2.1.2 ATOMIC TIME

FIGURE 2.2. FIRST ATOMIC CLOCK.

Another kind of time that can be gener- ated and used is atomic time. Whereas the

Universal Time scale is obtained by counting

cycles of the earth's rotation from some agreed-upon starting point, an atomic time

scale can be obtained by counting cycles of a signal locked to some atomic resonance.

In the late 1940' s, the National Bureau

of Standards announced the first atomic clock.

Shortly thereafter several national laboratories, including the International Time Bureau

(BIH), started atomic time scales.

In 1971 the General Conference of Weights

and Measures officially recognized the Atomic

Time Scale and endorsed the BIH time scale as the International Atomic Time Scale, TAI. The

role of the BIH in international coordination

is discussed later in this chapter. Since the

first of January 1972, the Atomic Time Scale

has been distributed by most countries in the

world.

Today, the time of day can be gotten to a very high order of accuracy because, unlike a coarse sundial, an atomic clock measures small

fractions of a second. Furthermore, it gives

us essentially constant units of time--it is a uniform timekeeper. Uniformity is important in technology, synchronization of power generators, and in the general problem of trying to make two things happen or start at the same

time.

Let's review for a moment the several

time scales we have discussed. First, the

universal time family is dependent on the

earth's spin on its axis. Second, ephemeris

time depends on the orbital motion of the

earth about the sun. And finally, atomic

time, which is very uniform and precise,

depends on a fundamental property of atoms.

Figure 2.3 illustrates the differences in these several kinds of time. Because of the

slow orbital motion of the earth, about one

18
(!to

ER

uti

L It?]

^

MEASURED T0 3ms IN 1 DAY

EPHEMERIS TIME A

(ET) V

MEASURED TO 50ms IN 9 YEARS

ATOMIC TIME ^ MEASURED TO 0.1 Ms IN 1 nun

FIGURE 2.3. CLASSES OF TIME SCALES AND ACCURACIES.

B. The New UTC System

The new UTC system came about as a solution to the problem of changing the frequency

rate each year or so. It was adopted in

Geneva in 1971 and became effective in 1972. Under the new system, the frequency driving

all clocks was left at the atomic rate with

zero offset. But by using a zero offset, the

clocks would gradually get out of step with

the day. The resulting situation was not unique because the year has never been an exact multiple of the day, and so we have leap years. Leap years keep our calendar in step

with the seasons.

cycle per year, measurement uncertainties

limit the accuracy of Ephemeris Time to about

0.05 second in a 9-year period. Universal

Time can be determined to a few thousandths of

a second or several milliseconds in one day.

Atomic Time is accurate to a few billionths of

a second in a minute or less. From these

numbers and the figure, it is easy to see why

scientists have been leaning toward a time

scale derived from an atomic clock.

Coordinated Universal Time

Prior to 1972, most standard frequency

radio broadcasts were based on a time scale

called Coordinated Universal Time (UTC). The

rate of a UTC clock was controlled by atomic

oscillators so it would be as uniform as possible. These atomic oscillators operated

at the same frequency for a whole year, but

were changed in rate at the beginning of a new year in an attempt to match the forthcoming

earth rotational rate, UT2. This annual

change was set by the BIH. For instance,

between 1965 and 1966, the relative frequency,

F, of standard frequency radio broadcasts was

set equal to -150 x 10- 10

. This meant that

all of the WWV frequencies, for example, were

lower than their nominal values. (See Chapter

3 for a full explanation of nominal values.)

However, the earth's rotational rate could not be accurately predicted, and so UTC would slowly get out of step with earth time.

This was a problem for navigators who needed

solar time--they had to apply a correction to

UTC, but it was difficult to determine the amount of correction.

Experience rapidly showed that it would

be an advantage to simplify the UTC system to

I avoid changing the rate each year. A decision

1 was made to broadcast the nominal values of

the frequency standards and to do away with

annual changes in rate. Thus, a way was developed to keep UTC in closer agreement with

solar time.

The same scheme was adopted to keep

clocks in step with the sun, and the "leap

second" was born. To make adjustments in the

clock, a particular minute would contain

either 61 or 59 seconds instead of the conventional 60 seconds. You could, therefore,

either have a positive or a negative leap

second. It was expected and proved true that

this would normally occur once a year.

The new UTC plan works like this. By

international agreement, UTC is maintained

within 9/10 of a second of the navigator's

time scale UT1 . The introduction of leap

seconds will permit a good clock to keep

approximate step with the sun. Since the

rotation of the earth is not uniform we can- not exactly predict when leap seconds will be added or deleted, but they usually occur on June 30 or December 31. Positive leap seconds were added on June 30, 1972, and December 31,

1972 through 1978.

NORMAL MINUTE

(NO LEAP SECOND ADDED)

57 59 EVENT —I

30 June, 23" 59"

Doling ol Event Shown

30 June, 23 h 59 m 59 5

s UTC

I July, 0

h 0"

MINUTE WITH LEAP SECOND ADDED

Doling ol Event Shown. 30 June, 23" 59 m 60 5' UTC

EVENT —>

30 June, 23" 59 m 1 I July, 0

h 0"

FIGURE 2.4. DATING OF EVENTS IN THE VICINITY OF A LEAP SECOND.

What does this mean to the user of a time

and frequency broadcast? It simply means that

the time he gets will never differ from UT1 by more than 9/10 of a second. Most users, such

as radio and television stations and telephone

19
FIGURE 2.5. STANDARD TIME ZONES OF THE WORLD REFERENCED TO UTC.

time-of-day services, use UTC so they don't

care how much it differs from UT1. Even most boaters/navigators don't need to know UT1 to better than 9/10 of a second, so the new UTC also meets their needs.

However, there are a small number of users who do need UT1 time to better than 9/10

of a second. To satisfy this need, most standard time and frequency radio stations

include a correction in their broadcasts which

can be applied to UTC to obtain UT1

On WWV, for instance, the corrections, in units of 0.1 second, are encoded into the

broadcasts by using double ticks or pulses

after the start of each minute. The amount of

correction is determined by counting the

number of successive double ticks heard each

minute, and the sign of the correction is

given by the location of the double ticks

within the minute.

We should keep in mind that the advantage

we gain by using the new UTC system prevents

us from simply subtracting the dates of two events to get the time difference between

them. We must, in fact, take into account any

leap seconds that were added or deleted. You should be especially cautious if the time

interval of interest extends backward into a period where the world time system was not operating on the new UTC time scale (prior to

1972).

2.1.3 TIME ZONES

All standard time and frequency stations

broadcast Coordinated Universal Time, which is

on
referenced to the Greenwich meridian. However, many users want to display local time.

If the time is being decoded from a time code

(as opposed to a voice time-of-day announcement), the problem can be solved by using

integrated circuit logic. In fact, special

chips have been manufactured that allow the

clock to display time from any of the world

time zones, even though it is receiving and

decoding DTC. Figure 2.5 shows a map of the

time zones currently in use.

Contrary to popular opinion, the National Bureau of Standards is not involved with

determining time zones in the United States. This responsibility has been given to the

Department of Transportation because the need

for designating time zones came about from the

use of railroads for interstate commerce. In- formation about the time zones in the U. S. can be obtained from the Department of Trans- portation, Washington, DC 20590.

To review then, we have the unfortunate

situation where the scientist wants a uniform

time scale (UTC) and the navigators require a clock tied to the earth's position (UT1) which

is non-uniform. Therefore, corrections must be

made to convert UTC to UT1

With this discussion, the reader can more easily understand why the new UTC system was

developed. It is a compromise between a purely uniform time scale for scientific application and the non-uniform measurement of the

earth's position for navigation and astronomy.

As mentioned before, we keep the calendar in

step with the seasons by using the leap years

and our clocks in step with the sun (day and

night) by using leap seconds.

2.2 USES OF TIME SCALES

2.2.1 SYSTEMS SYNCHRONIZATION

There are many applications for timing

where it does not matter if the clock in use

is on time or not, nor does it matter if the

seconds are uniform. A child's race is a good

example. If someone shouts "Go", the starting

time is synchronized and it doesn't matter if

it occurred on the second. In other words, it

is not essential that clocks be on the right

time-- it is only important that they all read

the same time--that is, they have to be synchronized.

It is only a matter of convenience and

convention to encourage the use of the correct

time. The convenience arises because of the ease with which lost time can be recovered.

If a locomotive engineer can get the time to reset his watch by simply calling his local

telephone company or asking another trainman,

he is able to maintain his schedule with ease.

In summary, there is an advantage and

benefit to keeping everyone synchronized and

on the correct time. In a way it is much like

the argument in favor of standards themselves.

If you stop to help a motorist by the side of

the road, your confidence that your tools will

fit his car is reassuring. In the same way,

by using the same time of day, annoyances

caused by errors are reduced.

Before we leave the subject of synchronization, we should mention that there are applications in astronomy, communications, and

navigation where synchronizations to an

21
accuracy of a millionth of a second or better

are required. Although the average person is not terribly concerned if he arrives at school, church, or a picnic on time to within

a few minutes or so, an airliner navigating

with a clock in error by one millionth of a second (microsecond) would have a position

error of a quarter of a mile.

This is also true internationally with

respect to high-speed digital communications

where several million characters can be sent

and received each second. Here the "time"

being used may be a few microseconds, so the

sending and receiving equipment must be

synchronized to this accuracy. If this

synchronization can be obtained from a local navigation or time and frequency station, it enables rapid, error-free data transmission.

2.2.2 NAVIGATION AND ASTRONOMY

Although most users like the uniformity

of atomic time, there is one application that

needs the non-uniformity of solar time. This

is in the area of celestial navigation where

the motion of the earth is a way of finding

out where you are on the earth.

Navigators who find their positions from

the stars are among the greatest users of

standard time broadcasts. Since the earth

makes one revolution in 24 hours, a person can

find his position (in longitude) if he knows

THE HARRISON

his local time and the time difference between

the Greenwich meridian and himself.

As an example, a person trying to find

his position uses an instrument like a sextant

to measure the local solar time wherever he might be on the earth. His problem then is trying to find out what the time is at the

Greenwich meridian. This is the same problem

that generated all the interest several

hundred years ago in developing a good chronometer. Thise clocks were used for many years

until radio came on the scene. Even today, most vessels do not leave home without a good

chronometer.

Because of this vital need for time of

day on the high seas, many countries operate

high frequency radio stations to serve navigators. The United States operates WWV in Fort Collins, Colorado, and WWVH in Hawaii.

Similar stations are operated by the Canadians

and the Japanese and many countries in Europe

and Asia. The U. S. Navy also broadcasts time

signals from a number of radio stations.

Most large countries also maintain observatories to measure and record astronomical

time. This is done by using telescopes, and

the official United States Observatory is operated by the Navy in Washington, D. C. Astronomical time is somewhat difficult to measure accurately. Errors of a few milliseconds are realized only after a whole eve- ning's sightings have been taken and averaged.

CHRONOMETER.

22
B I

(TAI £ UTC)

1 1 UT INFO £ / \ ACCURATE

LATITUDE \ FREQUENCY

CLOCK DATA STfl6L

'

£ STABLE CLOCK DATA FREQUENCY FREQUENCY i

1 Z_ \

USNO

NBS (TIME SERVICES DIV.

OPERATIONS, RESEARCH u. s. TIME £ FREQUENCY

£ PTTI MANAGEMENT COORDINATION D 1 V 1 S 1 ON

FOR DEPT. OF DEFENSE OPERATIONS £ RESEARCH

UTI CORRECT I

T£F CONTROL OF NAVY BROADCASTS

£ LORAN-C (DEPT.

OF TRANSPORTATION)

T£F CONTROL OF

NBS BROADCASTS,

(UWV, UWVH, WWV8

FIGURE 2.6. NBS, USNO, AND BIH INTERACTIONS.

2.3 INTERNATIONAL COORDINATION OF TIME AND FREQUENCY ACTIVITIES

2.3.1 THE ROLE OF THE INTERNATIONAL

TIME BUREAU (BIH)

Unlike standards such as length or mass, time of day can be received from many sources. Both Canada and the U. S. have primary frequency standards that act as flywheels for accurate timekeeping. In addition, the U. S. Naval Observatory makes astronomical observations for determining UTI and keeps accurate

clocks running for use by the Department of

Defense.

Other laboratories throughout the world

also maintain primary frequency standards for

their countries: The PTB in Germany, the

National Physical Laboratory in England, the

Radio Research Labs in Japan, and the National

Electrotechnical Institute in Italy, just co name a few.

Time and frequency information from all of these laboratories is collected by the

International Time Bureau (BIH) in Paris,

France, which is the agency responsible for

international coordination. There, the information is evaluated and the actual time cor- rections for each contributing clock are

determined.. By international agreement, all UTC time scales must agree with the UTC time

scale operated by the BIH to within ± 1 millisecond. The result is a uniform world time

system that differs from country to country by

only a few millionths of a second. So,

whether you get your time from CHU in Canada

or the PTB in Germany, it will differ from

U. S. time by no more than a few microseconds.

2.3.2 THE ROLE OF THE NATIONAL

BUREAU OF STANDARDS (NBS)

The National Bureau of Standards is in the Department of Commerce of the United

States and has been authorized by Congress to undertake the following functions: "The

custody, maintenance, and development of the

national standards of measurements and the

provisions of means and methods for making

measurements consistent with those standards,

including the comparison of standards used in scientific investigations, engineering, manu- facturing, commerce, and educational institutions, with the standards adopted or recognized by the Government."

The Time and Frequency Division, located

in Boulder, Colorado, is that part of the

National Bureau of Standards that carries out

the above functions related to time and fre- quency. There are several groups within the

Division, each having different responsibilities. The Frequency and Time Standards Group

operates the NBS standard of frequency and

time interval, as well as several time scales

based upon this standard. In addition to maintaining the standard, research efforts are carried out to improve it. This group also

offers a direct service for calibration of

osci 1 lators and clocks.

The Time and Frequency Services Group is responsible for distributing the standards and

for finding new and improved methods of dissemination. At this writing, the dissemination services consist of high frequency radio

stations WWV and WVA/H, low frequency radio

station WWVB, a telephone time-of-day service,

time and frequency calibration services using

network television, and a satellite time code.

This Group also provides services in the form

23
of literature and courses on how to use these

services and provides calibration and monitoring data on a regular basis to users of NBS

and other time and frequency services.

Research and development into new methods

of dissemination are being done in the Time

and Frequency Coordination Group. This Group

is also involved in national and international

time and frequency coordination activities.

A fourth group concerns itself with

frequency synthesis techniques and laser

stabilization methods.

In addition, services are available in the form of consultations, either by letter,

telephone, or visits to the Boulder Laboratories. Users of the time and frequency

services are encouraged to call upon us for

advice or assistance.

To summarize, the Bureau of Standards

maintains services based upon the primary

frequency standards. These services are constantly being improved and are designed to meet the needs of users at all levels of

accuracy for frequency and time calibration.

They range from the telephone time-of-day

service to the ultra-stable low frequency

radio service of WWVB which is used for pre- cise frequency calibration.

2.3.3 THE ROLE OF THE U.S. NAVAL OBSERVATORY

(USNO) AND THE DOD PTTI PROGRAM

The U. S. Naval Observatory (USNO) is in the Department of Defense (DOD). It's mission

with reference to time is:

"Make such observations of celestial

bodies, natural and artificial, derive and

publish such data as will afford to United

States Naval vessels and aircraft as well as to all availing themselves thereof, means for

safe navigation, including the provision of accurate time." The practical application of this mission is for the USNO to serve as the

operational reference for the official United

States time of day (as opposed to responsibilities for the unit of time and frequency in NBS).

The Time Service Division of the USNO pursues several activities: It operates

Photographic Zenith Tubes (telescopes) to determine Universal Time. It operates a large

bank of atomic clocks (more than 20) as the

basis of an extremely reliable and uniform

atomic time scale. It transports portable

atomic clocks worldwide for time synchronization to "assure worldwide continuity of

precision". It promptly publishes and distributes Time Service Announcements regarding measurements of, and information on, many

electronic systems useful for time dissemination. It operates under contract with

National Science Foundation (NSF), a connected

radio interferometer at the Green Bank

National Radio Astronomical Observatory for

the determination of UT with radio techniques.

The USNO contributes measurements, along

with about 60 other observatories and stan- dards laboratories throughout the world, to the Bureau International de l'Heure (BIH) in Paris, France, which combines the results into

final BIH values of UT1. The USNO is the only

organization in the United States that determines UT1 operationally. It distributes BIH Circular D for the BIH in the United States.

24
Within the DOD, the USNO has been given

management responsibilities to "accomplish

overall PTTI objectives at minimum cost, DOD Directive 5160.51 of 31 August 1971.

The DOD provides frequency and time

signals throughout the world. These include

many different radio frequencies, satellite

signals, and portable clocks. These can pro- vide very high resolution time synchronization, one-tenth microsecond or better at the

present time (1979) with 10 nanoseconds pos- sible in the near future. Specific examples

include the Transit system with worldwide

automatic timing capability to 50 us (single

pass, 1 sigma; averaging over a number of

passes can give about 5 us, 1 sigma).

The Global Positioning System is cur- rently under development and will provide a worldwide timing capability of about 10 ns. Loran-jC is operated by the U. S. Coast Guard.

It acts in a secondary mission for the dissemination of time for the USNO. It provides

time within ground wave coverage to about 1 us with a precision of about 0.1 us. Loran-C is the present operational system for the computation and dissemination of the internationally coordinated clock time references TAI and

UK.

The USNO accomplishes its global responsibilities with a system of cooperating

"Precise Time Reference Stations (PTRS)". Any

of these PTRS clocks can be used to gain access to the USNO Master clock by correcting

a reading against the PTRS clock with the published difference USN0MC-PTRS. The currently

designated PTRS's and the respective values

are given in Time Service Announcement" Series

16. Time Service Announcement Series 4 publishes Loran-C, Omega, VLF TV phase (time)

values and Time Service Announcement Series 7 has time scale information (atomic and astronomical, including polar coordinates). These

bulletins are available on request from:

Superintendent

U. S. Naval Observatory

Attention: Time Service Division

34th & Massachusetts Avenue, NW Washington, D. C. 20390

(Telephone: 202 254-4546, AV 294-4546)

(TWX 710-822-1970)

The three military services of the DOD have established focal points for matters of

Precise Time and Time Interval (frequency).

These focal points coordinate PTTI efforts and

provide pertinent information to their organizations and contractors. They also arrange

portable clock synchronizations for their

organizations. The focal points are:

1. Aerospace Guidance and Metrology

Center, Newark Air Force Station, Ohio (for

the U. S. Air Force).

2. Navy Electronic Systems Command,

Code 510, Washington, DC 20360 (for the

Navy).

3. U. S. Army Material Development & Readiness Command, DRCQA-PC, Alexandria, VA 20315 (for the Army).

25

CHAPTER 3. THE ALGEBRA AND CONVENTIONS FOR TIME AND FREQUENCY MEASUREMENTS

Users of time and frequency signals,

equipment, and publications need to have a language for keeping records, reporting per- formance, and reading the published literature. It should be easily and correctly

understood. Thus, a convention is needed for

the algebra dealing with frequency sources and clocks so that the meaning of data are understandable and not confusing.

You might ask, "Hasn't this problem come

up before?" or "How has it been resolved?"

Yes, it has come up before and it has been

resolved. The CCIR (International Radio

Consultative Committee) has defined the problem and published a recommended, standard

notation. In this section, we will explain

the notation with examples. Notice that this

convention regarding the algebra and especially the algebraic signs has been followed

throughput this book.

In many cases of time and frequency

measurement, the user knows that his clock is fast or that his frequency source is low with

respect to NBS, the USNO, or another oscillator. Intuition is a good guide and the careful worker will be able to confirm his sus- picions by reference to his data and the

published information he needs. But in some

cases, intuition is not enough and the adoption of a standard notation will eliminate

difficulties. Annotation of data sheets or charts should follow the CCIR recommendations

so that a person looking up something that

happened several months or years ago will not

be confused.

An example using the above equation would be:

A, - B, lpps Ipps

This time difference expression is ideally

suited to the kind of readings obtained with

time interval counters. This counter is explained in Chapter 4 and produces a readout

by having one pulse open the counter gate or START the count and another close the gate,

STOPPING the count. In the above equation,

clock A is the start pulse and clock B is the

stop pulse.

Can we ever get a negative algebraic sign

on the right-hand side of the CCIR equation?

Based directly on the counter measurements, we cannot because, if the pulse from clock B occurs 50 microseconds before the pulse from

clock A, the counter will simply wait for the next pulse and read 999,950 microseconds. It does not display negative numbers at all!

Remember, these clocks are ticking once per second and our counter looks only at this

seconds tick--it cannot see the minutes or

hours. We say then that our clocks have out- puts that are ambiguous to one second. This means that the counter reading of 999,950

microseconds is equivalent to -50. It means

the same thing. The problem of ambiguity is

i llustrated below.

The CCIR convention and notation is T [ CK listed here and can also be found in the publications of that group. The following mater- CL0CK ial was excerpted from Recommendation No. 459, tick 1970. The material has been rearranged for

clarity.

1. Algebraic quantities should be given

to avoid confusion in the sign of a difference in the time between

clocks or the frequency of sources.

2. It is a good idea to give the date

and place of a clock reading or frequency measurement.

3. The difference of the readings (a

and b) of clocks A and B is, by

convention:

A - B = a - b.

Another example, showing the correct notation that should be used, follows. This

could be the result obtained by carrying an NBS portable clock to the U. S. Naval Observatory and measuring it against the USNO

clock. The results of this comparison are properly written:

NBS PORTABLE CLOCK - UTC(USNO) = +12.3 MICROSECONDS

(7 JULY 68, 14h 35m UTC).

27
There can be no confusion about the meaning of

this measurement providing the whole expression is recorded. Do not simply make a claim

that the NBS portable clock leads, lags, is fast or slow, or ahead or behind of the USNO

clock. The algebraic expression should be

used!

It is also important to follow the

notation for frequency differences. Terms

like high, low, and offset should be avoided.

Since the algebra speaks for itself, it should

be used with a minimum of additional terms.

"fractional." We have reflected this through- out the text.

The relative frequency departure of

frequency source "c," for instance, from its

nominal value f is defined as: nc

F - 1 (which is dimensionless)

3. 1 EXPRESSING FREQUENCY DIFFERENCE

(IN HERTZ, kHz, MHz, ETC.)

The actual frequency of a source is indicated by the lowercase letter, f. (Refer to the glossary at the end of this book for a further explanation and definition.) A subscript can be used to identify the particular

frequency source; e.g., f for an oscillator

designated "c." Multiple letter subscripts

are sometimes used as in the case of NBS,

USNO, ABC, NBC, and CBS. With this notation,

we can write:

a b

where Af is the frequency difference in hertz,

kHz, Mhz, etc.

The significance of this seemingly simple

expression must be stressed. This is the

actual frequency coming out of the source

(e.g., oscillator) and not the nominal fre- quency, f , that is written on the manufacturer's nameplate or in the specifications.

However, it is useful to note the nominal

frequency, f , of the source when writing down

this measurement result.

3.2 RELATIVE (FRACTIONAL) FREQUENCY, F (DIMENSIONLESS)

The term "fractional," though in common

use throughout the English-speaking world, is being discouraged by the CCIR because of

difficulties in translating that term into

certain other languages. The CCIR prefers the

use of "relative" or "normalized" rather than

where F is the designation for relative fre- quency. This expression can be rearranged:

F c = — — (dimensionless)

^nc

It now has the form Af/f which is often used

in frequency measurements and computations.

Be very careful to note the exact order of the terms in the numerator!

3.3 RELATIVE (FRACTIONAL) FREQUENCY

DIFFERENCE, S (DIMENSIONLESS)

We can apply the formula in the above

equation to two separate frequency sources and

then take a difference in that quantity. It will be the relative frequency difference

between two sources. For example, consider

sources NBS and USNO.

S F NBS " F USN0'

where S is the relative frequency difference.

Let's assume that an NBS oscillator was measured against a USNO oscillator. The

results could be written as:

F NBS " F USN0

= +5 X 10 (? Ju1 y 1968 >

14 h 35 m to 20 h 30 m UK; USNO).

This, then, is a complete, unambiguous statement about the frequencies of two

28
sources. There is a general temptation to use

the resulting algebraic signs from such calibrations and make statements about such and

such a frequency source being low or high

relative to another. The examples using this

notation shown below illustrate the difficulty

in making such statements.

3.4 EXAMPLE OF ALGEBRA AND SIGN CONVENTIONS

IN TIME AND FREQUENCY MEASUREMENTS

3.4.1 TELEVISION FREQUENCY TRANSFER

MEASUREMENTS (covered in Chapter 7)

The NBS TIME AND FREQUENCY BULLETIN

reports the measured values of the three major

U. S. networks. On page 149 of this book, a portion of that bulletin shows typical data

obtained from the TV measurements. Let's look more closely at this data.

The heading for the data is labeled "RELATIVE FREQUENCY (in parts in 10 11 )."

Following the CCIR convention, then, these

data represent F. Rr , F NRr , and F

rRC; .

Expressing the first NBC reading in equation form, using the Af/f arrangement:

=

fNBC " f NBC(nominal) =

-11 NBC

f NBC(nominal)

fMRr = (5 x 10

5

- 0.15097) Hz.

The solution to the above equation yields:

4,999,999.84903 Hz

as the frequency of the NBC oscillator.

This computation reveals that the NBC frequency source is lower in frequency than

its nominal value of 5 MHz.

Let's apply the above equation to the NBS frequency standard itself. By definition,

since NBS has the standard of frequency, there

can be no average difference between its

nominal value and its actual value. So:

F NBS

=

°-

If we were to compute the relative difference

in frequency between NBS and NBC, the solution

would indicate:

S NBS-NBC

= F NBS

" F NBC

=

^ "("3019.4 x 10~ ),

S = + 3019.4 x 10 -11

.

What does this tell us? If we assume we are dealing with a frequency source at 5 MHz, we

can substitute that number for the nominal

frequency and solve for the actual frequency

of the signal being received from NBC via the

TV color subcarrier:

F =

fNBC ~ f NBC( nominal) =

-

5 x 10

6

Using the equality on the right:

f NBC

- (5 x 10

6

) = -3019.4 x 10" 11

x 5 x 10

6

,

f NRr - (5 x 10

6

) = -15097 x 10" 5

,

3.4.2 TELEVISION LINE-10 TIME TRANSFER

MEASUREMENTS (covered in Chapter 8)

Let's take an example using data obtained

from time measurements. Here, the CCIR con- vention, while clear enough, needs to be

applied with more care than usual. This is because people use intuition more readily when

dealing with time measurements. They feel at home with fast, slow, early, and late, and are prone to make mistakes.

Using the example on page 150 of this

book for the TV Line-10 measurements, the user

is expected to find the relative frequency for

his own oscillator. Notice that this problem

involves the use of time measurements to compute frequency! Figure 8.7, column one, gives time difference data of:

29
UTC(NBS) - LINE-IO(NBC)

= 02216.54 microseconds,

which follows the CCIR convention and indi- cates that a counter was started with the

pulse from the NBS clock and stopped with the next Line-10 pulse. The data in column two represent the same measurement made at a user's location, giving:

User Clock - Line-IO(NBC)

= 05339.9 microseconds.

From the above, column three is obtained, by

subtraction, giving:

UTC(NBS) - User Clock

= -3123.4 microseconds.

Since the user wants to know where his clock

is with respect to NBS, by convention the NBS clock is listed first in the equation. The

minus sign (-) indicates that the user's clock

pulse occurred before the NBS clock pulse.

This can be illustrated graphically (as it would appear on an oscilloscope, for example):

NBS CLOCK LINE-10

PULSE PULSE

3123.4 us I 2216.54 ps

5339.9 us

USER'S LINE-10

CLOCK PULSE PULSE

It also illustrates how Line-10 (or any common

time pulse) transfers information from NBS to the user. To get the exact time error between

the two clocks, path and equipment delays must

be taken into account. The final result could

easily change the sign from (-) to ( + ) since

path delays could contribute many thousands of

microseconds.

To use our derived relationship between

frequency and time measurements, we need the

change in time over the interval being con- sidered. As shown in figure 8.7, the clock

being measured moved -42.5 microseconds in four

days, or an average of -10.6 microseconds per

day. (This information will give us the

needed At/T. ) Now,

Af _ At

f T

^ = -(- 1°^ = 1.23 x 10" 10

. f 86,400 x 10

What does this mean? According to the CCIR convention this is

user' s osci 1 lator

or, stated correctly, the oscillator being mea- sured using Line-10 TV measurements has a fractional or relative frequency of +1.23 x 10- 10 . The plus sign (+) on F indicates that

the oscillator output frequency is higher than

its nominal value.

The actual frequency of the oscillator

can be calculated from the CCIR definition

usi ng:

and if we assume that the oscillator has a nominal frequency of 1 MHz, its actual frequency can be computed. In this case, it is 1,000,000.000123 Hz. If the nominal frequency

of the oscillator being tested were 2.5 MHz,

its actual frequency would be

2,500,000.0003075 Hz.

The next page gives the derivation of our equations and an example.

3.5 USING TIME TO GET FREQUENCY

But what about the frequency of the

oscillator used to drive the user's clock? Is it high or low in frequency with respect to NBS?

In our search for a method of relating

time measurements to frequency measurements,

we would want to know how far a particular

clock has drifted in a day. Notice that this

30
is a result that can be written, as microseconds per day. It tells us the form of the answer we are seeking. We want to know the

change in time (microseconds) during a spec- ified period (day). If the above were ex- pressed mathematically, it would be in the

form,

At

T

To this we want to relate our frequency mea- surements. Although we could, we typically do not measure the frequency error of our oscillator in hertz. It is usually a percentage or

a ratio. This method of rating oscillators is very common and results in having the numbers

come out as numerics, that is, without the

dimension of hertz. Algebraically, the frequency measurements have the form:

Af

f

Our task is to relate our Af/f frequency mea- surements to our At/T time measurements.

This can be done with calculus and is shown below. If the reader is not familiar

with calculus, he may want to skip to the

result, that states:

Af _ _ At

f T

'

Before we get to the derivation of this

equation by using calculus, let's look at a simple example that is not too rigorous, but

might offer encouragement that the math works. We have an oscillator with a nominal frequency

of 1 MHz, but it is actually running at 999,999 hertz. Let's find

*I and ^

f T

and see if they agree with the above equation.

Clearly, Af is -1 Hz, both by inspection and

by the CCIR definition of Af = f - f . Since

f = 1 MHz, then n

f 1 x 10

b

If we look at this 1 MHz oscillator in- stalled in a clock, we note that it will not "tick" once per second, but at a slightly different rate. Let's apply the CCIR formula for

clocks to this one and let's further assume it

is being compared to the NBS clock. If we start the clocks together, their difference

will be zero, but a second later:

NBS CLOCK - 1 MHz CLOCK = 1 MICROSECOND.

Ten seconds later, we will have a result that

is 10 microseconds and so on. We now compute

At/T. It is one microsecond per second or

10- 6 /l. This agrees with the CCIR notation,

and the formula for relating Af/f to At/T

works. This is hardly an exact proof nor is the calculus that follows strictly rigorous,

but it will serve for our purposes.

3.6 A MATHEMATICAL DERIVATION

In order to use time measurements to get

frequency information, we need to know the

relationships between time and frequency. A mathematical definition of frequency is:

where t is the period of the signal in seconds

and f is the frequency in hertz. This can

also be written as:

f = t\

If we differentiate the frequency expression

with respect to time and substitute we get:

df = - t

' 2 dt = - d| = - f \r t

and

df _ _ dt

f t

which can be written, for small changes, as:

Af _ _ At

f t

31
Both sides of this equation are dimensionless.

Af/f is, in the CCIR notation, F. At is the

change in time read over a measurement time,

T.

3.7 DEFINITIONS

f = actual frequency of source in hertz.

Af = frequency difference in hertz,

f = nominal frequency in hertz.

F = relative frequency (dimensionless).

S = relative frequency difference

(dimensionless)

32
CHAPTER 4. USING TIME AND FREQUENCY IN THE LABORATORY

Although simple devices for the measurement of electrical frequency were invented

during the 1800's, modern instruments have

only been around since the middle of the

twentieth century. Since World War II, how- ever, time and frequency measurement has advanced at a rapid rate. Measurement of time

intervals as small as one ten-billionth

(10- 10

) of a second and frequencies as high as one-hundred trillion (10 14

) hertz (cycles per

second) are now possible.

Such improvements, of course, are in keeping with the need for higher precision and

accuracy in today's technology. Not long ago, the highest precision required for timing measurements was on the order of a millisecond. Today, however, the need for precisions

of a microsecond is commonplace. For a variety of laboratory measurements, timing accu- racies to the nearest nanosecond are required.

Time interval and its reciprocal, frequency, can be measured with greater precision

than any other known physical quantity. By

allowing two oscillators to run for a sufficiently long time, we can compare their

average frequencies to a high degree of pre- cision. Conversely, we can measure time intervals with the same precision by counting

individual cycles in the output waveform of an oscillator, provided its frequency remains

constant. But a problem arises in attempting

to measure time intervals accurately.

Unlike two standard meter bars which can

be left side by side indefinitely while their

lengths are compared, time doesn't stand still

for measurement. No two time intervals can be

placed alongside each other and held stationary, so we must rely on the stability of an oscillator, a clock, a counter, or a similar

measuring device to measure or compare time

intervals.

In following the practices set forth in this chapter, one should keep in mind that the

end result of a measurement can be no better

than the performance of the equipment with

which the measurement is made. Frequent and

careful calibration of the measuring instru- ments against reliable standards is absolutely

necessary if maximum accuracy is to be

achieved.

In this chapter, we discuss the equipment

needed to make time and frequency measurements

in the lab, what the equipment is used for,

how it is used, and what the results of using

it will be. We do not discuss the reference

sources of time and frequency signals being

measured since they are fully discussed in later chapters.

4.1 ELECTRONIC COUNTERS

If you want to measure frequency or time

interval, the modern electronic counter is a good investment. It is easy to use, does a large variety of measurements, and is rapidly

going down in cost. The availability of new integrated circuits and displays has driven counter prices down while pushing their spe- cifications upward. A 50 MHz counter is only

a few hundred dollars or less at this writing.

There is another advantage in using counters. This is the written material available on their construction and use. Articles in Ham radio publications, electronic hobby magazines, and manufacturers' application notes and other literature are plentiful, easy to find, and easy to read. In this chapter, only

part of the available information is presented

in a tutorial way. The reader is urged to obtain and study other material.

FIGURE 4.1. ELECTRONIC COUNTERS.

Modern general-purpose counters have a variety of measurement features. Some have

provisions for the insertion of plug-in modules which adapt the counter to an even greater range of applications. Although counters

differ widely in specification and design

details, all have several major functional

sections in common. The basic parts of a typical counter are the time base, the main

gate, and the decade counting assembly (DCA).

The time base provides uniformly spaced

pulses for counting and control of the circuitry within the counter. The time interval

between pulses must be accurately known be- cause errors in the time base will appear

directly in frequency and time measurements. Most counters incorporate an internal quartz

crystal oscillator for a time base; some use the power- line frequency.

33
jJJJJJJ^_

INPUT SIGNAL

1 L

J L

GATE

CONTROL

DC A AND READOUT

DISPLAY

CONTROL

FIGURE h.2. SIMPLIFIED BLOCK DIAGRAM OF AN ELECTRONIC COUNTER.

The oscillator may be followed by decade

dividers to allow the user a choice of several

different time signals. By adding more dividers to the chain, it is possible to obtain

longer time intervals. By increasing the

crystal oscillator frequency, one may obtain

short time increments.

The main gate controls the time at which

the count begins and ends. The gate may be

actuated automatically by pulses from the time

base or manually by means of a switch on the

control panel. Pulses which pass through the

gate are routed to the decade counting assem- bly where they are displayed on the visual

numerical readout. After a pre-set display

period, the instrument resets itself and starts another count.

Other counter sections include amplifiers, pulse shapers, power supplies, and control circuits for the various logic circuits.

The input channels are usually provided with

level controls and attenuators to set the exact amplitude limits at which the counter

responds to input signals. Proper setting of

the level controls allows accurate measurement

of complex waveforms and also helps reject

noise or other unwanted signals.

4.1.1 SOURCES OF COUNTER ERROR

The errors associated with using counters

vary with the type of measurement. The most important source of error is the counter time

base. This is the internal oscillator that

controls the counting time or is itself counted and displayed. For accurate measurements,

a good time base is essential. Often, the

difference in cost of one counter over another

depends on the quality of this internal oscillator. You get what you pay for, so study the

time base specifications carefully when buying

a counter. Errors are discussed more fully

later in this chapter.

The second large source of error in coun- ters is the ± 1 count ambiguity. Notice in figure 4.2 that the counter gate opens and

closes to let the pulses through. Since the

time base that drives the gate is not related

in any way to the incoming pulses, sooner or later it is going to try to close or open the

gate at a time when the pulse to be counted is already present. So the lack of synchronism

between the signal counted and the gate will

result in an extra count sometimes and one

less count at other times. The effect on the counter display is the ± 1 count ambiguity.

If the correct answer is 100.8, you will see the counter display either 100 or 101 some of

the time. There are techniques for getting

around this problem. Such techniques are discussed in counter manuals and also in manufacturers' application notes. Although one extra count out of ten million is often not significant, that same one count with a display of 100 could be very important.

The third main source of error using a digital counter stems from noise on the signals or from setting the controls. It is easy

to be lulled into a sense of false security by

the fact that the counter is displaying bright

digits. But they may be wrong unless care is exercised in setting the controls so you know

what signal is going into the counter. Accurate and precise measurements demand care and

attention. Read the instruction manual care- fully and know what it is you are measuring.

Manufacturers often recommend the use of an oscilloscope in parallel with a counter. That

way, you can see what is going on. Errors in counter measurement are discussed more fully

later in this chapter.

4.1.2 FREQUENCY MEASUREMENTS

Modern counters can measure frequencies

up to several hundred megahertz directly. With

special methods of measurement, the frequency

34
w

INPUT

SIGNAL

DCA AND

READOUT

AMPLIFIER AND

PULSE SHAPER

J—

i TIME I GATE DISPLAY

BASE CONTROL CONTROL

FIGURE 4.3. DIAGRAM OF A COUNTER IN THE FREQUENCY MEASUREMENT MODE.

range of the counter may be extended into the

microwave region. The maximum counting rate

of the basic instrument is limited by the

inability of its switching circuits to follow

extremely fast input signals.

A. Direct Measurement

For direct frequency measurements, the

input signal is first amplified and converted

into a series of uniform pulses. The pulses

are then applied to the main gate, which is opened for a known time interval by the time

base (see fig. 4. 3)

The gate time may be 1 microsecond, 1 millisecond, 1 second, 10 seconds, etc., as determined by the setting of the time base

selector switch. During the gating interval,

the signal pulses pass through the main gate

and on to the DCA, where the pulses are actually counted. The count obtained in a given gate time measures the average input

frequency for that interval. The frequency is displayed on the readout and held there until

a new count comes along. The decimal point in the display is automatically positioned to give the correct readout.

B. Prescal ing

The high frequency limit of the counter

can be extended by a technique known as prescaling. This means dividing the frequency of

the input signal by some known factor before

the signal is gated into the counter. Pre- scalers are available commercially.

If the signal frequency is prescaled by a factor of D, the counter displays 1/D times

the actual frequency of the signal. By simple

multiplication, the counter reading may be

converted to the true value of the input

frequency.

As an example, let us suppose we wish to measure a frequency in the range between 200

and 300 MHz with a counter capable of measuring to only 50 MHz directly. By means of a scale-of-8 divider, we reduce the unknown

frequency to a value within the capability of

the counter. If the counter indicates a frequency of 35.180 MHz, the actual value of

the frequency being measured is 8 x 35.180

MHz = 281.44 MHz. In principle, any number

may be chosen for the divider D, but prescalers are usually designed to divide the input

frequency by an integral power of two. This

is because high speed counting circuits are binary; i.e., they divide by two. Integrated

circuits are becoming available which will

accept signals in the 100 MHz range and divide

by ten.

To avoid the necessity of multiplying by

the factor D, some prescal ers incorporate a provision for lengthening the counter's gate

time by the same factor. If the gating interval is increased D times while the input frequency is divided by D, the scale factor be- comes self-cancelling and the counter displays

the correct input frequency. Figure 4.4 depicts a counter equipped with a prescaler

which divides the signal frequency and the

time-base frequency by the same amount.

Prescal ing offers a convenient way of covering wide frequency ranges without elaborate equipment and/or tedious operating procedures. Accuracy of the prescaler-counter com- bination is the same as that of the basic

counter alone; but since the technique re- quires an increase in measurement time by

virtue of the scale factor D, it is slower

than the direct method.

C. Heteordyne Converters

Another way of extending the upper frequency limit of a counter is to use a heterodyne converter. The converter translates the

35
FREQUENCY -~

SIGNAL T FREQUENCY

PREAMPLIFIER

& TRIGGER

PRESCALER

AMPLIFIER

& SHAPER

J L

PULSE

RATE R/D

PULSE

RATE R

—J \ DCA

GATE

control!

DISPLAY

CONTROL

FIGURE h.k. PRESCALER-COUNTER COMBINATION.

unknown signal frequency downward by beating

it against a known signal of different fre- quency. The principle is the same as that

employed in the familiar super-heterodyne

radio receiver to produce an intermediate

frequency. Heterodyne converters are available either as plug-in modules or as external

accessories for many commercial counters. The

block diagram of a typical, manual ly- tuned converter is shown in figure 4.5. The beat,

or difference frequency, is applied to the counter and measured. The counter reading is added to the known signal frequency, and the

sum is the unknown signal frequency.

Looking at figure 4.5, a 10 MHz frequency

from the counter's internal time base is multiplied to 50 MHz and drives a harmonic

generator. The harmonic generator produces a large number of frequencies, each 50 MHz

apart. By means of a tunable cavity, the

desired Nth harmonic can be selected from

among the many available. In practice, it is best to pick the harmonic that is nearest to

but lower than the signal frequency. The

mixer is then within the passband of the video

amplifier as indicated by a movement of the

tuning meter. The mixer output [f - (N x 50 MHz)] goes to the counter input and is measured. The dial marking on the cavity tuning

control is added to the counter reading.

For example: Consider an input signal at 873 MHz. As the harmonic selector is tuned

upward, higher harmonics of 50 MHz are injected into the mixer. When the tuning dial

reaches 850 MHz (the 17th harmonic of 50 MHz),

the tuning meter will rise sharply and the counter will display a reading of 23 MHz. Thus, the signal frequency is the sum of the

dial setting (850) and the counter reading

(23) or 873 MHz.

If the dial setting and the counter reading are to be added, the tuning point must

always be approached from the lower side and

stopped at the first peak. If the tuning

control is turned too far, so that a harmonic

IF = f N X 50MHz

SIGNAL

FREQUENCY MIXER

HARMONIC

SELECTOR

N x 50MHz

(TUNABLE CAVITY)

VIDEO

(WIDEBAND)

AMPLIFIER

0

TUNING

METER

IF TO

-*- COUNTER

INPUT

llllllllllll

SPECTRUM HARMONIC

GENERATOR

50MHz FREQ

MULT

X5

1 0MHz FROM

TIME

BASE

FIGURE 4.5. TYPICAL MANUALLY-TUNED HETERODYNE CONVERTER.

36
SIGNAL

FREQUENCY

f

MIXER

Ill SPECTRUM

N x f

N x f AMPLIFIER

NULL

(ZERO-BEAT)

DETECTOR

n

HARMONIC

GENERATOR VFO

TO

COUNTER

FIGURE 4.6. TYPICAL MANUALLY-TUNED TRANSFER OSCILLATOR.

higher than the input is selected, the counter

reading must be treated as a negative number.

Equipment operating manuals recommend that

each measurement be repeated to be sure the answer is right.

The heterodyne technique offers nominally

the same accuracy as direct counting. Of all known frequency-extension methods, the heterodyne technique allows the operator to make

accurate microwave measurements in the least

time. Also, this method can stand large

amounts of frequency modulation on the measured signal without changing the accuracy of

the measurement.

D. Transfer Oscillators

Transfer oscillators can be used to extend the frequency measuring range of counters beyond the limit of prescalers and

heterodyne converters. Transfer oscillators

are usable at frequencies up to 40 GHz and

higher. Their wide bandwidth allows a single

plug-in unit to cover a great range of frequencies. Also, most versions allow accurate

frequency measurement of complex waveforms,

such as frequency-modulated signals and pulsed

rf.

A typical transfer oscillator consists of

a stable variable frequency oscillator (VFO),

a harmonic generator, a mixer, and a null detector. Like the heterodyne converter, the

transfer oscillator mixes the unknown signal

frequency with a selected harmonic of some

internal reference frequency. But the reference frequency is derived from the VFO, and

the counter is used (sometimes in conjunction

with a prescaler or converter) to measure the

VFO fundamental

In operation, the VFO is carefully tuned

until one of its harmonics matches the fre- quency of the signal being measured. The

frequency match is indicated by a zero beat on

the null detector. At this point the counter

reading f 1 is recorded. The VFO is then

slowly tuned upward or downward until the next adjacent harmonic produces a null. A new counter reading f 2 is noted. The harmonicnumber applicable to f t can be calculated from

the formula

The bars mean a negative sign is ignored;

i.e., the difference between fj and f 2 is always a positive number.

Having found both H 1 and f x

, the signal

frequency, f = H 1 f 1

. To illustrate this, let

us assume that the first null occurs when f 1 = 88.0 MHz. As the VFO is tuned slowly upward,

the next null is found at f 2 = 90.0 MHz. The

applicable harmonic number is

N = 90^0 = 90_0 = 45

1

I 88.0 - 90.0 I 2.0

and the unknown frequency is

f = 45 x 88.0 MHz = 3960 MHz.

The same result would be obtained if the

roles of fj and f 2 were interchanged; i.e., if the VFO were tuneci downward from 88.0 MHz to 80.0 MHz in arriving at the second null.

For any given f , many null points may be

noted as the VFO is tuned through its entire

range. Using 3960 MHz as an example, an adjacent null pair may be found at VFO set- tings of fj = 84. 25531 914. . . MHz and f 2 = 82.5

MHz. Because the fractional part of f x does

not end, however, the counter cannot display

that number accurately. N x = 47 can be com- puted from the formula (rounded off to the

37
nearest whole number since harmonic numbers must be integers). But f cannot be calculated with accuracy from the relation f =

N 1 f 1 because there is no way of knowing the

missing part of f x. In this case, it is better to calculate f from f2 since f2 is an exact number.

An error may arise if the operator cannot

tell exactly where the zero beat occurs. Here

the resolution of the null detector is all important. The total reading error is the sum of the error in the counter reading plus the error caused by imperfect null detection.

Typically, an uncertainty of several parts in

10 7 applies to microwave measurements made

with manually-tuned oscillators.

Some of the more elaborate transfer

oscillators employ feedback methods which lock

the VFO to an exact submultiple of f . The

phase lock feature essentially eliminates

human error caused by improper zero-beat detection. Most automatic transfer oscillators

also have provisions for extending the counter's gating interval by the factor N so that

the counter can display f directly. Figure

4.7 shows block diagrams of three automatic

transfer oscillators which are presently

available commercially.

SAMPLER

PULSE

DRIVER

PHASE

LOCK

VFO

GATE

EXTENSION

SELECTOR

SWITCH

PEAK TUNING

INDICATOR 0

FROM

TIME BASE

N (GATE)

SAMPLER

VCO

f,

AUTO

PHASE

CONTROL

HARMONIC

SELECTOR

SWITCH

SAMPLER

f OUT

VCO

fo

N (GATE) TRACKING

CONTROL

POWER

SPLITTER

HARMONIC

DETECTOR

MIXER

VFO

PHASE

DETECTOR

SWEEP

GEN

N (GATE)

FIGURE 4.7. AUTOMATIC TRANSFER OSCILLATORS.

38
TIME

BASE

INPUT

SIGNAL f

DCA AND

READOUT

I MA IN GATE.

GATE

CONTROL

DISPLAY

CONTROL

AMPLIFIER AND

PULSE SHAPER

FIGURE h .8. DIAGRAM OF COUNTER IN THE PERIOD-MEASUREMENT MODE.

Phase-locked transfer oscillators are normally used to measure only stable cw (continuous wave) signals and often are not applicable to complex waveforms containing frequency or pulse modulation. Even in those versions which are able to measure pulsed carriers, the accuracy is not as great as when

measuring continuous waves.

4.1.3 PERIOD MEASUREMENTS

The frequency of a signal can be calculated from its period. Sometimes it is easier

and better to measure the period of a signal

rather than its frequency. The basic circuits

of a frequency counter can be rearranged to make period measurements simply by exchanging

the roles of the time base and the input signal (fig. 4.8). This is done by means of a switch on the control panel. For period mea- surements, the output of the time base generator is counted during a time interval con- trolled by the period of the input signal f .

If, for example, f has a period of 1 ms

and the time base output is 10 s pps, the counter displays (1 x 10- 3 s) (1 x 10^ pulses/

s) = 100 pulses during each gate interval. If

the period of f is increased to 10 ms, the counter totals 1000 pulses. The counter read- out is proportional to the period of f and

can be designed to display the period directly

in units of time (microseconds, milliseconds,

seconds, etc. ).

Frequency obtained by period measurements

is often more accurate than direct frequency

measurements. This is true up to the point

where the frequency being measured equals the

frequency of the counter time base oscillator.

The disadvantage of period measurements is in interpreting the meaning of the counter display. It is not hertz or cycles per second--

it is time. To get hertz you have to divide

the answer into the number 1 (take the reciprocal). But this disadvantage is rapidly

disappearing due to the new integrated circuits now available. The division is per- formed by the counter circuits and you get the

best of both worlds—period measurements for

accuracy with the answer displayed in hertz.

The accuracy of a period measurement can

be improved further by using a multiple-period

mode of operation. In this mode, the output

of the signal amplifier is routed through one

or more decade frequency dividers before

H>

| loo oo y 1 o oo fioo } 10

MULTIPLE-PERIOD SELECTOR

10 10

GATE

CONTROL

DISPLAY

10 10 CONTROL

DCA AND

READOUT

FIGURE J».9. DIAGRAM OF COUNTER IN THE MULTIPLE-PERIOD MODE.

39
TABLE 4.1 READINGS OBTAINED WITH VARIOUS SETTINGS OF TIME BASE AND PERIOD MULTIPLIER CONTROLS

TIME BASE

PERIOD MULTIPLE

1 ms 0.1 ms 10 ps 1 us 0. 1 ps

X 1 2 ms 2.4 ms 2.40 us 2408 ps 2409.0 ps

X 10 2.4 ms 2.41 ms 2409 ps 2409.0 ps 2408.91 ps

X 100 2.41 ms 2.408 ms 2408.8 |js 2408.91 ps 2408.912 ps

X 1 ,000 2.409 ms 2.4089 ms 2408.91 us 2408.911 ps 2408.9113 ps

X 10,000 2.4089 ms 2.40890 ms 2408.912 ps 2408.9112 ps *408.91124 ps

^COUNTER HAS ONLY AN 8-DIGIT DISPLAY

reaching the gate control circuit. Thus the

main gate is held open for 10, 100, 1000, or more periods depending upon the number of

divider stages used. The more periods over which a signal is averaged, the better the

resolution. But you have to wait longer for

the answer.

To illustrate the advantage of multipleperiod averaging, consider a counter reading

of 415 kHz taken in the frequency-measurement

mode. The same input, if divided by 10 using

a period measurement with a 10 MHz time base

would display a six-digit reading of 2408.91

microseconds. By taking the reciprocal, f averages 415.125 Hz over ten complete periods.

We gained three decimal points by period

averagi ng.

The accuracy of the measurement may be

improved in two ways: by increasing the time

base frequency (a more costly counter) or by

averaging for more periods (waiting longer).

Table 4.1 shows the results of repeated period measurements of the same signal. The measurements were made with a counter having an 8-digit display and with various settings of

both the time base selector switch and the

period multiplier switch. The improved reso- lution at higher time base rates and at longer

gating intervals is quite apparent.

Noise is a major source of error in period measurements. A noisy input signal may

cause the main gate to open or close too soon

at one time and too late at another. In the

period measurement, assume that the counter is designed to trigger the main gate at a positive-going zero-axis crossing of the input

signal. Any jitter at this point produces

j erratic gating, as shown in figure 4.10.

The error resulting from noise is independent of frequency and may be reduced by

extending the measurement to more than one

period. A ten-period measurement, for in- stance, is ten times more precise than a single-period measurement because the same i uncertainty in the time of opening and closing 1 the main gate is distributed over an interval ten times longer.

4.1.4 TIME INTERVAL MEASUREMENTS

Electronic counters are widely used for

the measurement of time intervals, both periodic and aperiodic. They range in length from

less than a microsecond up to hours and even

days. At the end of the measured interval,

the elapsed time is displayed in digital form.

f Counters vary considerably in their timemeasuring ability. Some are designed to mea- sure the duration of a single electric pulse

while others measure the interval between the occurrence of two different pulses. The more versatile models have individually adjustable

start and stop controls which allow greater

control by the operator.

As with period measurements, the basic

components of the counter are connected in such a way that the output pulses from the

time base are counted. To select the interval

being measured, separate start-stop amplifiers j' and controls are used to open and close the

main gate.

40
TRIGGER POINTTRIGGER POINTTRIGGER J

POINT

GATING INTERVAL

TOO SHORT

GATING INTERVAL

CORRECT

-TRIGGER POINT

GATING INTERVAL

TOO LONG

TRIGGER

POINT

FIGURE 4.10. EFFECT OF NOISE ON TRIGGER POINT IN PERIOD MEASUREMENTS.

JJJJJJOSTART SIGNAL

_FL

STOP SIGNAL

MAIN

GATE

J L

AMPLIFIERS AND

SIGNAL SHAPERS

DCA AND

READOUT

FIGURE DIAGRAM OF COUNTER IN THE TIME INTERVAL MODE,

41
(A)

FIGURE 4.12. OPTIMUM TRIGGER POINTS FOR START-STOP PULSES.

During the time between the start and

stop signals, the main gate is held open and counts are accumulated according to the length

of the gate interval. At the end of the gate

interval, the readout assembly can be made to display the elapsed time directly in microseconds, seconds, minutes, or other appropriate units.

The start-stop input circuits are designed to produce sharp, uniform trigger

pulses from a variety of signals. Best re- sults are obtained if clean pulses are used. If the instrument has a trigger level control,

it is possible to select the exact input am- plitude at which a trigger pulse from that

channel is generated. If a slope control is provided, it is possible to set the trigger

point on either the positive-going or the

negative-going side of the input signal at will.

Careful setting of the trigger level and

slope controls is necessary to avoid measurement errors caused by noise, jitter, harmonics, and other forms of distortion on the

input signal. Figure 4.12 shows the best

triggering points for six different input

waveshapes. In each case, the desired trigger

point (as indicated by an x) is at the center

of the steepest, cleanest portion of the

signal. Note that for noise-free sinusoidal

(sine wave) signals as in waveform (B), the

optimum trigger points occur at the zero

crossings.

4.1.5 PHASE MEASUREMENTS

The time interval counter can be used to measure phase relationships between two input

signals of the same frequency. To do this, one of the signals goes to the start channel

while the second signal goes to the stop

channel. The level and slope controls are adjusted to trigger at the same point on both

input waveforms. For sinusoidal waves, the

best triggering point is the zero-crossover.

The time interval between corresponding points

on the two input waveforms is then read

directly from the counter. Time interval can be converted to a phase angle by the formula

START

STOP 0^1

DCA AND

READOUT

GATE

CONTROL

DISPLAY

CONTROL

FIGURE 4.13. USE OF T I ME- I NTERVAL UNIT FOR PHASE MEASUREMENTS.

42
JJJJO.L DCA AND

READOUT

J~~L

COMMON

INPUT

SIGNAL

STOP

J L

GATE

CONTROL

DISPLAY

CONTROL

FIGURE k.lk. TIME-INTERVAL COUNTER WITH START-STOP CHANNELS CONNECTED

TO COMMON SOURCE FOR PULSE-WIDTH OR PERIOD MEASUREMENTS.

a. snuncc AM/-I c \ - 360 x Time Interval .

<|> (PHASE ANGLE ) = degrees

Zn x Time Interval radians

where T is the known period of the two signals

expressed in the same units as the time

interval

necessary to connect both input channels to the same source (figure 4.14).

The smallest measurable time interval is limited by the resolution of the counter. In general, the resolution of time interval and

period measurements are affected by the same

factors. The presence of noise can seriously

impair the precision of pulse-width determinations.

4.1.7 COUNTER ACCURACY

4.1.6 PULSE-WIDTH DETERMINATION MEASUREMENTS

Trigger controls are seldom calibrated

with sufficient accuracy to measure the rise

time or decay time of a single pulse. It is almost always necessary to use an oscilloscope

to measure rise time. An oscilloscope used

with a counter is very useful. By selecting

one polarity of slope for the start trigger

and the opposite polarity for the stop trigger, you can measure the total duration of a pulse. The duration of pulse (A) in figure

4.12, for example, could be determined by

selecting point x on the positive slope for

the start trigger and point x on the negative

slope for the stop trigger. Some time interval counters bring the gate start-stop pulse

out on connectors for oscilloscope viewing.

Carrying the idea one step further, the

period of waveform (B) in figure 4.12 could be

measured by choosing the positive zero-crossing as the trigger point for both the start

and stop pulses. When measuring pulse width

or period with a time interval counter, it is

Depending upon the characteristics of the

counter and its method of use, the accuracy of

measurements may range from 1 part in 10 2 to a few parts in 10 10 or better. Although the

possible causes for inaccurate measurement are

many, the main sources of error are (1) timebase instability, (2) uncertain gating, and

(3) faulty triggering. The total measurement

error is the algebraic sum of all individual

errors.

A. Time Base Error

The time base oscillator will change

frequency slightly during the course of a measurement. This is due to temperature

changes, line voltage changes, internal noise,

and crystal aging. Temperature effects can be

minimized by allowing the instrument to warm

up before use and by keeping the counter in a constant- temperature location. If the line

voltage fluctuations are severe, it may be

necessary to operate the counter from a regu- lated power source. Check the operating

manual for the manufacturer's recommendations.

43
(X10

6

10

)

HOURS

FIGURE 4.15. TYPICAL TIME BASE STABILITY CURVE.

Any noise generated internally by the

time base oscillator produces a jitter in its

output frequency. This may cause erroneous

counts, especially when a measurement is per- formed during a brief time interval. As the measurement time is made longer, the jitter

tends to average out. For this reason, the

jitter is often called "short-term" instability. A typical time base stability graph is shown in figure 4.15. Note that the vertical

axis is labeled Af/f, which is equal to F, the

relative frequency of the time base. A com- plete explanation of this notation is given in Chapter 3.

In modern counters, the amount of oscillator noise is generally so small that short- term stability is specified only for the most precise instruments. When reading specifications, it should be realized that the averaging time has an important bearing on the

actual meaning of stability figures. For example, an oscillator having a stability

specification of 5 x 10- 10 (per minute) might

be noisier than one with a specification of 5 x 10- 9 (per second). To be absolutely meaningful, short-term stability figures should be

compared for the same averaging period, pre- ferably of only a few seconds.

Long-term instability, on the other hand,

refers to a slow, often predictable drift in the average frequency of an oscillator due to gradual changes in the crystal. Quartz cry- stals exhibit drift rates typically of 10- 10 to 10- 6 per day as a result of crystal aging.

After a brief warmup period when the oscillator is first turned on, a high-quality

crystal has a linear frequency drift, the

slope of which reveals the crystal aging rate.

Because aging produces larger and larger error as time goes on, it is necessary to recalibrate the oscillator if the time base is to be accurate. For utmost accuracy it may be wise to check the oscillator calibration

immediately before and after a critical mea- surement. However, on inexpensive counters,

this may be impractical.

For measurements which require a better

time base accuracy than that of the counter's

internal oscillator, you can use an external

frequency source to drive the time base gen- erator. Most counters have a built-in con- J nector and switch for using an external oscillator. The internal oscillator is automatically disconnected when the time base is switched to an external source.

B. Gate Error

As previously mentioned, an uncertainty

of ± 1 count is inherent with all conventional

electronic counters because the time base

j pulses are usually not synchronized with the ! signal being measured. Because of this, it is possible for the reading to be in error by one count even if there are no other sources of

error! As an example, figure 4.16 shows how

the same signal might produce a count of

either 8 or 9, depending upon when the gating

interval started.

It is obvious that the percentage error

becomes smaller as more pulses are counted. A one count matters less for 1,000,000 than for

1,000. This explains why long gating intervals permit greater accuracy of measurement.

44
GATING INTERVAL

8 COUNTS

SIGNAL

PULSES

JJJ_LJ_L_L_LJ^L-L

9 COUNTS

GATING INTERVAL

FIGURE ^. 16. CONSTANT GATING INTERVAL WITH AMBIGUITY OF + 1 COUNT.

If we count a 1 MHz frequency for 1 second, the ± 1 count error is one part per

million. If either the measured frequency is increased to 10 MHz or the gating interval is lengthened to 10 seconds, the error goes down

to 1 part per ten million. In measurements of

high frequencies, made directly or with the

aid of heterodyne converters, the uncertainty

of ± 1 count may be less important than the counter's time base error. The same is true

of prescalers which incorporate gate extension

features.

Previously, we noted that low frequencies

can be measured more accurately by period measurements than by direct frequency measurements. For very high frequencies, however,

the reverse is true. At some point, the

effect of gate error must be identical for

either mode of operation. The chart in figure

4.17 depicts the crossover point as well as the accuracy which can be expected both from

multiple-period measurements and direct frequency measurements up to 100 MHz.

C. Trigger Errors

Trigger errors usually arise from the

presence of noise or other modulation on the

gate-control signal. This source of error has

been mentioned previously in conjunction with

period and time interval measurements. For frequency measurements, the trigger error is almost always negligible because the gatecontrol signal, obtained directly from the

time base, can be virtually noise-free.

For period and time interval measurements, the trigger error (e tr ) is expressed

mathematically as the ratio of the gate time

deviation (At) to the total duration (T) of

the gating interval. Trigger error is also

proportional to the fraction of peak noise

voltage (E ) over peak signal voltage

FIGURE 4.17. ACCURACY CHART FOR PERIOD AND FREQUENCY MEASUREMENTS.

45
where k is a proportionality constant determined by the nature of the modulation.

Although noise can sometimes be removed

from the input signal by external devices;

e.g., limiters, filters, etc., its effects can be reduced more conveniently by multipleperiod averaging. As more periods are averaged, the effects of both the ± 1 count ambiguity and the trigger error diminish proportionately.

Most counters incorporate a Schmitt trig- ger or similar circuit to generate the trigger

pulses. Because of a slight hysteresis or lag

in switching action which characterizes such

circuits, the rise and decay of the trigger

pulses are not started at exactly the same

voltage. Instead, the width of the trigger

pulse is determined by two voltage levels —one that establishes the point at which the pulse

CONTINUOUS SINE WAVE INPUT

(FOUR PULSES produced)

HEAVY AMPLITUDE MODULATION

(THREE PULSES PRODUCED)

FIGURED. 18. UNDIFFERENTIATED

begins to rise and the other that dictates the

point where the pulse starts to fall. A com- plete pulse is produced, therefore, only when

the input signal crosses both critical levels.

Figure 4.18 shows how counts may be lost or gained as a result of signal modulation.

The shaded area in each example is the hysteresis zone of the trigger circuit. Incorrect

counts are likely to result whenever the input

waveform reverses direction within the shaded

zone.

To measure the time interval between the occurrence of a sharp pulse and the next positive-going zero crossing of a continuous

sine wave, it would be logical to start the counter with the pulse and adjust the stopchannel control (s) to trigger at zero volts on the positive slope. If the trigger level control for the stop channel is improperly

LOW-FREQUENCY RIPPLE SUPERIMPOSED

(THREE PULSES PRODUCED)

I

I

1 i kd

1

I J 1i

i

i

i

U|

i

I

I !!

f

i

i

i I

1

HIGH-FREQUENCY INTERFERENCE

(FIVE PULSES PRODUCED)

SCHM I TT-TR I GGER WAVEFORMS

46
-CALIBRATION LOW — (TRIGGER LEVEL TOO HIGH)

U— TIME INTERVAL TOO LONG H

CALIBRATION CORRECT

- CALIBRATION HIGH

- (TRIGGER LEVEL TOO LOW)

U— TIME INTERVAL TOO SHORT-*]

I

1

FIGURE it. 19- TIME ERROR PRODUCED BY IMPROPER CALIBRATION

OF TRIGGER LEVEL CONTROL.

calibrated, however, the stop pulse may be

produced at a point that is actually positive

or negative when the control is set at zero. This means that the measured time interval

will be too long or too short as shown by

figure 4. 19.

Of course, an opposite error will result

if the trigger level control for the start

channel reads too low or too high. If the

controls for both channels are misadjusted in opposite directions, the combined error will

be cumulative. Furthermore, the effect of

trigger calibration error is made worse if

instead of a steep wavefront the input signal

>T/2

T/2

<T/2

has a gradual slope at the selected triggering

point.

A low-frequency sine wave is convenient

for checking the calibration of trigger con- trols. With the counter in the time interval

mode, the gating interval switch in the XI po- sition, and both input channels adjusted to trigger at zero volts on the same slope, the

counter should display exactly one period of

the test signal. Next the slope selector for

the stop channel is turned to its opposite po- sition without adjusting any other controls.

If the counter reading changes by exactly one- half period when the opposite slope is

(a) LEVEL TOO HIGH

(b) LEVEL CORRECT

(c) LEVEL TOO LOW

FIGURE A. 20. SINE WAVE METHOD OF CHECKING TRIGGER LEVEL CALIBRATION.

47
DIGITAL

COUNTER

D TO A CONVERTER ANALOG CHART

STRIP RECORDER

FIGURE k. 21. DIGITAL-TO-ANALOG ARRANGEMENT FOR CHART RECORDING.

selected, the trigger level setting is correct. If the reading changes by more than

one-half period, as in (a) of figure 4.20, the

stop channel's trigger level is too high. If the change is less than one-half period, as in (c), the trigger level is too low. The start trigger can be checked in a similar fashion by

holding all other controls fixed and switching

the slope selector for the start channel only.

4.1.8 PRINTOUT AND RECORDING

Some counters have provisions for con- necting a printer to get a permanent record of

the readings. Using a digital-to-analog con- verter, the counter can also be used to drive

a chart recorder. The printer must be compatible with the counter if a valid printout is to be obtained.

used wires, mirrors, and light beams instead

of electronic circuits, but it could display

waveforms of low-frequency alternating

current.

The modern cathode-ray tube was developed

in the early 1920' s. Since then, the electronic oscilloscope has evolved into an in- strument that can display high-frequency waveforms and store images for days. Measurements

or comparisons of frequencies and time intervals are among the major uses of the oscilloscope.

There are four main parts: (1) the

cathode-ray tube and its power supply, (2) the

vertical deflection amplifier, (3) the horizontal deflection amplifier, and (4) the timebase generator. A block diagram is shown in figure 4.22.

4.2 OSCILLOSCOPE METHODS

The cathode-ray oscilloscope is probably

one of the most useful laboratory tools available to the electronics technician, engineer,

or scientist. Whole volumes have been devoted

to its many uses.

The basic physical principles of the

oscilloscope have been known for years. An early oscillograph (as it was then called) was

in operation during the 1890' s. This device

The role of the time-base generator is fairly involved. In order to display faithfully a signal that is a function of time, the

spot of light must traverse the cathode-ray

tube screen at a uniform rate from one side to the other and then return to the original side

as quickly as possible. The voltage that

causes this horizontal sweep is generated by

the time-base generator. The waveform is a ramp or "sawtooth" as shown in figure 4.23.

The frequency of the ramp voltage is variable so that waveforms of any frequency

VERTICAL

INPUT VERTICAL

AMPLIFIER

HORIZONTAL INPUT

TRIGGER INPUT

CRT

POWER

SUPPLY

HORIZONTAL

AMPLIFIER

c

) TIME-BASE

GENERATOR

c

FIGURE It. 22. SIMPLIFIED BLOCK DIAGRAM OF A CATHODE-RAY OSCILLOSCOPE.

48
FIGURE it. 23. WAVEFORM OF HORIZONTAL DEFLECTION VOLTAGE FROM TIME-BASE GENERATOR.

within the limits of the oscilloscope can be

displayed. The time-base generator has a control to change the frequency of the ramp. How fast the spot moves across the screen is of interest to the operator; therefore, the

frequency control is usually labeled in seconds (or fractions of seconds) per centimeter.

When the frequency of the traverse is high enough, the persistence of the phosphorous and the retention capability of the human

eye combine to make the moving spot appear as

a solid line across the screen. If a periodic

wave is applied to the input of the vertical

amplifier and the time-base generator is properly adjusted, the waveform will be displayed as a stationary pattern on the cathoderay tube screen.

4.2.1 CALIBRATING THE OSCILLOSCOPE TIME BASE

If there is much doubt as to the accuracy

of the time-base generator, it should be calibrated. Use a signal that is more accurate

than the time-base signal being measured. The

calibration is done in much the same way as the frequency measurement, but in reverse. If you have a crystal oscillator that operates in the same frequency range as the sweep time

base being measured, proceed as follows:

Display the calibration source signal on the screen, and adjust the time-base switch to the setting that should give one cycle per

centimeter. If the source is 100 kHz, for

example, the time-base step switch should be set to 10 microseconds/centimeter. If the

time-base generator is off frequency, there

will be more or less than one cycle per centi- meter displayed on the screen. Adjust the

variable frequency control of the time-base

generator until the display is correct.

The adjustment may or may not hold for

other positions of the step switch, so it is best to recalibrate each position. Many

laboratory oscilloscopes have built-in calibration oscillators which can be used in the same way as an external frequency source. Refer to the oscilloscope manual for details.

EXTERNAL

FREQUENCY

SOURCE

CRT

POWER

SUPPLY

CRT

VERTICAL

AMPLIFIER

TIME-BASE

GENERATOR

FIGURE 4.24. TIME BASE CALIBRATION HOOKUP USING AN EXTERNAL FREQUENCY SOURCE.

49
1 CM

FIGURE 4.25. SINE WAVE DISPLAY AS VIEWED ON OSCILLOSCOPE.

4.2.2 DIRECT MEASUREMENT OF FREQUENCY A. Lissajous Patterns

Suppose that we want to measure the frequency of a sine wave when it appears as shown

in figure 4.25. By counting the number of

cycles and the number of scale divisions

(centimeters), we see that five cycles occur

in 15.5 centimeters. Now we observe the timebase generator dial, first making sure it is in the calibrated mode. If the dial reads 0.1 millisecond per centimeter, for instance, the

frequency computation would be:

5 cycles 1 cm 1000 ms 3230 Hz. 15.5 cm 0.1 ms 1 s

Alternatively, we may divide the display (in

cycles per centimeter) by the time-base rate

(in seconds per centimeter):

cycles ^ s_ _ cycles cm

cm cm cm

Hz.

Direct frequency measurements with the

oscilloscope have an accuracy limit of about 5 parts per thousand. Accuracy of the measurement is limited by the ability of the operator

to read the screen and by the accuracy of the

time base. In general, one should expect to

do no better than about 0.5%. Still, if you

want to know whether you are looking at 60 Hz or 120 Hz, or if you are checking a radio

carrier frequency for harmonics, the accuracy

is good enough.

4.2.3 FREQUENCY COMPARISONS

Unlike direct frequency measurements , frequency comparisons with the oscilloscope

can easily be made with a precision of one part per million. A precision of one part per

100 million or even one part per billion can

be achieved by using extra equipment described

1 ater.

The optical-mechanical oscillograph was developed in 1891, but the idea of a Lissajous

pattern is even older, having been demonstrated by a French professor of that name in 1855.

If we apply a sine wave simultaneously

to the horizontal and vertical inputs of an oscilloscope, the spot will move in some pat- tern that is a function of the voltages on each input. The simplest Lissajous pattern

occurs if we connect the same sine wave to

both inputs. The equipment hookup is shown

in figure 4.26. The resulting pattern is a straight line. Furthermore, if both signals

are of the same amplitude, the line will be inclined at 45° to the horizontal.

FIGURE 4.26. EQUIPMENT HOOKUP

FOR A 1:1 LISSAJOUS

PATTERN.

If the input signals are equal in amplitude and 90° out of phase, a circular pattern

results. Other phase relationships produce an elliptical pattern. The phase difference in degrees between the horizontal sine wave and

the vertical sine wave can be determined as shown in figure 4.27.

The phase relationship between signals

that generate linear, circular, or elliptical

patterns is given by:

• L A

sin 6 = „ ,

50
-0.866 9 = 150° 210° 9-180°

FIGURE k.27. ELLIPTICAL LISSAJOUS PATTERNS FOR TWO INDENT I CAL FREQUENCIES

OF DIFFERENT PHASE.

RESULTANT IMAGE

12 0-36

HORIZONTAL 271- DEFLECTION

VOLTAGE

33 FIGURE 4.28. DEVELOPMENT OF A 3:1 LISSAJOUS PATTERN.

51

where 8 is the angle between the two sine waves, and A and B are relative distances

measured along the vertical axis of the trace.

The actual determination of frequency

ratio is done in a straightforward manner. The number of loops touching, or tangent to, a horizontal line across the top of the pattern

is counted. Likewise the number of loops

tangent to either the right or left side is counted. The frequency ratio is then computed

from the formula

where: f = the frequency of the vertical

v deflection voltage,

f. = the frequency of the horizontal deflection voltage,

T, = the number of loops tangent

to the horizontal line, and

T = the number of loops tangent

to the vertical line.

Figure 4.28 shows the graphical construction of a 3:1 pattern. Note that the ratio of

T./T is 3:1.

h v Lissajous patterns become quite complicated when the two frequencies are widely

separated. Some oscillograms for higher

ratios are shown in figure 4.29. Two versions

of each pattern are shown. Version (A) is the

typical closed-lobe pattern, and version (B)

is the double image that results if phase

relations cause the pattern to be folded and

exactly superimposed upon itself.

The usefulness of Lissajous patterns

depends upon a number of things. Consider a

1:1 pattern. If we are trying to synchronize

two power generators at 60 Hz and our pattern

is changing through one complete cycle every

second, we have achieved synchronization to 1 Hz or about 1.7%--not very good. If our frequency was 100 kHz, the same change of one revolution per second in our pattern would

give a resolution of 1 x 10- 5

. This shows

that the higher the frequencies we are working

with, the better synchronization becomes.

In comparing or synchronizing precision

oscillators, a frequency multiplier may be

useful. Every time our signals are multiplied

in frequency by a factor of ten, the precision

of measurement improves by a factor of ten. When the two signals are multiplied and then

fed to a phase detector, they produce a lowfrequency signal equal to the difference in

hertz between our two multiplied signals.

This difference signal, Af. may then be com- pared via Lissajous pattern against the sine wave from a precision low-frequency oscil1 ator.

If we again use two 100-kHz signals as an example but multiply their frequencies to 100 MHz before comparing them, we achieve a mea- surement precision of 1 x 10- 8

. This arrange- ment is shown in figure 4.30.

The phase detector may be a doublebalanced mixer of the type shown in figure

4.31. These mixers are available com- mercial ly.

Another system for comparing two oscillators is indicated in figure 4.32. A phase- error multiplier increases the phase or frequency difference between the oscillators by some factor, say 1000, without frequency

100kHz FREQUENCY

MULTIPLIER

OSCILLATOR (X1000)

1 00MHz

100kHz FREQUENCY

MULTIPLIER

OSCILLATOR (X1000) 1 00MHz

CALIBRATED

LOW FREQUENCY

OSCILLATOR

1000 AF

FIGURE 4.30. ARRANGEMENT FOR MEASURING SMALL FREQUENCY DIFFERENCES WITH OSCILLOSCOPE AND FREQUENCY MULTIPLIERS.

53
INPUT INPUT B

DC OUTPUT

FIGURE 4.31. DOUBLE-BALANCED MIXER.

OSCILLATOR

100 kHz

OSCILLATOR

100 kHz

100 kHz

PHASE ERROR

MULTIPLIER

(X1000)

FIGURE 4.32. ARRANGEMENT FOR FREQUENCY COMPARISON USING

OSCILLOSCOPE AND PHASE-ERROR MULTIPLIER.

multiplication. The output from the phase- error multiplier can be connected directly to the oscilloscope. The resulting Lissajous

pattern will allow oscillator synchronization

with the same precision as provided by the

frequency multiplier technique. If care is taken, calibrations with errors as small as

1 x 10- 9 or 1 x IO- 10 can be accomplished.

A principal use of the 1:1 Lissajous

pattern is for synchronization of oscillator

frequencies. If the pattern is drifting, no matter how slowly, it means that a frequency

difference exists between the oscillators.

Neither the oscilloscope nor the operator can

tell which oscillator is at the higher frequency. The Lissajous pattern is the same regardless of which frequency is higher. What

can be done, though, is to slightly change the

frequency of one of the oscillators. If the rate of pattern drift increases when the frequency of oscillator #1 is raised, then oscillator #1 is at a higher frequency than oscillator #2. The opposite happens if the pattern

drift slows down when the frequency of oscillator #1 is increased.

Calibration of variable oscillators such

as rf signal generators and audio oscillators

is easily done using Lissajous patterns of

higher than 1:1 ratio. Figure 4.33 shows a

method of calibrating an rf signal generator

using a fixed 100-Hz frequency source.

Starting with a 1:1 Lissajous pattern

between the standard oscillator and the signal

generator, mark the calibration points on the

generator dial each time the pattern becomes

stationary as the dial is turned. Each

stationary pattern represents a definite

frequency ratio which can be evaluated by one

of the methods explained previously. Patterns

of ratios up to 20:1 can be recognized if the

signal generator is stable enough.

100kHz CRYSTAL

OSCILLATOR

FREQUENCY

MULTIPLIER

HORF SIGNAL

GENERATOR

FIGURE 4.33. ARRANGEMENT FOR CALIBRATING

A SIGNAL GENERATOR.

54
If the oscillator has a base frequency of

1 MHz with internal dividers to 100 kHz, calibration points can be found on the signal

generator dial every 100 kHz from 0.1 MHz to 2 MHz and every 1000 kHz from 2 MHz to 20 MHz. This range is adequate for many rf signal

generators. Multipliers can be used to extend

the markers into the VHF range if necessary,

but the calibration points will be farther

apart.

Not much can be learned from a drifting

pattern of high ratio, and even the stationary

pattern may be confusing. A complete rotation

of the pattern in one second means a 1-Hz

deviation by the higher frequency no matter

what the ratio. But for ratios other than

1:1, the change in the lower frequency is not

as great.

With a 10:1 pattern, for instance, a drift of one revolution per second could mean

that the higher frequency is off by 1 Hz, or

it could mean that the lower frequency is off

by 0.1 Hz. Unless one of the two frequencies

is assumed to be accurate, there is no way of

telling from the Lissajous pattern which

frequency is in error. Only their relative

difference can be measured by this method (see

Chapter 3).

B. Sweep Frequency Calibration:

An Alternative Method

Most frequency comparisons made in a standards laboratory are between oscillators

that have very small frequency differences. A

1:1 Lissajous pattern between oscillators

having a relative frequency difference of 1 x

10- 11 would take over a day if the comparison

were made at 1 MHz! There are faster ways to measure the rate of change on the oscilloscope. These methods allow the difference

frequency to be determined in much shorter

times than is possible with a slow moving

Lissajous pattern.

The output wave of an oscillator to be

measured is displayed on the oscilloscope.

Next, the oscilloscope sweep frequency oscillator is calibrated. Now, if the oscilloscope

is triggered by a reference oscillator, the

pattern or waveform will appear to stand still

if the frequencies of the two oscillators are nearly equal. Actually, the pattern will

slowly drift. The rate of drift in microseconds per day depends on the frequency difference between the oscillators. The direction

of drift tells us which oscillator is high or

low in frequency with respect to the other.

When a sine wave of an oscillator frequency is displayed and the oscilloscope is triggered by a reference oscillator, the movement of the pattern can be used for calibration. This pattern will move to the left

or right if the unknown frequency is -high

or low with respect to the reference oscillator. The question is, how fast is the

pattern moving? How many microseconds is it moving in how many hours or days? Remember,

the sweep is calibrated in microseconds or milliseconds per centimeter.

One could make a measurement of the

pattern position at, say, 10:05 am, then come back at 2:30 pm for another look. Suppose in the meantime, it moves exactly three centi- meters and the sweep speed is set at one microsecond per centimeter. What does this

tell us?

ELAPSED TIME = 2:30 pm - 10:05 am

= 4 hours 25 minutes = 0.184 day.

PHASE DRIFT RATE = 3-00 cm x 1 . 00 us/cm

0.184 day

= 16.3 us/day.

This result tells us how much our oscillator moved in time (16.3 microseconds) during

an interval of one day. The result is in the

form At/T (see Chapter 3 for a further explanation). Expressed as a numeric, At/T = 1.89

x 10- 10 since there are 86,400 x 10 6 microseconds in a day.

We can relate this to a frequency nota- tion because Chapter 3 tells us that

_ At _ Af

T f

For this oscillator, the relative frequency,

F = Af/f = -1.89 x 10- 10

. The negative sign

tells us that the measured frequency was lower

than its nominal value. In this case, it is assumed that the reference frequency is equal

to its nominal value. Notice also that the

oscilloscope has performed a measurement much

like the time interval counter mentioned

earlier in this chapter.

©T FREQUENCY

OSCILLATOR REFERENCE

TO BE CALIBRATED -o o

FIGURE 4.34. VIEWING FREQUENCY DRIFT.

55
4.2.4 TIME INTERVAL MEASUREMENTS

Time- interval measurements are quite easy

to perform with the oscilloscope. Usually,

one time pulse is displayed on the screen while a different time pulse is used to trig- ger the oscilloscope. Another method is to display both time pulses on a dual-trace

scope. Triggering can be achieved from either

of the two displayed pulses or from a third

time pulse. An adjustable rate divider is handy for this purpose.

A point worth remembering is that the

accuracy of the time- interval measurement is inversely proportional to the length of the

interval. If the time pulses are 10 milliseconds apart, for instance, the measurement

can be performed to probably no better than

± 50 microseconds. But if the pulses are only

100 microseconds apart, the measurement can be performed with an accuracy of ±0.5 microsecond. In both cases, however, the relative

uncertainty is the same (± 5 parts in 10 3

).

For time pulses spaced close together,

the pulse width may occupy an appreciable

percent of the time interval; therefore, large errors can be introduced by having the wrong

trigger slope or improper trigger amplitude.

Unless care is taken by the operator, such errors can nullify the accuracy that is otherRF VOLTMETER

wise attainable in the measurement of brief

time intervals.

The main advantage in using an oscilloscope for time interval measurements is that

one can see the pulse waveform. For this

reason, time intervals between odd-shaped or noisy pulses are better measured with the

oscilloscope than with other types of instruments. Time intervals in the submicrosecond

range can be measured easily and economically

with high-frequency oscilloscopes, but long

intervals are more accurately measured with an electronic counter, as described earlier.

Always refer to your oscilloscope manual; it

contains additional useful information.

4.3 WAVEMETERS

A wavemeter is an adjustable circuit that

can be made to resonate over a range of frequencies as indicated on a calibrated dial.

The dial may be marked in units of frequency

or wavelength. A typical wavemeter covers a frequency range on the order of one octave. Resonance is detected by observing the re- sponse of a meter or other device incorporated

into the measuring instrument or by noting a disturbance in the system under test.

INCANDESCENT LAMP

THERMOCOUPLE

MILLIAMMETER

SECONDARY WINDING SECONDARY WINDING

WITH LAMP WITH METER

FIGURE 4.35. WAVEMETER RESONANCE INDICATORS.

56
RF INPUT

>

1 OPR

XTAL

OSC

J

MIXER

CALIB

VARIABLE

FREQUENCY

OSCILLATOR

AUDIO

AMPLIFIER

RF OUTPUT

TERMINAL

ZERO-BEAT

INDICATOR

FIGURE 4.36. BLOCK DIAGRAM OF HETERODYNE FREQUENCY METER.

Wavemeters are sometimes classified as transmi ssion types or as absorption types

depending on the way they are connected to the

frequency source being checked. A trans- mission-type wavemeter has connectors at both

input and output ends so it can be put into

the circuit under test. An absorption- type

(or reaction-type) meter, on the other hand,

is designed for loose coupling via the electromagnetic field of the frequency source.

The simplest LC absorption wavemeter for

measuring radio frequencies consists of a coil

and a variable capacitor in series. The

capacitor shaft has a calibrated dial so the resonant frequency of the LC circuit can be

read off as the capacitor is adjusted.

The simple wavemeter is used by holding

it near the frequency source being measured.

It is then tuned while watching for a sharp

reaction by the source. The frequency is read

from the calibrated dial. The reaction is caused by the absorption of a small amount of

rf energy from the source by the wavemeter

when the two are at the same frequency. In measuring the frequency of a small transmitter, one might look for a flicker in the power

stage current as the wavemeter is tuned

through resonance. Because of the loose

coupling and the small power consumed by the

wavemeter, the energy taken from high-power

sources might go unnoticed; therefore, an indicator is built into the wavemeter itself.

A small lamp is sometimes used with the wavemeter for rough measurements of frequencies at high power levels. A neon lamp in parallel with the wavemeter 1 s tuned circuit or

an incandescent lamp in series with the tuned

circuit will glow if enough power is absorbed

from the frequency source. Better measurements can be made if the lamp is replaced by a meter. Figure 4.35 depicts indicator arrangements used with absorption wavemeters.

In operation, the wavemeter is tuned for

maximum brilliance of the lamp or for peak

deflection of the meter. A resonance indi- cator J_n series with the tuned circuit must

have very low impedance so as not to degrade

the Q (fractional linewidth), and hence the

resolution, of the wavemeter. An indicator

connected across the tuned circuit must have a high impedance for the same reason. The resonant elements themselves should exhibit

the lowest losses possible, since the accuracy

with which frequency can be measured by these

means is mainly dependent on the Q of the

tuned circuit.

4.4 HETERODYNE FREQUENCY METERS

The heterodyne frequency meter is basically a mixer, one input of which is driven by

a reference signal from a self-contained,

calibrated, tunable oscillator. It also

includes an audio amplifier with means for

operating a zero-beat indicator, such as headphones, an ac voltmeter, or an oscilloscope. Utilizing the familiar heterodyne

principle, it compares the internal oscillator

frequency with the unknown frequency of an external source.

The unknown signal is applied through the

input terminals to the mixer stage, where it combines with the reference signal to produce

a beat note. The internal oscillator is then

carefully tuned for a zero beat. Its frequency is then read directly from the calibrated tuning dial. If the unknown frequency

falls within the fundamental tuning range of

the oscillator, its value will be the same as the oscillator frequency under the zero-beat

conditions.

Because the nonlinear mixer gives rise to harmonics, it is also possible to detect beat

notes between the fundamental of one signal

and harmonics or subharmonics of the other.

If the frequency of the external signal

matches a known harmonic of the oscillator

fundamental, the dial setting must be

57
RADIO

TRANSMITTER

HETERODYNE

FRE^UE

ITER TV

WHIP

ANTENNA

RADIO

RECEIVER

LOUDSPEAKER

FIGURE 4.37. ARRANGEMENT FOR CHECKING TRANSMITTING FREQUENCY WITH HETERODYNE FREQUENCY METER AND RECEIVER.

multiplied by the appropriate harmonic number

N. On the other hand, if the unknown frequency

equals a subharmonic, the dial reading must be divided by N. Thus, from a knowledge of harmonic relations, heterodyne measurements can

be made of frequencies far above or below the

fundamental tuning range of the frequency

meter.

Figure 4.36 shows the block diagram of a basic heterodyne frequency meter. The internal variable-frequency oscillator must have

good stability. This calls for close temperature regulation, complete shielding, and rug- ged construction. The oscillator should also

be thoroughly buffered to reduce frequencypulling tendencies when strong signals are applied from the external source, or when

other factors would cause the oscillator load

to change.

Some heterodyne meters are equipped with

a whip antenna so the internal oscillator

signal can be radiated over short distances.

For checking the frequencies of transmitters

or other high-power sources, a safe method is to tune in the transmitter with a radio

receiver and then adjust the frequency meter

until a zero beat is heard through the

receiver's loudspeaker. The unknown frequency

can be measured just as though it were coupled

directly to the frequency meter.

The overall accuracy of measurement

depends on the calibration accuracy of the

tunable oscillator and the resolution attainable through the zero-beat process. At high

frequencies, the error introduced by imperfect

zero-beat detection can be ignored. Provided

the temperature is kept nearly constant, the

variable-frequency oscillator may retain its

accuracy within 0.1% over long periods of time

without recal ibration.

Most heterodyne meters contain a stable

quartz crystal oscillator for use in gener- ating markers at fixed check points on the

tuning dial. By checking the dial calibration

at the point nearest the frequency to be

measured, it is possible to achieve an accu- racy approaching one part per million after

the instrument has reached a stable operating

temperature.

Heterodyne frequency meters combine wide

frequency coverage with good accuracy in a portable instrument. They can be obtained for

operating over fundamental frequency ranges from approximately 100 kHz to 200 MHz. Accu- racy varies from about 0.01% at low frequencies to 0.00025% or better at very high frequencies. Harmonics up to 3 GHz may be gen- erated with a precision of 1 x 10- 6

.

Because they are tunable devices, the

heterodyne frequency meters offer better noise

immunity and signal sensitivity than wideband

instruments such as oscilloscopes and counters. The heterodyne meter is one of the most

useful of all frequency-measuring instruments

for checking transmitter frequencies, for

calibrating rf signal generators, and for

other applications which do not require a high

degree of signal purity.

4.5 DIRECT-READING ANALOG FREQUENCY METERS

Analog frequency meters operate by trans- forming the unknown frequency into a different

physical quantity-such as voltage or current

ampl itude--which can be measured directly by

conventional instruments. Thus, the output

indicator might be an ordinary mi Hi ammeter whose deflection is somehow made proportional

to the input frequency.

4.5.1 ELECTRONIC AUDIO FREQUENCY METER

A meter- type instrument that reads directly in audio frequency units and requires no manipulations of any kind is shown in figure

58
/\A,

INPUT

OVERDRIVEN

AMPLIFIER

(LIMITER)

-OIL

M I LL I AMMETE R

FIGURE ^.38. ELECTRONIC AUDIO FREQUENCY METER,

4.38. The input signal is shaped into a rec- tangular waveform, differentiated, rectified,

and then applied to a mil 1 i ammeter. The current spikes have the same frequency as the

input signal; but since their peak amplitude

is constant, the meter deflection is proportional only to the number of spikes that occur

per unit of time. Deflection, therefore, is determined by the frequency of the input

signal so the meter scale can be calibrated

directly in hertz.

Operation of the instrument is no more complicated than the operation of an electronic voltmeter. Besides its simplicity of

operation, the instrument has an additional

advantage in that frequency indications are independent of both waveform and amplitude of

the input signal over very wide limits. Its overall accuracy is comparable to that of the meter movement, typically 1 to 5 percent.

4.5.2 RADIO FREQUENCY METER

The operating range of the audio frequency meter may be extended to the VHF region or higher by adding a series of decade frequency

scalers ahead of the instrument. Figure 4.39

shows a diagram of such a device which is

capable of measuring radio frequencies up to 50 MHz directly.

The audio frequency meter is preceded by

four mixer stages, each excited by a signal

derived from a standard 1 MHz crystal oscillator. The injection frequency for the first

mixer consists of the 25th to the 49th harmonic of 1 MHz as determined by the tuning of

harmonic selector A.

The difference between the input frequency and the selected harmonic is then applied

to a second mixer, where i L is heterodyned

against a subharmonic of 1 MHz as determined

by the tuning of harmonic selector B. The

process continues until the input frequency is eventually reduced to a value that falls

within range of the direct-reading audio

meter.

The instrument is operated by manually

tuning each harmonic selector and mixer in succession for peak deflection on the associated tuning meter. Each harmonic selector

dial is calibrated so that the input frequency

can be read directly from the dial settings in conjunction with the reading of the audio

meter. For example, if optimum tuning occurs when dial A is set at 32, dial B is at 7, dial

C is at 5, dial D is at 9, and the audio meter

INPUT >-

TUNING METER TUNING METER TUNING METER TUNING METER 0

TUNABLE

MIXER

25-49 /

HARMONIC

SELECTOR

1 MHz

1 MHz XTAL

OSC

0

TUNABLE

MIXER 0

TUNABLE MIXER

0-1 MHz J.

HARMONIC

SELECTOR

0

TUNABLE

MIXER

0-100 kHz

HARMONIC

SELECTOR

0.1 MHz

• 1 0

AUDIO FREQUENCY —{/)

METER

0-10 kHz

HARMONIC

SELECTOR

10 kHz

no

1 kHz

no

FIGURE 4.39. DIRECT-READING RADIO FREQUENCY METER.

59
is reading 170, then the input frequency is 32.759170 MHz. Typical accuracy of the

instrument with an analog audio meter is

5 x 10- 7 at VHF. If the audio meter is re- placed by an electronic digital counter, the

accuracy may be improved to 2 x 10- 8 or better.

4. 6 FREQUENCY COMPARATORS

Frequency comparators are versatile

devices for making frequency measurements and

comparisons. These instruments combine phase error multipliers, phase detectors, integrated

circuits and frequency multipliers into useful

and accurate laboratory aids. The block

diagram of this instrument is shown in figure

4.40. The heart of the frequency comparator

is a variable-multiplication phase error multiplier of special design.

The phase error multiplier, like all electronic devices, is subject to noise.

Through each stage of error multiplication,

the noise at the input is amplified. Even- tually the noise out of the phase error multiplier would be stronger than the signal, so to maintain a useful signal-to-noise ratio the

multiplication is usually limited to 10,000.

Probably the most important part of this

instrument is the frequency discriminator

which is based on a tuned-circuit. The output

is a dc voltage proportional to the difference

between the frequency of the discriminator

input signal and the resonant frequency of the

tuned circuit. The Q of the tuned circuit

determines the sensitivity of the discriminator.

As an example of the Q required, consider

a discriminator operating at 1 MHz with an error multiplication of 10,000. If the com- parator is to respond to a frequency difference of 1 part in 10 12

, the discriminator will

have to detect a frequency difference of Af =

10- 12

x 10 4 x 10 6 Hz = 0.01 Hz. If 0.01 Hz corresponds to 1% of full-scale meter deflection, the bandwidth of the discriminator must

be 100 x 0.01 Hz = 1 Hz. This requires a Q of

10 6. One commercial comparator solves this

problem by using a quartz crystal as the tuned

circuit for the discriminator.

The comparator usually contains phase

detectors that also are connected to the out- put of the phase error multiplier chain. The

phase detectors have various integrating

times, or bandwidths, that allow one to optimize the accuracy of the particular measurement being made. As shown in figure 4.40, the

comparator can be used with an external chart

recorder, an electronic digital counter, or an osci 1 loscope.

4.7 AUXILIARY EQUIPMENT

Some specialized types of equipment,

though not complete systems for the measure- ment of frequency or time, are nevertheless

very useful as auxiliary devices to increase

the versatility, accuracy, and range of other

measuring instruments. These auxiliary devices include frequency synthesizers, phase error multipliers, phase detectors, frequency

multipliers, frequency dividers, signal aver- agers, and phase-locking receivers.

4.7.1 FREQUENCY SYNTHESIZERS

Not all frequency comparisons and mea- surements are performed at even frequencies

like 100 kHz, 1 MHz, 5 MHz, etc. Many electronic applications involve odd frequencies.

An example is the television color subcarrier

frequency of 63/88 x 5 MHz. VHF and UHF communications channel frequencies are another

example.

There are two general types of synthesizers, each with points in its favor. The

direct synthesizer uses internal frequency

multipliers, adders, subtracters, and dividers

in combination to produce an output frequency.

The indirect method of synthesis uses a phaselock oscillator to provide the actual output

signal. The spectral purity of the latter

R INPUT

F

X INPUT

PHASE ERROR MULTIPLIER

XI 0

1 TO XI

4

F R +10"AF

FREQUENCY

DISCRIMINATOR

PHASE DETECTOR

0

N AF

TO CHART

"RECORDER

DC VOLTMETER

CALIBRATED IN 'PARTS IN 10 N "

TO COUNTER

OR OSCILLOSCOPE

FIGURE k.kO. BASIC FREQUENCY COMPARATOR.

60 1
method is more a function of the phase-lock

loop than of the spectrum of the locking

signal. Both types of synthesizers are widely

used in modern electronic equipment for the

generation of frequencies.

The synthesizer has two specifications

which determine its usefulness in any application. One is range and the other is reso -

1 ution . If the instrument has a range of 10 MHz and a resolution of 0.1 Hz, any frequency

up to 10 MHz may be generated in steps of 0.1

Hz. The dials or buttons on the front of the

instrument display the output frequency.

These controls interconnect the various

multipliers, mixers, etc. that are necessary

to produce the output frequency.

As mentioned previously, the measurement

and comparison of odd frequencies is simplified by the use of a frequency synthesizer.

As an example, assume that one wishes to determine the frequency of a crystal oscillator. The output frequency may be measured

with an electronic counter if one is available. Barring the availability of a counter,

a Lissajous pattern may be displayed on an oscilloscope with some reference frequency;

but the pattern on the oscilloscope might b( too complex for an accurate interpretation.

On the other hand, if a frequency syn- thesizer is available, a low-ratio Lissajous

pattern may be obtained. Once the pattern is stabilized, the oscillator frequency may be

read off the synthesizer dials. If the pattern is other than 1:1, the oscillator fre- quency may be computed by using the dial value

and the pattern ratio. Similarly, phase or frequency stability of odd-frequency oscillators may be determined using a synthesizer

in conjunction with phase detectors, oscilloscopes and chart recorders.

4.7.2 PHASE ERROR MULTIPLIERS

Oscilloscopes, phase detectors, chart

recorders, and similar devices used to make

frequency comparisons may be limited by their

frequency response. This can be a serious

handicap when frequency multiplication is used

to speed up calibrations. A phase error multiplier, however, multiplies the frequency

di f f erence between two signals without changing the frequencies themselves.

A phase error multiplier is shown in figure 4.41. It consists of one or more decade frequency multipliers in conjunction

with one or more mixers whose outputs are the

difference frequency between their input

signal s.

The reference frequency f is multiplied

by nine to produce 9f . The unknown frequency

f is multiplied by ten to produce lOf . But

f may be redefined as f = f ± Af, where Af

x

J x r is the difference frequency. Then 1 Of

x = 1 Of

p ± lOAf, and the mixer output will be 1 Of

r ± lOAf - 9f = f ± lOAf.

r r

The difference frequency is now 10 times

what it was initially. This process can be

repeated, as shown in figure 4.41, until the

noise gets too big. Four such decades seem to

be about the limit. This is equivalent to multiplying two 1-MHz signals to 10 GHz. The

output of the phase error multiplier is now treated the same way as the input would have

been, but the greatly amplified frequency

difference simplifies and quickens the mea- surement process. An application of the phase error multiplier is discussed in the sections

on Lissajous patterns (4.2.3A) and frequency

comparators (4.6).

UNKNOWN

IN

REFERENCE r R IN *

MULT.

X10

MULT,

X9

r— MIXER-

<8>-

9F,

MULT

X10

MULT.

XI 0

F R +1 OAF

OUTPUT

o „

AF = |F R -F

X

|, F

x

= F R±

AF

FIGURE kM. PHASE ERROR MULTIPLIER.

61
4.7.3 PHASE DETECTORS

Why use a phase detector? If two oscillators differed by 1 Hz and we com- pared their frequencies, we'd see a 1 Hz input

signal. But what if they only differed a by 0.001 Hz? A chart record of their

difference frequency or "beat note" would take 1000 seconds to trace a cycle. We soon find ourselves dealing with fractions of a cycle. A cycle has 360 elec- input

trical degrees. If we measure the phase b of the two signals, then we can calibrate

our chart or oscilloscope in terms of phase. This is simply a way of looking

at fractional hertz--parts of a cycle.

SHAPER "A"

FLIP-FLOP

SHAPER "B"

INTEGRATOR

DC 1

a. Block Diagram.

n

For example: Consider two 1 MHz oscillators that differ in frequency by

1 part in 108. This means that it will

take 100 seconds for a cycle of their

beat note to occur. On a chart, we would see 1 cycle (equals 1 microsecond

at 1 MHz) in 100 seconds. In 10 seconds,

only one-tenth of a microsecond change

will have occurred. These small changes

of phase can be used to calibrate frequency differences. Of course, if the

signals we are comparing differ by large

amounts, the phase changes very rapidly.

There are two main types of phase

detectors--the linear type and the non- linear type. They are discussed sepa- rately in the following paragraphs. The

linear phase detector gives a linear out- put as the relative phase of the input

signals change. They are often used with

phase error multipliers and strip chart

recorders. In a previous section, it was noted that linear phase detectors often cannot accept very high frequencies.

This is a good reason for combining a phase detector with a phase error

multiplier.

A block diagram of a typical linear phase

detector is shown in figure 4.42a. Examination shows that the heart of this type of

phase detector is a flip-flop. The action, or duty cycle, of the flip-flop determines the

output voltage of the detector. An upper

frequency limit for the detector occurs be- cause of the flip-flop. If the shaped signals

from the two inputs are less than 0.1 microsecond apart, for instance, the flip-flop

might not react at all. So the dc voltage to the chart recorder will not be a true time

record of the phase difference. In fact, what

happens is that the sawtooth output shown in figure 4.42b begins to shrink. The waveform

shifts up from zero and down from full-scale.

This shrunken response could be calibrated,

but it is much more convenient to use the

phase detector at a frequency which it can

Jl Jl Jl

SHAPER "A" OUTPUT JT

i i SHAPER "B" OUTPUT FLIP FLOP

i INTEGRATOR q--ra-..a-l-fin

FLIP-FLOP AND INTEGRATOR OUTPUT

SLOPE PROPORTIONAL TO FREQUENCY DIFFERENCE

TIME TYPICAL PHASE DETECTOR OUTPUT

b. Waveforms.

FIGURE h.kl. LINEAR PHASE DETECTOR.

handle. This is where a phase error multiplier proves helpful. With the phase error multiplier, one is able to have very sensitive

full-scale values of phase shift, say 0.01 microsecond, with inputs in the 100- kHz range.

A nonlinear phase detector is essentially

a phase discriminator like that used in FM receivers. The circuit diagram of a typical

nonlinear detector is presented in figure

4.43. These detectors are available with

cable connectors or in a printed circuit board

package. They are very small.

Because the response characteristic is nonlinear, this phase detector is not used

extensively with chart recorders. It has,

however, one important characteristic—its

output voltage can reverse polarity. In other

62
FIGURE 4.43. SCHEMATIC DIAGRAM OF NONLINEAR

PHASE DETECTOR,

words, the output is a dc voltage proportional

to the sine of the phase difference between

the input signals, and the polarity of the

output voltage depends on the lead or lag con- dition of the reference signal with respect to the unknown signal phase.

If the input signals are different in frequency, the output voltage is periodic like

a sine wave and there is no dc component.

This ac output can be measured with a frequency counter, or it can be applied to an oscilloscope to form a Lissajous pattern.

There are commercial meters available to make phase measurements. Several of these can compare signals at very high frequencies directly without the need to use phase error

multipliers, etc. By using sampling techniques, comparisons up to 1000 MHz can be

made. Read the equipment manuals carefully to note the linearity and accuracy of any instrument so that the results can be correctly

understood.

Analog dividers lend themselves to rf applications in conjunction with receivers and

transmitters. Digital types are found in electronic counters, computers and slewable

dividers.

A. Analog or Regenerative Dividers

Figure 4.44 shows the block diagram of an analog or regenerative divider. Frequency f is supplied to the input of the divider. After

amplification, this signal becomes one of two inputs to a mixer stage. The mixer output at f/10 is multiplied to 9f/10 and then fed back

to the other input of the mixer.

The gain of the various stages is very

critical. The regenerative divider is actu- ally an oscillatory circuit though not selfoscillating. If the loop gain around the

feedback circuit is too high, the unit will

continue to divide even with no input. The

output frequency in this condition is determined by tuning of the various circuits. If the circuit gain is too low, the unit will not function at all. The proper condition, as one would suppose, is where the unit divides under

conditions of input signal but ceases operation when that input signal is removed.

Some commercial units have their gain set

so that the dividers are not self-starting.

This feature is designed to eliminate the

problems of division with no input signal.

Also, since the unit isn't self-starting, the

output phase should not shift due to momentary

loss of input or dc power. Once the divider

stops, it remains stopped until a start button

is pressed.

4.7.4 FREQUENCY DIVIDERS

Frequency dividers are special forms of

frequency synthesizers. They may be grouped

into two general types: analog dividers that

use regenerative feedback, and digital dividers that use binary arithmetic together with

logic functions provided by bistable circuits.

INPUT © F

B. Digital Dividers

Simple bistable circuits such as flipflops may be combined to perform digital

division. Any flip-flop, whether assembled

from discrete components or integrated circuits, performs the same basic function. The

output assumes two possible states, first one

FREQUENCY

MULTIPLIER

X9

FIGURE h.kk. ANALOG OR REGENERATIVE TYPE OF DECADE FREQUENCY DIVIDER.

63
INPUT

PULSES

FIRST

STAGE

RESET

LINE

SECOND

STAGE

THIRD

STAGE

FOURTH

STAGE

OUTPUT

AND

RESET

PULSE

DECODING

(COINCIDENCE)

GATE

FIGURE k.kS. BLOCK DIAGRAM OF A FOUR-STAGE FLIP-FLOP DIVIDER.

and then the other, when the circuit is triggered by two successive input pulses. Thus, we have division by two.

If the output of one flip-flop is con- nected to the input of another, the output of

the second flip-flop will be a division by

four. A third flip-flop would allow division

by eight, and so on. Each succeeding flipflop provides an additional division by two. The divisors are 2, 4, 8, 16, 32, etc.

A decade (-H0) divider is formed by using extra circuitry. One such divider uses in- ternal feedback to supply six extra pulses

to a four-stage flip-flop chain, thereby

forcing the circuit into supplying an output

pulse after only ten input pulses rather than

the normal sixteen.

New digital logic circuits are being

produced so rapidly and at such low cost that

we cannot list all the possibilities here.

Every manufacturer has literature available

that gives applications for their circuits. You can assemble dividers, synthesizers, and even digital crystal oscillators to give you

just about any frequency and waveform you

desire. There are circuit "chips" that will

generate triangular waves and pulse bursts.

Complete phase- lock loops are available in small packages. There are low-power circuits

and high voltage dividers and drivers for a variety of indicators and displays.

4.7.5 ADJUSTABLE RATE DIVIDERS

When using an oscilloscope to observe

periodic waveforms, such as pulses or short

bursts of sine waves extracted from radio

receivers, it is often advantageous to trigger

the oscilloscope from a separate source. The

problem arises as to how to position the

pattern for best viewing or measurement. An

2 3 5

INPUT PULSES njnjnjnjn<jnj~Lru FIRST STAGE OUTPUT

( + ) SECOND STAGE OUTPUT

(.+) THIRD STAGE OUTPUT

(-) FOURTH STAGE OUTPUT

FIGURE k.h(>. FOUR-STAGE FLIP-FLOP DIVIDER WAVEFORMS.

64
adjustable digital divider can solve this

problem.

Assume that we have a digital divider

which accepts a 1-MHz input signal, counts 10 6 cycles of this input, and then delivers a narrow output pulse. If this process is repeated continuously, the output pulses will occur once every second. If we use the 1-pps

output to trigger an oscilloscope, we may need

to shift this pulse around in time to see a desired waveform.

If during the counting cycle, we were to add an extra pulse to the input pulse train,

the dividers would count this additional pulse

along with the normal pulses. The divider

chain would count along as usual until the

number 1,000,000 is reached. At that instant,

the unit would send out its normal seconds

pulse. But since we added one extra pulse to the input train, the output pulse would occur

1 microsecond earlier than it would have

otherwise. If we add a pulse to the input

train every second, the output pulse would

advance in time by 1 microsecond per second.

If we subtract one pulse from the input train

each second, the output pulse would be retarded 1 microsecond per second. By adding or subtracting pulses at different rates, we can

make the output pulses move rapidly or slowly

forward or backward. This is how the adjustable divider works. It can be built up easily

with modern digital circuitry.

4.7.6 SIGNAL AVERAGERS

If the same point on the seconds pulse is examined each time the pulse occurs, an average voltage for that point will emerge. This

is because the signal amplitude at the point

is constant, and ultimately the random noise

voltage at that point will average out to

zero. The length of time required for the

average signal value to appear will depend on the amount and nature of the noise.

The signal averager examines many points

on a pulse. The instantaneous value of voltage at each point is stored in a memory circuit comprised of a bank of high-quality

capacitors. Each time the pulse occurs, the same points are examined and stored in the same memory elements. After a sufficient

length of time, each capacitor will have

stored the average value of voltage from its

respective point on the pulse.

Each of the memory capacitors is in series with an electronic switch. The

switches are closed and opened by a control

circuit, which operates each switch in succession so that only one is closed at any given

time. Resistors between the input and output

amplifiers allow the time constant of the

memory elements to be optimized with respect

to the length of the pulse.

As each switch is closed, its associated

capacitor charges to the instantaneous voltage

present at the point on the waveform being

examined. That voltage is simultaneously

displayed on an oscilloscope. The waveform of

the displayed pulse is composed then of many

discrete voltage levels read from many storage

capacitors.

As one can imagine, the entire process must be carefully synchronized. The switch

controller is triggered simultaneously with

the oscilloscope trigger, and the switching

rate is adjusted nominally to allow a display

of all capacitor voltages during one sweep of

the oscilloscope beam.

The signal averaging technique allows one

to see weak signals in noise without loss of

signal bandwidth. Commercial instruments are available using the analog method described

above and also digital techniques. After the

signal has been recovered from the noise,

INPUT

R C TIME CONSTANT

RES I 5T0RS

.11111 —

SWITCH CONTROLLER —-FH

OUTPUT OSCILLOSCOPE

ONE SWITCH CLOSED

AT A TIME FIGURE k.kl. SIMPLIFIED DIAGRAM OF A SIGNAL AVERAGER

65
measurements can be made to determine the time

of arrival, etc. A local clock "tick" can be

used to trigger the start of the averaging

process.

4.8 PHASE LOCK TECHNIQUES

There are many books and articles on this

subject. As mentioned, even integrated circuits are available to phase lock signals.

Here, we only want to acquaint the reader with

the concept of the phase-locked loop and what

it will do for our time and frequency measurement problems. If further information is desired, the reader should seek out texts and

application notes on the subject.

The phase-locked loop is made by con- necting a phase comparator, integrator, and

voltage-controlled oscillator (VCO). Any one

of these blocks can be implemented in many

ways. Functionally, a crystal oscillator that

reacts to an input voltage by changing its

frequency is the same as a dc motor that

speeds up or slows down as its input voltage

i s changed.

The net effect of a phase-lock loop can

be one of several things. It can change the

input frequency. For example, if the input is

1 MHz, we could put a 100 kHz oscillator in for a VCO and a multiplier in the circuit and

have a divider. Or, we could use a 10 MHz VCO

and a divider in the circuit and have a multiplier. We could also change the power level. That is, a very small input signal could be made to control a powerful output stage. This

would give us a power amplifier.

Typical applications for a phase-locked

loop involve the idea of filtering the input

signal. Usually a low-pass filter does this

for us. The VCO is connected after the filter. This prevents high frequency signals

(where the noise usually is) from affecting

the VCO. So, if the input is a 1 MHz signal

with noise, we can remove the noise and keep

the 1 MHz signal. It's not really that easy

or simple, but just an example of what is happeni ng.

Since very narrow bandwidths can be

achieved, we could separate one signal from

another even though they are very close

together. If you read articles written in the

field of time and frequency technology, men- tion is often made of using phase-locked loops

to remove noise from signals after they have

arrived over telephone lines, etc.

An important application of phase-lock

techniques is in the VLF tracking receiver.

The application of the loop is to measure the

phase of a local oscillator with respect to a received signal at VLF frequencies. Here the

phase-locked loop filters provide an analog

phase output and even synthesizes the necessary frequencies so the receiver can be tuned

throughout the VLF band.

The block diagram of a simple phasetracking receiver is shown in figure 4.48. Basically, this receiver adjusts the phase of

a local frequency standard to agree with that

of a received radio signal. The phase adjust- ment is performed by a phase snifter, which

also accepts the output signal of the local

standard. This shifted signal becomes the

input signal for an internal frequency synthesizer whose frequency is set equal to the

received frequency. Comparison between these

two signals is then made by a phase detector

which in turn controls the action of the phase

shifter in such a way as to maintain the phase

difference, e, at a nearly zero value.

A time constant in the servo loop limits

and controls the rate of phase change that can

be accomplished by the shifter. This time constant permits the receiver to have a very narrow bandwidth. Commercial receivers fea- ture adjustable time constants which allow the

effective bandwidth to be reduced to as low as

RF

VLF OR LF

F RF PHASEF RF FREQUENCYF

s ELECTRONIC

OR MECHANICAL

PHASE

SHIFTER

LOCAL

FREQUENCY

STANDARD RECEIVER >

*RF

COMPARATOR < SYNTHESIZER

E

=

<t> RF

-< e PHASE SHIFTER CONTROL

I r

FIGURE k.k8. SIMPLE PHASE-TRACKING RECEIVER.

66
0.001 Hz. With bandwidths this narrow, the

receiver can follow a signal that is much

weaker than the neighboring noise levels.

Another way to view the action of a phase-tracking receiver is to consider it a signal-averaging process. The long time constants (100 seconds or more) characteristic

of these receivers may be thought of as aver- aging times. The phaselocked output frequency

of the receiver is a faithful representation

of the received frequency and phase. Only the

noise has been removed.

4.9 SUMMARY

There are many types of equipment available for use in making time and frequency

calibrations. Perhaps the most important are

the electronic counter and the oscilloscope.

There are also many techniques that can be

used, depending on the application. We again

encourage users to discuss their needs with

manufacturers of time and frequency equipment

and to read the available application notes and other literature.

Now that we have discussed the use of

time and frequency in the lab, we will turn our attention to some of the services that

provide accurate signals for calibration.

These services range from high and low frequency radio broadcasts to newer services that

use network television as a transfer standard

to satel 1 ites.

Obviously, you will not want to use all of the services. The magnitude of your mea- surement problems, your accuracy requirements,

the manpower available, and your budget are

all contributing factors in choosing a service

that best meets your needs. Hopefully, the

following chapters will provide enough information to allow you to make a wise decision.

67

CHAPTER 5. THE USE OF HIGH-FREQUENCY RADIO BROADCASTS

FOR TIME AND FREQUENCY CALIBRATIONS

High frequency (HF) shortwave radio

broadcasts are one of the most popular sources

of time and frequency information. HF signals

from stations such as WWV (Ft. Collins, Colorado), WWVH (Kauai, Hawaii), and CHU (Ottawa,

Canada) are readily available and provide

essentially worldwide coverage. In addition,

the signals can be picked up with relatively

inexpensive receiving equipment.

5.1 BROADCAST FORMATS

As previously mentioned, there are many

HF radio stations worldwide that can be used

for time and frequency measurements. Some of

them are described in this chapter.

5.1.1 WWV/WWVH

Although HF broadcasts are extremely

popular, there are drawbacks to using them. To date, no practical method has been found to automate the use of HF signals. Also, the

equipment requires human operators.

HF signals rely primarily on reflection

from the sky (ionosphere) to arrive at a distant point. Changes in the density and

height of the ionosphere (or reflecting

region) change the distance a time signal must

travel. This produces a characteristic of

high frequency or "short" waves called fading.

The signals that leave the transmitter take a number of paths to reach the receiver. As these signals arrive at a distant point, they

have shifted in phase by different amounts. Sometimes they recombine to produce a stronger

signal, but at other times, they almost cancel

each other and no signal is available.

It is this recombining-canceling pattern

that produces the characteristic fading at HF. This also changes the apparent time of a received pulse or "tick." The arrival time of

a timing pulse may vary from day to day, even

for measurements made at the same time of day.

Accuracies approaching 1 millisecond can typically be achieved by taking measurements

each day and averaging over several days.

This means that a user must have a clock and

the necessary equipment to keep good time and measure small time differences. It also means

he must be dedicated and willing to expend the

effort to do the job.

Good signal reception is essential for accurate frequency or time measurement using

HF broadcasts. Signal strength and geographical location will determine the necessary

receiver and antenna requirements. For example, a directional antenna may be necessary so that it can be oriented to favor the trans- mission mode which consistently provides the

strongest received signal. Accuracy of mea- surements using HF signals is typically 10 milliseconds for time and 1 x 10- 6 for

frequency.

Standard time and frequency stations WWV

and WWVH broadcast on carrier frequencies of

2.5, 5, 10, and 15 MHz. WWV also broadcasts

on 20 MHz. The stations get all of their signals from a cesium beam frequency source. They each use three "atomic clocks" to provide

the time of day, audio tones, and carrier frequencies. The rates or frequencies of the

cesium oscillators at the stations are con- trolled to be within one part in 10 12 (1000

billion) of the NBS frequency standard located

in Boulder, Colorado. Time at the stations is held to within a few microseconds of the NBS atomic time scale, UTC(NBS).

These cesium standards drive time code

generators through various dividers, multipliers, and distribution amplifiers. The time

code generators generate audio tones and time

ticks. Frequency multipliers provide the

radio carrier frequencies which are then

amplitude modulated.

The seconds pulses or "ticks" transmitted

by WWV and WWVH are obtained from the same frequency source that controls the carrier

frequencies. They are produced by a double

sideband, 100 percent modulated, signal on each RF carrier. The first pulse of every

hour is an 800-mi 1 1 i second pulse of 1500 Hz. The first pulse of every minute is an 800-mi llisecond pulse of 1000 Hz at WWV and

1200 Hz at WWVH. The remaining seconds pulses

are brief audio bursts (5-mi 1 1 isecond pulses

of 1000 Hz at WWV and 1200 Hz at WWVH) that

TONE 0.010

SEC 0.005

SEC 0.025

SEC * -TONE

1000 Hz

1200 Hf WWVH

FIGURE 5.1. FORMAT OF WWV AND WWVH

SECONDS PULSES.

6Q
sound like the ticking of a clock. All pulses occur at the beginning of each second. The

29th and 59th seconds pulses are omitted.

Each tick is preceded by 10 milliseconds

of silence and followed by 25 milliseconds of

silence to avoid interference from other tones

and to make it easier to hear the tick. The

total 40-mi 1 1 i second protected zone around

each seconds pulse is illustrated in figure

5.1. This means that the voice announcements

are also interrupted for 40 milliseconds each

second. This causes only a small audio distortion. The ticks have priority and must be

received clearly.

Transmitting equipment at each station

consists of high-power linear amplifiers which

are connected to separate antennas. The several WWVH antennas are directional arrays except

for the 2.5 MHz antenna. All the antennas at WWV are omnidirectional. The directional

property of the WWVH antennas is intended to minimize "interference" between the two sta- tions and to improve coverage in the Western

Paci f ic.

The complete broadcast format for WWV and

WWVH, showing exactly what is broadcast during

each minute of the hour, is shown on page 215

of Chapter 12. See Table 5.1 for other

detai Is.

5.1.2 CHU

Canada has many frequency and time ser- vices that are quite similar to those of the

U. S. The Canadian HF broadcast station near Ottawa has the call letters CHU. Its signals

can be heard over much of the U. S. and are a valuable alternative to the WWV signals. It has the same propagation characteristics and

the same receiving techniques are used.

The CHU signals differ from those of the

U.S. stations in two ways. They are not in

the standard frequency bands (see below) and

they use a different format. The format is principally voice and ticks. At CHU, dual cesium frequency sources are used to generate

the carriers and the seconds pulses. Two

systems are used to complement each other in cases of maintenance or failure. The output

from the cesium oscillator is fed to a solid

state frequency synthesizer which produces the

3330, 7335, and 14670 kHz carrier signals for

the transmitters.

The three excitation frequencies are fed

into a transmitter room and terminated in a panel where each frequency is coupled into two transmitters (main and standby) operating on that frequency. A 100 kHz signal is fed into one of two digital clocks where it is divided

into seconds pulses of 1000 Hz tone. The

clock gates out the 51st to 59th second pulses

of each minute to permit the voice announce- ments to be inserted, and also gates out the

29th pulse to identify the half-minute point.

The same 100 kHz signal is fed into a "talking clock" where it is divided down to 50 Hz and amplified to a power level that will

operate a synchronous motor. The motor drives

a magnetic drum on which the voice announcements are recorded. Two talking clocks are

used. One announces the station identification and time in English, and repeats the time

in French. The other announces the station

identification and time in French, and repeats

the time in English. The format is shown in figure 5. 2.

In 1975, CHU converted its operation to single sideband. It now broadcasts upper sideband with full carrier. This is called the

3A3H mode. Users can still get the carrier as

a frequency standard and an ordinary AM radio

will allow reception of the audio signals.

A time code is now being tested on CHU. It appears in the 31st to the 39th seconds

pulses of each minute. The modulation is the

CHU DATA TRANSMISSION SEQUENCE

58th MIN.

2$ 27 21 21 30 31 —\ /

so 1 TT-4+KVOICE RECORDING

'HOURS MINS."

48 43 50

I I I

1

i

! >W~4tKVOICE RECORDING

"HOURS — EXACTLY'

n i i i I s I i I i ii ii i: ii

t 1st to 10th PULSES (INCL.)

I OMITTED

1OO0 CYCLES (1 SEC.) OF 1000 Hz.

NOTE: VOICE RECORDINGS ALTERNATE IN FRENCH AND ENGLISH: "CHU CANADA, EASTERN STANDARD TIME HOURS MINUTES HEURES MINUTES" (EVEN MINUTES) "CHU CANADA, HEURE NORMALS DE L'EST HEURES MINUTES HOURS MINUTES" (ODD MINUTES) "CHU CANADA, EASTERN STANDARD TIME HOURS EXACTLY" HEURES PRECISES" (ON THE HOURI

FIGURE 5.2. CHU BROADCAST FORMAT.

70
TONE OF

KHz

VOICE ANNOUNCEMENT

20 SEC) MONDAY TO SATURDAY

111 HOURS PER DAY]

0

30 SECOND PULSES

ONLY

100 MILLISECONDS

50

45

PULSE

5 MILLISECONDS)

GEOALERT

MESSAGE

SECOND SATURDAY AND SUNDAY

(4 HOURS PER DAY)

HOURLY SCHEDULE DAILY SCHEDULE (TIME IN 1ST) MINUTES SCHEDULE

FIGURE 5.3. TRANSMISSION SCHEDULE OF ATA OVER A DAY, HOUR, AND MINUTE.

commercial 300 baud FSK (frequency shift

keying) at frequencies of 2025 and 2225 Hz. The code is a modified ASCII in which each

11 -bit character contains two BCD digits, with

one start bit and two stop bits. The first

digit is the number 6 for identification,

which should be verified in the receiving

clock, and the remaining 9 digits give the

day, hour, minute and second of UTC. The

entire message is then repeated in order that

the receiving clock can check for identical

messages before updating. The code ends, and

update occurs at 0.500s. This 0.500s must be

added in, along with the time zone hour, to give the correct time. The same code is also

available by telephone. Details can be obtained by writing to the Time and Frequency

Section, National Research Council, Ottawa,

Ontario, Canada. Other details of CHU's

service are shown in Table 5.1, page 77.

If you are outside the United States or Canada, the following HF stations are available for time and frequency measurements.

5.1.3 ATA

ATA, which broadcasts on 5, 10, and 15 MHz, is located in Greater Kail ash, Delhi,

India. It is operated by the National Physical Laboratory in New Delhi. The transmission

schedule is shown in figure 5.3.

58th SEC 59th SEC 60th SEC 1st SEC 2nd SEC

5 995

|

995 I 5 —//-

995

-//—

100 1-452-1

5 ®4 5

JONE|lKH_z)_VVWVAVvV

10 A/WW

5 10 AAMAAAM/Vjo_n eii khz >_

TONE(lKH_z)_\A/vVW\/WV

10 A/VVCWVV

100 10 AAAAAAAA/Wi°mm;

TIME IN MILLISECONDS

FIGURE 5.4. TIME FORMAT OF ATA SIGNAL.

71
ATA gives time ticks for every second,

minute, and quarter-hour (i.e., Oth, 15th,

30th, and 45th minute of every hour). The

second pulses consist of a group of 5 cycles

of 1 kHz of 5 milliseconds duration. The

minute pulses, started at the Oth second, are

a 1 kHz signal of 100 milliseconds duration.

At the beginning of every quarter-hour, a 1 kHz tone starts; it lasts for four minutes

(i.e., from the Oth to 4th, 15th to 19th, 30th

to 34th, and 45th to 49th minute).

The second and minute pulses are maintained during the tone period by interrupting

the tone for 25 and 120 milliseconds respectively and introducing second and minute

pulses. The second and minute pulses are preceded and succeeded by 10 millisecond

intervals as shown in figure 5.4.

5.1.5 IBF

IBF is operated by the Istitute Elettrotecnico Nazionale Galileo Ferraris in Torino,

Italy. It transmits on 5 MHz.

Seconds pulses consist of 5 cycles of 1 kHz modulation. Seven pulses mark the minute.

Voice announcements (given in Italian, French,

and English) are given at the beginning and

end of each emission. Time announcements are given by Morse Code every 10 minutes beginning

at 0 hours 0 minutes.

The UT1 correction is given by double

pulses. Additional details are shown on Table

5.1.

A voice announcement, comprising the

station call sign and Indian Standard Time is made about 20 seconds prior to the beginning

of each quarter-hour. Other details of ATA

are shown i n Table 5.1.

5.1.4 I AM

The Istituto Superiore delle Poste e delle Telecomunicazioni (ISPT) in Rome, Italy,

operates HF radio station IAM on 5 MHz. IAM broadcasts time signals and standard frequencies.

The transmission is effected for 6 days a week (week-days only) in accordance with the

diagram in figure 5.5.

The second pulse is made up of 5 cycles

of a 1000 Hz standard frequency. Minute

pulses consist of 20 cycles.

The UT1 correction is given by double

pulses. Additional details are shown on Table 5.1.

5.1.6 JJY

The Radio Research Laboratories in Tokyo,

Japan, operate radio station JJY. Like WWV

and WWVH, JJY broadcasts Coordinated Universal

Time on 2.5, 5, 10, and 15 MHz. The types of

information broadcast are also similar to WWV/WWVH.

JJY transmissions are continuous except

for interruptions between minutes 25 and 34. The call letters are announced six times an hour at 9, 19, 29, 39, 49, and 59 minutes

after each hour. The 1 Hz modulation pulses

are of 5 milliseconds duration (8 cycles of

1600 Hz). The first pulse in each minute is

preceded by a 600-Hz tone 655 milliseconds

long. The format is modulated by 1000 Hz for

intervals of 0 to 5, 10 to 15, 20 to 25, 30 to

35, 40 to 45, and 50 to 55 minutes, excluding

the periods of 40 milliseconds before and

after each second pulse. The hourly modulation schedule, waveform of second pulses, and

identification of minute signal by preceding

marker are shown in figure 5.6.

TS I TS

0730 0735

1030 1035

I A3 ANNOUNCEMENT

0745 0750

1045 1050

I TS I TS

0800

1100

0805

1105

0815

1115

0820

1120

0830

1130

TIME SIGNAL IN A3

TIME IS GIVEN IN SLOW-SPEED TELEGRAPHY AT 0735, 0750, 0805, 0820, 1035, 1050, 1105, AND 1120 HRS UTC.

DURING SUMMER TIME, THE EMISSIONS ARE ADVANCED BY 1 HOUR.

FIGURE 5.5. TRANSMISSION SCHEDULE OF IAM.

72
(1) Hourly modulation schedule

19 29 39 49 59 mm.

10 15 20 25 30 35 40 45 50 55 0 mm.

second pulses and 1000Hz tone

no emission D

ij] second pulses only

second pulses and

identifying signals

(2) Wave form of second pulses

1000Hz tone

or non modul. non modul. 1600Hz tone , 1000Hz tone non modul. . . or non modul.

.- 40 ms - 5 ms 40 ms —

exact time (second) FIGURE 5.6. JJY EMISSION

SCHEDULE.

(3) Identification of minute signal by preceding marker

1000Hz tone

or non modul. 59s 600Hz tone 00s 1000Hz tone

or non modul.

5 ms

/ ^40-,^40-r- 655 ms —_300ms-H ,-40-

5 ms

(4) Identifying signals

a) Call sign twice by the Morse code

b) Time in J. S. T. once by the Morse code

c) Call sign twice in voice

d) Time in J. S. T. once in voice

e) Radio propagation warning by the Morse code

(N: Normal, U: Unstable, W: Disturbed)

Example: DUT1 = +0.5s

57 58 59 0 1 2 3 4

5 ms 655 ms 45 ms

(1600Hz) (600Hz) (1600Hz) Second Preceding Lengthened second FIGURE 5.7. METHOD OF

pulse marker for minute pulses INDICATING

DUT1 ON JJY

Example: DUT1 = -0.2s

6 7 8 9 10 11 12 13 14 15 16 17 18s

73
DUT1 is given once each minute to the nearest 0.1 second. The magnitude and sign

are indicated by the number and position of

the lengthened second pulses of 45 ms duration

of 1600 Hz. Identifying signals and the

method of indicating DUT1 are shown in figures

5.7. For additional details, see Table 5.1.

5.1.7 OMA

Station OMA in Li bl i ce , Czechoslovakia,

operates on 2.5 MHz. The station transmits

seconds, minutes, and five-minutes marks

formed by interrupting the 1-kHz subcarrier

for 5 milliseconds, 100 milliseconds, and 500

milliseconds, respectively. The modulation

schedule, as follows, is repeated each hour:

Minute 00-01, 15-16, 30-31, 45-46:

10 times OMA in Morse Code, 800 Hz

tone.

Minute 01-05, 16-20, 31-35, 46-50:

1 kHz standard frequency transmitted

from 6 hours to 18 hours UT only;

replaced by seconds pulses the rest

of the day.

Minute 05-15, 25-30, 35-40, 50-60:

Seconds pulses 5 milliseconds long,

each 60th pulse is 100 milliseconds,

each 300th pulse is 500 milliseconds. From 18 hours to 6 hours UT,

the 5 millisecond pulses replace the

1 kHz tone.

Minute 20-25:

Carrier frequency transmission without modulation.

Minute 40-45:

Transmission is interrupted.

At the end of the 15th, 30th, 45th, and

60th minute of the hour, a short time signal

is transmitted consisting of six second pulses

100 milliseconds long. See Table 5.1 for

further detai Is.

5.1.8 VNG

The Australian Telecommunications Commission provides HF standard frequency and time

signals broadcasts from radio station VNG at

Lyndhurst, Victoria. This service transmits

precise time signals and radio frequencies on

a schedule designed to provide an Australiawide coverage. The transmitting schedule

is shown below.

SCHEDULE OF FREQUENCIES

TIMES OF EMISSION FREQUENCY

UTC AEST AEDT MHz

0945- 2130 1945- 0730 2045-•0830 4 5 2245- 2230 0845- 0830 0945-•0930 7 5 2145- 0930 0745- 1930 0845-•2030 12 0

UTC: Universal Coordinated Time

AEST: Australian Eastern Standard Time

AEDT: Australian Eastern Daylight Time

A. Time Signals

Seconds markers are transmitted by doublesideband amplitude modulation of the carrier

and consist of various length bursts of 1000

Hz tone. The start of a tone burst, which

commences at a zero-crossing of the sine wave (increasing carrier power), marks the start of

a seconds i nterval

B. Time Code

Seconds markers are normally 50 milliseconds of 1000 Hz. Seconds markers 55 to 58 are

5 milliseconds of 1000 Hz. Seconds marker 59

is omitted.

Minute marker (seconds marker 60) is 500

milliseconds of 1000 Hz. During the 5th,

10th, 15th, etc. minutes, seconds markers 50 to 58 are 5 milliseconds of 1000 Hz.

During the 15th, 30th, 45th, and 60th

minutes, a station identification announcement

is given without interruption to the time

signals. During the first 15 seconds of each

minute, a group of emphasized seconds markers

(emphasized by 50 milliseconds of 900 Hz tone immediately following the normal seconds markers) gives the UT1 correction. The time code

format is shown in figure 5.8.

C. Announcements

The announcement gives station identification (call sign and frequencies) in English

74
SECONDS MARKERS ARE NORMALLY 50 MILLISECONDS LONG.

SECONDS MARKER 59 IS OMITTED. MINUTE MARKER (SECONDS MARKER 60) IS 500 MILLISECONDS LONG.

STATION IDENTIFICATION ANNOUNCEMENT (IN

ENGLISH) IS GIVEN DURING THE 15TH.30TH, 45TH AND 60TH MINUTES OF THE HOUR WITHOUT INTER- RUPTION TO THE TIME SIGNALS.

DURING THE 5TH , 10TH, 1 5TH , ETC., MINUTES,

SECONDS MARKERS 50 TO 58 ARE 5 MILLISECONDS

LONG. DURING NORMAL MINUTES, SECONDS MARKERS

55 TO 58 ARE 5 MILLISECONDS LONG.

FIGURE 5.8. VNG TIME CODE.

and is approximately 30 seconds long, finishing approximately 10 seconds before each quarter hour. The speech is "notched" to allow

the seconds markers to continue and has spectral components around 1000 Hz reduced to avoid erroneous operation of tuned-relay time

signal receivers.

D. DUT1 Code

The difference between UTC and UT1 (DUT1)

is given by the number of consecutive emphasized seconds markers, each one representing

0.1 second. DUT1 is positive when the first

emphasized marker of a group is seconds marker

1 (i.e., the marker following the minute marker). DUT1 is negative when the first emphasized marker of a group is seconds marker 9. DUT1 is zero if no seconds markers are

emphasi zed.

E. Accuracy

Carrier frequencies and 1000 Hz tone as emitted by VNG are maintained within 1 part

in 10 10 of Telecom Australia's standard of

frequency (24 hour average value). As re- ceived, the accuracy may be degraded to the

order of 1 part in 10 7 due to the ionosphere.

Time interval as emitted (i.e., elapsed

time between any two seconds markers) has the

same accuracy as the carrier frequencies

except for intervals which include step

adjustments.

Time of day as emitted is maintained

within 100 microseconds of Telecom Australia's

standard of Coordinated Universal Time,

UTC(ATC). As received, the signals may

exhibit jitter of the order of 1 millisecond

rms due to the ionosphere.

The Telecom Australia standard referred

to above is based on cesium beam frequency and

time standards at the Research Laboratories in Melbourne. This standard is maintained such

that its value of frequency and time interval

is within a few parts in 10 12 of the international definition of time interval. Timeof-day signals generated by the Telecom

Australia standard are maintained within about

100 microseconds of the international standard

of UTC as determined by the BIH.

F. Frequency and Time Generating Equipment

The generating equipment is at Lyndhurst

and includes precision quartz oscillators,

frequency synthesizers, time code generators,

announcing machines, and supervisory alarms.

A very low frequency signal, sent via

landline from the Telecom Australia standard

in Melbourne, controls the frequency of the

operative quartz oscillator within 1 part in 10 11

. Regular visits with a portable clock ensure that the VNG signals typically remain

within 20 microseconds of UTC(ATC). General

information is shown in Table 5.1.

5.1.9 ZU0

Radio station ZU0 is operated by the

Precise Physical Measurements Division of the

National Physical Research Laboratory (NPRL)

in Pretoria, South Africa. The signals are generated at NPRL and broadcast by the Post Office transmitting station at Olifantsfontei n.

75
EXAMPLES

I. DUTI = + 0.3 second lengthened

FLLJLJLi 0 I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 seconds

2. DUTI = -0,4 second l engthened

8 9 10 II |2 13 14 15

FIGURE 5.9. DUTI CODE ON ZUO.

A. Carrier Frequencies and

Times of Transmission

ZUO transmits on 2.5 and 5 MHz. There is also a 100 MHz transmission which will not be

discussed here. Carrier frequencies are normally derived from a cesium beam frequency

standard at NPRL.

B. Standard Time Intervals and Time Signals

The transmitters are amplitude modulated

by the time signals, the time difference

(DUTI) code, and the Morse Code announcements.

The signals consist of one pulse per second,

each pulse consisting of 5 cycles of 1000 Hz. The first pulse in every minute is lengthened

to 500 mi 1 1 i seconds.

Morse Code announcements are made during

the minute preceding every fifth minute. They

consist of the call sign ZUO (repeated three

times) and the Universal Time at the next

minute. The correct time is indicated by the

beginning of a time pulse. Double-sideband

modulation is employed. Modulation of the

first half cycle of each pulse is positive.

C. Standard Audio Frequencies

No continuous frequencies are broadcast,

but in many cases the interrupted 1000 Hz tone, which constitutes the time signal

pulses, may be used as a standard frequency.

The frequency of the Morse Code pulses is 600

Hz.

D. Accuracy

Time intervals and standard frequencies

correspond to the adopted cesium beam frequency of 9 192 631 770 Hz. The frequency from

which all other ZUO frequencies are derived

can be relied on to be within 1 part in 10 11 of the mean atomic frequency, as determined by

the BIH. Standard time intervals have the same accuracy; i.e, 1 x 10- 11

, with an additional uncertainty of ± 1 microsecond.

Time signals are Coordinated Universal

Time (UTC) and are kept within 1 millisecond

of the signals of other coordinated time sta- tions. To keep UTC in approximate agreement

with UT1 , leap seconds are inserted when directed by the BIH. Inserted seconds are known

as positive leap seconds, and omitted seconds

as negative leap seconds.

E. DUTI Code

The approximate value of the difference

UT1 - UTC, to the nearest 10th of a second, is denoted DUTI; i.e., DUTI = UT1 - UTC. DUTI is indicated during the f i f teen- second period

following each minute mark. This is done by a group of lengthened second markers, the number

in the group being the value of DUTI in tenths

of a second. If the group begins with the

first second marker after the announced minute, DUTI is positive. If the group begins

with the ninth second marker, DUTI is negative. If there are no lengthened second markers, DUTI is zero. Examples are shown in figure 5.9.

76
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5.2 RECEIVER SELECTION

This and the following sections discuss

the use of HF broadcasts for frequency and

time calibrations. For convenience, we will

refer to mainly to WWV; however, the discussions also apply to other HF broadcasts.

The reception and use of shortwaves can

be quite tricky. However, for many locations,

simple receivers and antennas work well most

of the time. Also, there are many separate

signals available from WWV and WWVH. By

choosing from among the many combinations of

frequencies and distances involved, a solution

usually can be found for any problem.

For those users who must have the very

best signal most of the time, this section

discusses several methods available to solve

reception problems. The user is also encouraged to use other references on HF propagation, antennas, and receiver selection and

operation.

Almost any shortwave receiver can be used

to receive HF signals for time and frequency

calibrations. However, for accurate time and

frequency calibrations, especially in noisy

locations, it is important to have a good

antenna-ground system and a receiver with

optimum sensitivity, selectivity, image re- jection, and frequency or phase stability. A block diagram of a typical high-performance

receiver is shown in figure 5.10.

The first requirement of a good receiver

is sensitivity. Therefore, a tuned RF amplifier is desirable because it increases sensitivity. The next requirement is selectivity.

This is the ability of a receiver to reject

nearby signals that interfere with the desired

signal. Selectivity is achieved by heterodyning (translating) the incoming signal to a much lower fixed frequency (usually 455 kHz)

called the intermediate frequency (IF). The

IF amplifiers are fixed-tuned for maximum gain

with a relatively narrow frequency pass band.

Often, either crystal or mechanical filters

are used in the IF stages to provide an even narrower pass band of frequencies to reject

interference.

The third feature of a good receiver is the ability to reject interference from an undesired signal at what is called the image

frequency. Since the IF signal is the difference between the local oscillator frequency

and the incoming signal, there are always two different incoming signals that can produce

the same IF signal. One is above and the

other below the oscillator frequency.

For example, a receiver with an intermediate frequency of 455 kHz tuned to WWV at

10.0 MHz would have an oscillator frequency

of 10.455 MHz (10.455 - 10.0 = 455). However,

if another signal with a frequency of approximately 10.910 MHz is present, another "difference" signal of 455 kHz is produced (10.910

- 10.455 = 455). Thus, if two signals are separated by twice the intermediate frequency

and the oscillator frequency falls between the

two signals, an unwanted image signal results.

Table 5.2 lists various dial settings and

corresponding oscillator frequencies that can receive WWV signals as image frequencies if the receiver IF is 455 kHz. V

RF

AMPL

1ST

MIXER

XTAL

0SC

6

PRESELECTOR

ON

OFF

o

-o-<—o- AGC

I F AMPL

2ND

MIXER

IF AMPL DET

VF0

b

MAIN TUNING

SIGNAL STRENGTH

ill/A METER

SPEAKER

6

OUTPUT

FIGURE 5.10. BLOCK DIAGRAM OF A TYPICAL HIGH-PERFORMANCE HF RECEIVER.

81
TABLE 5.2 IMAGE FREQUENCIES FOR WWV/WWVH

OSCILLATOR IMAGE DIAL OSCILLATOR IMAGE DIAL FREQUENCY FREQUENCY FREQUENCY FREQUENCY FREQUENCY FREQUENCY

(MHz) (MHz) (MHz) (MHz) (MHz) (MHz)

2.045 1 .590 2.500 14.545 14.090 15.000

2.955 3.410 2.500 15.455 15.910 15.000

4.545 4.090 5.000 19.545 19.090 20.000

5.^55 5.910 5.000 20.455 20.910 20.000

9.545 9.090 10.000

10.455 10.910 10.000

Images occur because the IF signal is very low in frequency (i.e., 455 kHz) relative

to the high frequency of the incoming signal

(i.e., 10 MHz). Thus, the RF amplifier may

have insufficient selectivity to reject interference which is relatively close to the

desired signal. In the above example, the

unwanted signal is only 0.910 MHz away from 10 MHz and, if strong, might easily pass through

the RF stage into the mixer and produce an image IF signal

To solve this problem, a high quality

receiver usually employs double conversion or two mixer stages. It heterodynes in two steps

to make image rejection easier. The first

mixer stage converts the incoming signal to a high intermediate frequency (usually greater

than 1 MHz). Any images produced will be separated by more than 2 MHz from the desired

signal. For example, if a receiver has a first mixer IF signal of 1.5 MHz, the image

signal will be twice the IF signal or 3.0 MHz above the desired signal. Since 3.0 MHz is now relatively large compared to the incoming

signal, the RF stage pass band is usually

selective enough to reject the undesired

images. For the required additional selectivity and gain, a second mixer stage is added

to produce a lower second IF frequency, thus

solving the image problem.

A local crystal oscillator can be used in these conversion processes to provide tuning

stability. Other information regarding welldesigned receivers can be found in such

reference material as the Radio Amateur'

Handbook .

If all the above seems too confusing,

don't worry. Many small, inexpensive imported

and domestic receivers work very well. In fact, several receivers have special buttons

that you can push and the receiver automatically tunes itself to WWV. If timing is a special problem in your home or business, of course check out the receiver very carefully

before you purchase it. Image and interference

signals come and go so they may not persist

long enough to be a serious problem. The single best option to look for in a receiver is the provision for an external antenna. This

may be just a short wire on the windowsill or

an elaborate rooftop installation. Fancy

knobs and adjustments won't help much if you

can't get a good signal.

5.3 CHOICE OF ANTENNAS AND SIGNAL CHARACTERISTICS

A good antenna is important if you want

strong, noise-free signals. However, follow

the basic rule of "using what you have available." Many receivers with built-in antennas work quite satisfactorily. Often, adding a short piece of wire to the existing receiver antenna will bring in a stronger signal that

can be heard well enough to set your watch.

But for the serious listener or the

metrology lab that depends on WWV or other HF signals for calibrations, something more is needed. In the material that follows, the

rules for antenna selection and installation

are discussed with respect to choices available. Companies that sell WWV receivers often

offer antennas. These have been proven by use and the instructions included with them will

help a lot in getting a good usable signal for

frequency and time calibrations.

The signal power as received at great

distances from HF stations may be relatively

weak. As the distance is increased, the signal decreases in strength so an antenna with

maximum efficiency produces the best results.

Radio waves arrive at different vertical

angles, called the wave or radiation angle.

The wave angle of an arriving signal depends

on the distance between the transmitting and

receiving stations and also on the height of

the ionosphere. The closer the receiving

station and the higher the ionosphere, the

82
larger the wave angle. Therefore, an antenna

should be selected and oriented that favors

the wave angle (elevation) as well as the

direction (azimuth). Determining wave angles

is discussed more fully in Section 5.6.

Many HF stations transmit on a number of

frequencies. Because of changes in ionospheric conditions, which sometimes adversely

affect the signal transmissions, most receivers are not able to pick up the signals on all frequencies at all times in all locations.

Except during times of severe magnetic disturbances, however--which make radio transmissions almost impossible—users should be able

to receive the signal on at least one of the

broadcast frequencies. As a general rule,

frequencies above 10 MHz provide the best

daytime reception while the lower frequencies

are best for nighttime reception.

An all-band antenna capable of covering

the entire HF band is commercially available.

These are used for around-the-clock communication purposes where the entire HF band must

be used for maximum reliability. Such an antenna is very large and cumbersome as well

as expensive. Depending on location, an antenna and a receiver capable of receiving

two or more frequencies may be required. Any

of the different WWV frequencies transmitted

may be received depending upon distance, time

of day, month, season, and sunspot cycle.

Each frequency will be discussed with the type

of antenna most suitable for its reception.

Although WWV and WWVH are not currently broadcasting on 25 MHz, this frequency is also

di scussed.

In the following discussion, mention is made of sunspot cycles. These cycles occur

with a periodicity of about every eleven

years. Propagation characteristics at high

frequencies are greatly affected by the sun- spot cycles. One solution to the problem is to select another frequency that will work and

to ignore the ones that won't. For those

users who want to understand the propagation

effects and to counteract the coming sunspot

cycles by antenna selection and placement, we recommend further reading on this subject.

Information on sunspots is available from the

Space Environment Lab, NOAA, U. S. Department

of Commerce. Mini mums in the sunspot cycle

occurred in 1954 and 1965. The corresponding

maximums were in 1958 and 1969, and another

will probably occur in 1980.

ANTENNAS FOR THE 2.5 TO 4 MHz RANGE .

Signals at these frequencies have a very

short range during the day because of groundwave propagation limitations. Daytime use is limited to locations within one to two hundred

miles from the transmitter. They become

useful at night, however, especially during

the winter season in the higher latitudes

where longer nights prevail. Reception is then possible over distances of several

thousand mi les.

The vertical quarter-wavelength monopole antenna has a radiation pattern that favors

reception at low wave angles and is very

effective in receiving long distance skywave

signals normally arriving at angles of 20

degrees or less. This antenna is not effective for reception of short-range skywave

signals with large wave angles, but it is useful for receiving weak groundwave signals.

For nighttime reception on paths up to several thousand miles, the skywave is pre- dominant and a horizontal half-wavelength

antenna is recommended. The antenna should be

located a quarter-wavelength or higher above

bround and separated from possible interfering

reflecting obstacles. The quarter-wavelength

vertical antenna and the half-wavelength

horizontal dipole antenna are shown in figure

5.11. To get antenna dimensions for CHU or other frequencies, refer to antenna manuals

such as those published by the American Radio

Relay League.

ANTENNAS FOR THE 4 TO 5.0 MHz RANGE .

These frequencies can be received at greater distances than the 2.5 MHz frequency

throughout the day or night, especially during

minimum sunspot cycle. Reception is possible

up to 1000 miles under ideal conditions, but

under normal conditions, daytime propagation

conditions limit its useful range. Therefore,

reception is usually limited to less than 1000

miles during the day. During the night, 5 MHz

is a very useful frequency for long-range

reception except during maximum sunspot cycle.

It is excellent during early dawn and early

evening in the winter months when the signal

path is in darkness. Horizontal dipole antennas are a good choice for this band. Users should also consider the possibility of a multiband antenna.

ANTENNAS FOR THE 7 TO 10.0 MHz RANGE .

At these frequencies, reception over great distances is possible during the day or

night. They can be classified as intermediate

frequencies but again, they are dependent upon the sunspot cycle. During minimum sunspot

cycle, great distances can be covered during

the day when higher frequencies cannot be

received. During maximum sunspot cycle, these

are probably the best frequencies to use during the night when the lower frequencies cannot be heard. During maximum sunspot

cycle, reception is limited to short distances

83
PULLEY

GUY AS /\ NEEDED

STRA I

INSULATORS

MYLAR SASH CORD

#14-#12 AWG

COPPER CLAD STEEL WIRE

70 OHM TWIN

LEAD OR COAX

TO RECEIVER

FREQUENCY

MHz

1/2 \ HORIZONTAL

DIPOLE ANTENNA

LENGTH HEIGHT

2.5

56. 1 m 15 m TO 30 m

187' 50' TO 100'

5-0 28.07 m 9 m TO 15 m 93'- 7" 30' TO 50'

10.0

14.05 m 7- 5m TO 15m

46' -10" 25' TO 50'

FIGURE 5.11. QUARTER-WAVELENGTH ANTENNA AND HORIZONTAL HALF-WAVELENGTH ANTENNA.

84
during the day with limitations comparable to those noted for 5.0 MHz. These frequencies

also provide daytime reception at fairly close

range, 200 miles or so, and can be used when 5 MHz reception is poor. The half-wavelength

horizontal antenna should be selected for

short distances. A quarter-wavelength vertical antenna is suitable at greater distances

(see figure 5.11).

ANTENNAS FOR THE 14 TO 15 MHz RANGE.

These frequencies are most favorable for

long range, daytime reception. They are not usable for short-range reception except during

periods of sunspot maximum. However, for long range reception, they are the most favored

frequencies during both sunspot cycle conditions. Under average conditions, the maximum

wave angle is limited to 30 degrees or less

depending on the density of the ionized layer.

During maximum sunspot cycle, reception

may even be possible during the night in some

locations. During minimum sunspot cycle, they

are useful only during the daylight hours and

dawn and dusk periods. Both horizontal and

vertical antennas are suitable.

ANTENNAS FOR 20 MHz .

The 20 MHz frequency is normally the best

to use for daytime reception and will be opti- mum at either noon or a few hours past noon. Signals at this frequency arrive at very low wave angles and are useful only for long dis- tance reception. During minimum sunspot

cycles, reception is poor but improves during

the winter. During maximum sunspot cycles,

the reception is excellent at night and during

the day. The vertical dipole that favors low wave angle radiation has been used at this

frequency with favorable results. Construction details for a 20 MHz antenna are shown in figure 5. 12.

ANTENNAS FOR 25 MHz .

The 25 MHz frequency is best during daylight hours except during the summer. Reception is especially good during maximum sunspot

cycle and very poor during minimum sunspot

cycle. It is used only for long distance

reception because a low radiation angle is required for the signal to be returned back to the earth from the ionosphere. Design details

of a different vertical antenna for use at 25 MHz are also shown in figure 5.12.

1" DI AM ALUM

TUB INC

INNER CONDUCTOR SOLDERED TO LUG S FASTENED TO MACHINE SCREW

«10 GA COPPER SOLDERED TO B R A I 0 & CONNECTED TO RADIALS

WITH SPLIT-NUT BOLTS

"10 GA COPPER

RADIALS (ONLY 4 SHOWN

FOR CLARITY).

Frequency

MHz 1A X VerticalH Gnd Radial

L HeightA

15 h.7 m l5'-7"

A. 9 m l6'-5"

S.h m

18'

20 3-5 m

1 l'-9"

3-7 m

12' -V

3-9 m

13 l

25

2.8 m 9'-5"

2.95 m

9' -10"

3-3 m

1 1

'

FIGURE 5.12. MODIFIED HALF-WAVELENGTH VERTICAL ANTENNA FOR USE AT 15, 20, AND 25 MHz.

85
VERTICAL ANTENNA CONSTRUCTION .

The vertical antenna which is favorable

to low wave-angle reception is preferable to the horizontal half-wavelength antenna and can readily be constructed as shown in figure

5.12. The dimension of the ground radial s and

orientation as shown should be used to yield

approximately a 50 ohm antenna impedance. For

a 70-90 ohm antenna, the half-wavelength

dipole mounted vertically will yield approximately the correct impedance. In order to prevent interaction between the feed line and

the lower half of the dipole, which disturbs

the radiation pattern, extend the feed line

horizontally outward several feet from the antenna before dropping it vertically to the

ground.

5.4 USE OF HF BROADCASTS FOR TIME CALIBRATIONS

Now that we have a receiver and an antenna, we can proceed to make time calibrations.

If your accuracy requirements are low, you can

use the voice time-of-day announcements. How- ever, if you want higher accuracy, you will

have to measure the seconds ticks or decode

the WWV/WWVH time code.

5.4.1 TIME-OF-DAY ANNOUNCEMENTS

Time of day is available from many sources

in the United States and Canada. Radio and

television stations mention the time frequently

and, in fact, use the time of day to schedule

their own operations. The telephone companies

offer time-of-day services in most locations.

But where does the time come from?

Most time-of-day services in the U. S. start at the National Bureau of Standards. An NBS telephone service is available by calling

(303) 499-7111. In addition, WWV and WWVH broadcast voice time-of-day announcements once

each minute. This is also the case for CHU where the time announcements are given alternately in French and English. Using the WWV

voice announcement and the tone following the

words, "At the tone, XXXX hours XXXX minutes

Coordinated Universal Time," a person can check a wall clock or wristwatch to within a fraction of a second. The UTC time that is announced can be converted to local time by

using the map in Chapter 2.

5.4.2 USING THE SECONDS TICKS

For accurate time recovery, we cannot

just listen. We must rely on electronic equipment. By using an electronic oscilloscope to

see the ticks, the user can set clocks with

much greater accuracy than is possible by only

listening to the ticks. An oscilloscope is a necessity when dealing with noisy signals that

fade.

The following describes ways to get time

of day with resolutions better than 1 millisecond (1000 microseconds) by using an oscilloscope. Under very favorable conditions, it

is possible to establish time synchronization

to about 100 microseconds. In each case, path

and equipment time delay corrections are necessary for accurate results.

In order to achieve the best results in any of these measurement methods, the following guidelines are recommended:

1. Make your measurements at the same time every day.

2. Avoid twilight hours when the ionosphere is the least stable.

3. Use the highest carrier frequency

which provides consistently good

reception.

4. Look at the received signals on the

oscilloscope for a few minutes to judge the stability of propagation

conditions and select that part of

the timing waveform that is most

consistent.

Since receiver and propagation time delays are

so important, they will be discussed before we

go into the details of performing timing

cal ibrations.

A. Receiver Time Delay Measurements

For accuracy in time-setting, the receiver delay must be known. Typical delays in receiver circuits are usually less than a millisecond. However, they should be known to the user who wants the very best timing accuracy. The actual delay in the receiver will vary with tuning and knob settings.

To measure the receiver time delay, the

following equipment is required: An oscilloscope with accurately calibrated, externallytriggered time base; an HF signal generator;

and an audio signal generator. The equipment

is connected as shown in figure 5.13.

The main tuning dial of the receiver

should be set at the exact position for re- ceiving WWV/WWVH signals. This is because the

receiver delay time varies with slightly different receiver dial positions. Therefore,

the receiver tuning should be set and marked

86
RECE I VER

IN OUT

OSCILLOSCOPE

VERT

AF GEN HF SIGNAL

GENERATOR

MOD RF

1 kHz

FIGURE 5.13. EQUIPMENT SETUP FOR RECEIVER TIME DELAY MEASUREMENTS.

where the maximum signal is received. The

frequency of the HF signal generator is then

adjusted for peak receiver output.

The audio signal generator is set to a 1 kHz output frequency. A high accuracy 1 kHz

signal is not required. The HF generator is externally modulated by the 1 kHz signal. The

oscilloscope sweep rate is set to 100 microseconds/division with positive external triggering from the 1 kHz signal. The vertical

amplifier gain is set high for a large vertical deflection. The vertical position control

is adjusted for zero baseline with no input

signal

Initially, the 1 kHz signal generator is connected to the vertical input of the oscilloscope. The trigger level control is adjusted so that the trace crosses over or touches

the horizontal center line at the left. The

horizontal position control can be adjusted so that the signal crosses over the first division on the left as shown in figure 5.14. The crossover point of the undelayed signal will serve as the zero delay reference point.

/

I

H

—1— \

—1—

/ 1 DELAY

i »>

DELAYED ZERO CROSSOVER

ZERO CROSSOVER

TIME

UNDELAYED

I

FIGURE 5.14. OSCILLOGRAM OF DELAYED AND UNDELAYED 1 kHz SIGNAL.

Without touching any of the oscilloscope

controls, the 1 kHz signal is disconnected

from the vertical input and replaced with the

delayed 1 kHz signal from the receiver output.

Since a receiver delay is almost always less

than 1 millisecond (1 cycle of a 1 kHz signal), there is little chance of ambiguity on which cycle to measure. The delay is equal to the sweep time from the reference undelayed crossover to the first delayed crossover

poi nt.

If the delayed signal is found to be of

opposite phase to the reference 1 kHz signal,

the receiver has an inverted output signal.

However, the receiver delay time remains

unchanged and the only difference will be that

the output seconds pulse will be inverted with

a negative leading edge. For receiver delays

of less than 500 microseconds, the sweep rate can be set at 50 microseconds/division.

This technique is a way of producing a local signal that approximates the timing signal. It uses the same frequency on the dial

and uses the same tone frequency, h two- channel scope is very useful since you can then display the tone before it enters the RF signal generator and after it comes out of the

receiver. A study of the block diagram (fig.

5.13) will no doubt suggest other variations

on this technique to the user.

B. Time Delay Over the Radio Path

In order to make a calibration, a user must know the path delay from the transmitter

to his receiver antenna. This is often difficult due to the fact that several "modes" of

propagation are possible at high frequencies.

A discussion of propagation delay calculations

can be found in section 5.6.

For time setting of highest accuracy, a user would have to actually measure the path

delay using a portable clock. This is an in- volved and costly operation that few users would deem justified for their needs. How- ever, once the path delay is determined, it

87
need not change appreciably if certain precautions are observed. For timing accuracies

that approach 100 microseconds, the location

and routing of antennas and lead-ins are important as are receiver settings. The

general rule is to "not change anything". If changes must be made, careful records of readings before and after will help in re-establishing the system with a correct

delay value.

In fact, any user of time signals at high

levels of accuracy must take extreme care in

how the equipment is adjusted, maintained, and

operated. Typical users estimate their path

delays and obtain results with accuracies

approaching one millisecond. The above para- graphs are intended for those users who want

to obtain the very best accuracy. They are

not typical for the large percentage of users.

It also goes without saying that a wealth of

information can be obtained by reading the

literature supplied with the timing equipment.

C. Using an Adjustable Clock to Trigger the Oscilloscope

This method, sometimes called the Direct

Trigger Method, is the simplest and requires

an oscilloscope with an external sweep trigger

and accurately calibrated time base, and a receiver with electronic audio output. The

clock must be adjustable so its 1 pps output

can be advanced or retarded. Equipment con- nection is shown in figure 5.15.

A local clock pulse at 1 pps second rate

is used to trigger the oscilloscope sweep. At some time interval later during the sweep, the

seconds pulse appears on the display as shown.

The time interval from the start of the sweep

to the point where the tick appears is the

total time difference between the local clock

and the transmitting station. By subtracting

the propagation and receiver time delays from

the measured value, the local clock time error

can be determined. The equation to determine

time error at a receiving location is: V

Time Error = t - t = TD - (TD + TD )

r t

v p r

-

where: t = time at receiving station

t. = time at transmitting station

Z (WWV, WWVH)

TD = total time difference (measured by oscilloscope display)

TD = propagation path time delay

" (computed)

TD = receiver time delay (measured)

NOTE: The units (usually milliseconds) should

be the same.

The receiver is tuned to the station and

the oscilloscope sweep rate set at 0.1 s/

division. Listening to the broadcast will

help judge the quality of reception and fading. The tick will typically appear as shown

in figure 5.16. CHU signals can be used as well (see fig. 5.17). If the tick is one division or more from the left side of the

scope display, the time of the local clock is corrected until the tick falls within the

first division from the left side. If the

local time tick is late, the received tick

will be heard before the sweep starts. If this is the case, the local clock should be

adjusted until the tick appears. After the

local seconds pulse has been properly adjusted

and appears within the first division (0.1

second in time), the sweep rate is increased

to say, 5 ms/division. Using this greater

resolution, the local clock is adjusted until

the leading edge of the received pulse starts

at a time equal to the propagation delay time

plus the receiver delay time after the trigger

as shown in figure 5.18.

The sweep rate should be increased to the

highest rate possible without allowing the

total sweep time to become faster than the

combined propagation and receiver delay time

HF RECEIVER

SECONDS

PULSE ^

OSCILLOSCOPE

VERTICAL TRIGGER

1 pps LOCAL

FREQUENCY

SOURCE

OR CLOCK

OR

TICK

-«

FIGURE 5.15. BLOCK DIAGRAM OF EQUIPMENT CONNECTION FOR DIRECT

TRIGGER METHOD OF TIME SYNCHRONIZATION.

88
FIGURE 5.16. WWV TICK AT A SWEEP RATE OF 0.2 SECOND/DIVISION.

FIGURE 5.17. CHU TIME TICK AT A SWEEP RATE

OF 0.2 SECOND/DIVISION.

TICK

*

I I

FIGURE 5.18. WWV TICK AT A SWEEP RATE OF

0.1 SECOND/DIVISION.

TICK

WWV TIME TICK: TRIGGER TIME DELAY: 19,000 MICROSECONDS

TOTAL TIME DELAY: 20,740 MICROSECONDS

FIGURE 5.19. SECOND ZERO CROSSOVERS OF SECONDS PULSE AT A SWEEP RATE

OF 1 MILLISECOND/DIVISION.

WWVH TICK RECEIVED TWO MILES AWAY: RECEIVER DELAY: 320 MICROSECONDS

TOTAL TIME DELAY: 1.160 MILLISECONDS

0.833 ms SECOND ZERO CROSSOVER

89
less the 5 mill i seconds to allow for the

length of the received seconds pulse.

With a sweep rate of 1 ms/division, for

example, greater resolution can be realized by

measuring the second zero crossover point of

the 5 ms received tick. Although the leading

edge of the seconds pulse as broadcast from

these stations is "on time", coincident with

the NBS or CHU standard, it is difficult to measure because of the slow rise time at the

beginning of the burst and distortion due to propagation. For this reason, the second zero crossover should be used. The second zero crossover of the WW or CHU pulse is delayed

exactly 1000 microseconds and the WWVH crossover is delayed 833 microseconds as shown in figure 5.19. This is called the cycle

correction.

At a sweep rate of 1 ms/division, any changes in arrival time (jitter) are readily

apparent. After watching the pulses for a period of a minute or two, select a cycle that

is undistorted and relatively larger in amplitude. In determining the time at a receiving

location, include the delay of the chosen zero crossover point, then add the cycle correction

to the propagation and equipment delay using

the following relationship:

Time Error = t - t. r t = TD "(TD + TD + cycle correction)

Where cycle correction time

= 1000 microseconds per cycle (WWV or CHU)

= 833 microseconds per cycle (WWVH)

As an example, assume an operator at a distant receiver location is interested in comparing his time to that of WWVH. The pro- pagation and receiver time delays were mea- sured as 11.7 milliseconds and 300 microseconds respectively. Since the total delay is 12.0 milliseconds (11.7 ms + 0.3 ms), the

oscilloscope sweep rate was set at 2 ms/

division for a total sweep time of 20 ms

slightly greater than the propagation delay + receiver delay + 5 ms total. The second zero crossover of the tick was observed and mea- sured 12.5 ms after the sweep was triggered by

the WWVH clock.

From these data, the time at the receiver

site was calculated as -0.333 ms with respect

to the WWVH time as broadcast, or:

Time Error = t - t. r t = 12.5 - (11.7 + 0.3 + 0.833)

= -0.333 ms, or -333 microseconds,

Where: TD = 12.5 ms

TD = 11.7 ms

P TD = 0.3 ms

r

Note that if a receiving station is located at a distance greater than 3,000 km (1863 miles) from the transmitter, the pro- pagation time delay will exceed 10 ms. (The

radio path delay works out to be about 5 microseconds per mile. At 1863 miles, the

delay would be at least 9.315 ms. It is usually greater than this due to the fact the

HF radio signals bounce off the ionosphere.)

This forces use of a scope sweep time of 2 ms/

division and lowers the measurement resolution. The next section describes a method of measurement to overcome this difficulty.

D. Delayed Triggering: An Alternate Method

that Doesn't Change the Clock Output

To improve the resolution of measurement,

the oscilloscope sweep must be operated as

| fast as possible. The user does have an option: He can generate a trigger pulse inj dependent of his clock. He then positions the j pulse for maximum sweep speed and makes his measurement. But then he must measure the

difference between his clock and the trigger

pulse. Note: This can be accomplished by

using an oscilloscope with a delayed sweep circuit built in or with an outboard trigger

generator. The latter method is discussed

J here, but the delayed sweep scope could be used as discussed in Chapter 4. Reference to

f the instrument manual will aid in using that

j technique.

On a typical digital delay generator, a i delay dial indicates the delay between the f input local clock tick and the output trigger

pulse. If the user already has a variable i rate divider to produce delayed pulses, a time j interval counter can be used instead of the

delay generator. In either case, the amount

of trigger delay must be accounted for in measuring the total time delay (TD) of the

received tick with respect to the local master

clock.

Measurements should be made at the same time every day (within ten minutes) for con- sistent results. A time of day should be selected when it is approximately noon midway j between the transmitting station and the re- ceiver's location. For night measurements, a time should be chosen when the midpoint is near midnight. Measurements should not be ' made near twilight.

The equipment can be connected as shown

in figure 5.20. A commercially available

90
TIME TICK

o EXT

TRIGGER

OSCILLOSCOPE

V ERTI CAL

INPUT !

TICK

DELAYED 1 pps

100 kHz

CONTROLLE D DELAY GEN

OR DIVIDER

& CLOCK

TIME

INTERVAL

COUNTER*

FREQUENCY

SOURCE

OSC & CLOCK

1 pps

*S T E TEXT

FIGURE 5.20. EQUIPMENT SETUP FOR DELAYED TRIGGER METHOD OF TIME SYNCHRONIZATION.

frequency divider and clock can be used in place of the controlled delay generator. A time interval counter is then used to measure

the output of the delayed clock to the master

clock. The output of the delayed clock is used to trigger the oscilloscope. The initial

procedures described in the previous method

also apply to this method and, therefore,

should be referred to in setting time with

WWV/WWVH.

With the oscilloscope sweep adjusted to 1 ms/division, the trigger pulse should be

delayed by an amount equal to the propagation

delay in milliseconds. Initially, any fractional milliseconds in the delay can be

neglected. The sweep should be adjusted so that it begins exactly at the left end of the

horizontal graticule and is vertically

centered.

The second zero crossover point of the

tick (figures 5.21, 5.22) should be observed

and carefully measured. With the sweep at

1 ms/division, the delay of the second zero crossover on the oscilloscope is measured to iiinimm

Mil mmimmmm\

mm wmimmmm

SECOND ZERO CROSSOVER 540 ys SECOND ZERO CROSSOVER

TRIGGER DELAY TIME: TOTAL TIME DELAY: 19,000 MICROSECONDS

20,740 MICROSECONDS

TRIGGER DELAY TIME: 20,200 MICROSECONDS

TOTAL TIME DELAY: 20,740 MICROSECONDS

FIGURE 5.21. SECOND ZERO CROSSOVER OF WWV TICK AT A SWEEP RATE OF

1 MILLISECOND/DIVISION.

FIGURE 5.22. SECOND ZERO CROSSOVER OF WWV TICK AT A SWEEP RATE OF 100 MICROSECONDS/DIVISION (4 SWEEPS)

91
the nearest one-tenth of a millisecond and

added to the trigger delay, resulting in an approximate total time delay. If the local clock 1 pps time is exactly coincident with

the UTC(NBS) seconds pulse, the total measured

time delay will be approximately equal to the

sum of the propagation delay time, the receiver delay time (typically 200 - 500 microseconds), and the cycle correction (1000 microseconds for WWV or CHU and 833 microseconds

for WWVH).

To further increase the resolution of delay measurement, the oscilloscope sweep rate

can be increased to 0.1 ms/division (100

microseconds/division) and the trigger pulse

from the generator adjusted to be approximately 500 microseconds less than the total delay

time previously measured. At these settings,

the second zero crossover of the tick will be somewhere near the midscale of the oscilloscope face.

The vertical centering of the sweep should be rechecked and centered if necessary.

The tick is measured to the nearest 10 microseconds (figure 5.21). The result should be within ± 100 microseconds of the result ob- tained at the 1 ms/division sweep rate. If the result of this measurement falls outside

this tolerance, then the procedure should be repeated again b" measuring the total time

delay at a sweep rate of 1 ms/ division. To obtain the time, the equations described

earlier should be used.

E. Using Oscilloscope Photography for

Greater Measurement Accuracy

By film recording five or more overlapping exposures of the WWV/WWVH tick, an average of the tick arrival time can be estimated

tI SECOND ZERO CROSSOVER

FIGURE 5.23. WWV TICK AT A SWEEP RATE OF

1 MILLISECOND/DIVISION

(5 OVERLAPPING EXPOSURES).

with more accuracy. The exposures are made

when consistently strong and undistorted ticks

appear on the oscilloscope. To determine the

time, the average of the second zero crossover

point of the tick is measured using the same procedure explained above.

In making measurements using this technique, an oscilloscope camera using selfdeveloping film is mandatory. The camera shutter is placed in the time exposure position so that it can be opened and closed manually. The lens opening of the camera, the

oscilloscope trace intensity, and the scale

illumination must be determined by experiment. Refer to your camera manual.

One of the previously described proce- dures is followed to obtain the seconds tick. At a sweep rate of 1 ms/division, the shutter

is opened before the sweep starts and closed

after the sweep ends. This is repeated each

second until five overlapping exposures are completed (figure 5.23). The pictures should

be taken when the ticks begin to arrive with

the least distortion and maximum amplitude.

This procedure can also be used at a faster sweep rate of 100 microseconds/division

with the second zero crossover point appearing

approximately at midpoint of the trace. (One

complete cycle of the tick should be visiblefigure 5.24.) Overlapping exposures of the

ticks are taken and an average reading is obtained from the photograph.

5.4.3 USING THE WWV/WWVH TIME CODE

Among the several tones, ticks, and voice

signals offered on WWV and WWVH, there is a time code. If you receive and decode this

signal, you can automatically display the day

SECOND ZERO CROSSOVER

FIGURE 5.24. WWV TICK AT A SWEEP RATE OF

100 MICROSECONDS/DIVISION

(OVERLAPPING EXPOSURES)

92
of the year, hour, minute, and second. Of

course, it is not nearly that easy-there is a lot more to it than simply receiving the code.

To avoid errors, you must design your decoder/

clock careful ly.

Where is the code hidden? You will find

it very near the carrier signal--only 100 Hz

away, in fact. This is shown in figure 5.25. Also shown are other "parts" of the WWV spectrum. Although 10 MHz is used as an example,

the same distribution applies to the other

frequencies if you simply change the carrier.

This signal, 100 Hz away from the main

carrier, is called a subcarrier. The code

pulses are sent out once per second. With a good signal from a fairly high-quality receiver, you can hear the time code as a low rumble

in the audio. It can also sometimes be heard

on the NBS telephone time-of-day circuits.

A. Code Format

Now that we know where the pulses are and

how the are sent, what do they say? First of

all, they follow a specific format. In this

case, it is a modified IRIG-H format similar

to that shown in figure 1.2 of Chapter 1. After suitable identifiers are sent, the bits

that make up the units--tens and hundreds for

minutes, hours and days--are sent sequentially.

Certain pulses in succession comprise

binary-coded groups representing decimal numbers. The bi nary- to-decimal weighting scheme

is 1-2-4-8 with the least significant binary

digit always transmitted first. The binary

groups and their basic decimal equivalents are

shown in the following table:

BINARY GROUP

WEIGHT: 12 4 8

0 0 0 0 10 0 0 0 10 0 110 0 0 0 10

10 10

0 110

1110

0 0 0 1 10 0 1

DECIMAL

EQUIVALENT

0

12

34

5

6

78

9

In every case, the decimal equivalent of a BCD group is derived by multiplying each binary

digit times the weight factor of its respec- tive column and then adding the four products

together. For instance, the binary sequence

1010 in the 1-2-4-8 scheme means (lxl)+

(0 x 2) + (1 x 4) + (0 x 8) = 1 + 0 + 4 + 0 =

5, as shown in the table. If fewer than nine

decimal digits are needed, one or more of the

binary columns may be omitted.

In the standard IRIG-H code, a binary 0 pulse consists of exactly 20 cycles of 100-Hz

amplitude modulation (200 milliseconds duration), whereas a binary 1 consists of 50 cycles of 100 Hz (500 milliseconds duration).

In the WWV/WWVH broadcast format, however, all tones are suppressed for 40 ms while the

seconds pulses are transmitted.

Because the tone suppression applies also

to the 100-Hz subcarrier frequency, it has the

effect of deleting the first 30-mi 1 1 i second

portion of each binary pulse in the time code.

Thus, a binary 0 contains only 17 cycles of

100-Hz amplitude modulation (170 milliseconds

duration) and a binary 1 contains 47 cycles of

100 Hz TIME CODE

s:o

Q. CL

FREQUENCY

FIGURE 5.25. THE WWV SPECTRUM AT 10 MHz.

93
100 Hz (470 milliseconds duration). The leading edge of every pulse coincides with a positive-going zero crossing of the 100-Hz

subcarrier, but it occurs 30 milliseconds

after the beginning of the second.

Within a time frame of one minute, enough

pulses are transmitted to convey in BCD language the current minute, hour, and day of

year. Two BCD groups are needed to express

the hour (00 through 23); and three groups are needed to express the day of year (001 through

366). When representing units, tens, or hundreds, the basic 1-2-4-8 weights are simply

multiplied by 1, 10, or 100 as appropriate.

The coded information always refers to time at the beginning of the one-minute frame. Sec- onds may be determined by counting pulses

within the frame.

Each frame commences with a unique spacing of pulses to mark the beginning of a new

minute. No pulse is transmitted during the

first second of the minute. Instead, a one- second space or hole occurs in the pulse train

at that time. Because all pulses in the time

code are 30 milliseconds late with respect to

UTC, each minute actually begins 1030 milliseconds (or 1.03 seconds) prior to the leading

edge of the first pulse in the new frame.

For synchronization purposes, every ten seconds a so-called position identifier pulse

is transmitted. Unlike the BCD data pulses,

the position identifiers consist of 77 cycles

of 100 Hz (770 milliseconds duration).

UT1 corrections to the nearest 0.1 second

are broadcast via BCD pulses during the final ten seconds of each frame. The coded pulses

which occur between the 50th and 59th seconds

of each frame are called control functions.

Control function #1, which occurs at 50 seconds, tells whether the UT1 correction is nega- tive or positive. If control function #1 is a binary 0, the correction is negative; if it is a binary 1, the correction is positive. Control functions #7, #8, and #9, which occur re- spectively at 56, 57, and 58 seconds, specify

the amount of UT1 correction. Because the UT1 corrections are expressed in tenths of a second, the basic binary-to-decimal weights are multiplied by 0.1 when applied to these

control functions.

Control function #6, which occurs at 55 seconds, is programmed as a binary 1 throughout those weeks when Daylight Saving Time is in effect and as a binary 0 when Standard Time

is in effect. The setting of this function is changed at 0000 UTC on the date of change.

Throughout the U. S. mainland, this schedule

allows several hours for the function to be

received before the change becomes effective

local ly--i . e. , at 2:00 am local time. Thus,

control function #6 allows clocks or digital

recorders operating on local time to be pro- grammed to make an automatic one-hour adjust- ment in changing from Daylight Saving Time to Standard Time and vice versa.

Figure 5.26 shows one frame of the time

code as it might appear after being rectified,

filtered, and recorded. In this example, the

leading edge of each pulse is considered to be the positive-going excursion. The pulse train

in the figure is annotated to show the characteristic features of the time code format.

The six position identifiers are denoted

by symbols P lf P 2 , P 3 , P 4 , P 5 , and P 0

. The

minutes, hours, days, and UT1 sets are marked

by brackets, and the applicable weighting fac- tors are printed beneath the coded pulses in each BCD group. With the exception of the

position identifiers, all uncoded pulses are set permanently to binary 0.

The first ten seconds of every frame

always include the 1.03-second hole followed

by eight uncoded pulses and the position

identifier Pi. The minutes set follows P x and

consists of two BCD groups separated by an uncoded pulse. Similarly, the hours set follows P 2

. The days set follows P 3 and

extends for two pulses beyond P 4 to allow | enough elements to represent three decimal ; digits. The UT1 set follows P 5 , and the last

j pulse in the frame is always P 0 .

In figure 5.26, the least significant I digit of the minutes set i s (0 x 1 ) + (0 x 2) + (0 x 4) + (0 x 8) = 0; the most significant \ digit of that set is (1 x 10) + (0 x 20) +

(0 x 40) = 10. Hence, at the beginning of the

1.03-second hole in that frame, the time was exactly 10 minutes past the hour. By decoding j the hours set and the days set, it is seen that the time of day is in the 21st hour on the 173rd day of the year. The UT1 correction ' is +0.3 second. Therefore, at point A, the correct time on the UT1 scale is 173 days, 21 hours, 10 minutes, 0.3 second.

B. Recovering the Code

The time code will appear at the detector

in any receiver tuned to WWV or WWVH. How- ever, due to the limited low frequency pass- band of most receivers, it may or may not be heard at the speaker or on headphones. Re- ceivers are deliberately designed to suppress I lower frequencies to limit the power supply ; hum (60 Hz) and its second harmonic (120 Hz).

The best approach would be to start at the r receiver-detector and "strip" out the 100 Hz 'i with a narrow filter. If the 100 Hz is then » amplified and detected, a code stream can be

f produced and used to drive a decoder/display, r

94
FORMAT H, SIGNAL HOOl, IS COMPOSED OF THE FOLLOWING:

1) 1 ppm FRAME REFERENCE MARKER R = (P 0 AND 1.03 SECOND "HOLE")

2) BINARY CODED DEC II1AL TIME-OF-YEAR CODE WORD (23 DIGITS)

3) CONTROL FUNCTIONS (9 DIGITS) USED FOR UTi CORRECTIONS, ETC.

4) 6 ppm POSITION IDENTIFIERS (P THROUGH P ) 5) 1 pps INDEX MARKERS 5

TIME FRAME 1 MINUTE

I I I I I I I I I

INDEX COUNT (1 SECOND) 30^/^ 40

i 1 I I I u| I I 1 I I I I I I I I I I I

50 ill

ON TIME POINT A

1 SECOND- w—H k p

^7—1 -03 SECOND

-RJ HOLE IN CODE

FOR 0.8 SECOND

PULSE

p p

3 w—H k p p5 #6-1 c-^ k- p laMJUumjuijinji^

l——uni ir<; ——

-MINUTES- •DAYS- UTtCORRECTION

P

0 -P

W

c

UTC AT POINT A

= 173 DAYS 21 HOURS

10 MINUTES

UTI AT POINT A = 173 DAYS 21 HOURS

10 MINUTES

0.3 SECONDS POSITION IDENTIFIERS (0.770 SECOND DURATION) WEIGHTED CODE DIGIT (0.470 SECOND DURATION) WEIGHTED CONTROL ELEMENT ( 0.470 SECOND DURATION) CONTROL FUNCTION #6 J|{narY ZERO^URING ^STANDARD '^tImE DURATION OF INDEX MARKERS, UNWEIGHTED CODE, AND UNWEIGHTED CONTROL ELEMENTS = 0.170 SECONDS BEGINNING OF PULSE IS REPRESENTED BY POS I T I VE - GO I NG EDGE. 9/75

FIGURE 5.26. WWV/WWVH TIME CODE FORMAT.

WWV decoders can be purchased or homebuilt. Some hobby and radio amateur publications have carried articles on using the WWV

time code. At this writing, the NBS is not

able to supply decoder designs. As such material becomes available, it will be announced

in the NBS Time and Frequency Bulletin, published monthly.

Because signals such as those transmitted

by WWV tend to fade, it is possible to lose

some of the code bits. As these errors occur,

any clock driven directly from a WWV receiver

would display incorrect time. This can be overcome by having the decoder circuit detect

the errors. One scheme for error detection

simply decodes one frame of the incoming code

and stores it electronically as digital bits.

The next frame is received and stored in another location. If the two frames do not differ by exactly one minute, an error has

occurred. The first data are discarded and

another try is made until several successful

decodes have been made. This can be carried

out to three, four, or five successive, successful decodes to reduce the possibility of error.

Meanwhile, the clock can be made to operate from a simple crystal oscillator. It will operate from that crystal until the time

code has been successfully decoded and is used

to reset the clock to the correct time if

necessary. When you consider how accurate

crystal clocks are, the code really doesn't

have very much to do, but it will reset the

clock as needed and automatically adjust for

Daylight Saving Time. It will also increase

your confidence in the clock as a timekeeper.

There are 60 minutes and therefore 60 frames

in the WWV time code every hour. This means

that there are 1440 frames in a day. A number

of successful decodes should be received in a day. Typically, this means that the crystal

clock would be corrected several times. This

should be more than adequate for good timekeeping.

An electronic means can be provided to receive and decode Coordinated Universal Time

but display local time as needed.

95
5.5 USE OF HF BROADCASTS FOR FREQUENCY CALIBRATIONS

In addition to the widely used time ser- vice of WWV and WWVH, standard frequencies are also available to the broadcast listener.

With a general purpose HF receiver capable of

tuning these stations, a calibrating frequency

is readily available for comparison and measurement.

Frequency or its inverse, time interval,

is an important quantity in physics, electronics, and in our everyday lives. From tuning

musical instruments to setting the radio and

TV dials, frequency enters our lives in many

ways. If we say we are going to perform a frequency calibration, the question arises, on what? If we calibrate the frequency source in our watch, we have to adjust the balance wheel

(or the crystal). Radio listeners often want

to check the dial settings on their shortwave

receivers. That way, they know if there is an error in the dial when they are seeking a certain station whose transmitter frequency

is known.

There are two ways to use a frequency

source such as WWV. On the one hand, you can measure your frequency source against WWV and

write the difference down for reference, or you can actually readjust the source of frequency you have to make it agree with WWV.

WWV and WWVH broadcast low frequency

tones at 440, 500, and 600 Hz. These can be

used with an oscilloscope or electronic counter for checking audio oscillators, musical

instruments, and other devices that operate at low frequencies. The accuracies obtainable

are usually satisfactory for these applications but would not be sufficient for setting

a crystal oscillator.

In the material that follows, several

calibration methods are discussed. Generally,

WWV-type signals are used to calibrate lower

accuracy frequency sources with 1 part in 10 7 being about the limit under good conditions.

Also, the HF radio services lend themselves

well to the user who is willing to turn the

knob and change his oscillator frequency.

These methods are best used to set two sources

equal rather than to measure the numerical

difference between two frequency sources.

Direct frequency comparison or measure- ment with WWV can usually be accomplished to about one part in 10 6

. This resolution can be improved by long-term (many weeks) time com- parison of clocks operated from a frequency

source rather than direct frequency comparison. Four methods of calibrating frequency

sources using the broadcasts of WWV/WWVH are discussed: (1) beat frequency method; (2)

oscilloscope Lissajous pattern method; (3)

oscilloscope pattern drift method; and (4)

frequency calibrations by time comparisons.

STANDARD FREQUENCY

BROADCAST

V

Nth HARMONIC.

22pf ±Z C

SQUARE WAVE,

1 00kHz

SIGNAL STRENGTH

V\ ) METER

HF RECEIVER

OFF

° AGC

ON o*-o SWITCH

SQUARE

WAVE

GENERATOR

SPEAKER

100 kHz

CRYSTAL

0SCI LLAT0R

FIGURE 5.27. EQUIPMENT SETUP FOR BEAT FREQUENCY METHOD OF CALIBRATION.

96
5.5.1 BEAT FREQUENCY METHOD

The beat frequency or heterodyne method

of frequency comparison with standard radio

frequencies is a simple technique commonly

used by radio operators to calibrate trans- mitters and tune receivers. A frequency off- set of about 1 part in 106 can be accurately

determined. Thus, a 1 MHz signal that is calibrated in this way can have an expected

error of 1 Hz.

Figure 5.27 shows an arrangement for

calibrating a 100 kHz oscillator. A 100 kHz signal containing harmonics is coupled to the

receiver input along with the WWV signal from

the antenna.

This method works by heterodyning or mixing a known and accurate frequency (e.g., WWV RF signal) with the output of an oscillator. The mixing is accomplished by the con- verter circuit in any superheterodyne receiver.

The difference frequency of the two RF signals

can be amplified and detected. The result is an audio output signal called the beat frequency or beat note.

The frequency of this beat note is the

difference of the two input frequencies. When

the two frequencies are made equal, their difference decreases to zero and is called "zero

beat." When zero beat is reached, the oscillator is equal in frequency to the WWV fre- quency.

To calibrate a frequency source or crystal

oscillator with an output frequency lower than

that broadcast by WWV, the correct harmonic

equal to the WWV signal is required. For example, if a 100 kHz signal is to be calibrated with the WWV 5 MHz carrier frequency,

then it must also contain a harmonic fifty

times itself. Thus, a signal to be calibrated

must be a submultiple of the WWV carrier

frequency.

Theoretically, a sine wave does not contain any harmonics. In practice, though, all sine wave signals contain sufficient harmonics

to produce a beat note. A square wave signal,

on the other hand, is very rich in harmonic

content and is ideal for generating harmonics

to calibrate receivers and transmitters in the

HF and VHF band.

A simple method of generating a square wave from a sine wave is by clipping the signal with a diode clipping ci rcuit—shown in figure 5.28. To obtain a strong harmonic signal for beat notes requires a large amplitude

signal to produce heavy clipping. A better

method is to digitally condition the 100 kHz signal to produce square waves.

If the receiver input impedance is low

(50 -100 ohms), a 10 to 20 picofarad (pf)

capacitor can be used to couple the high frequency harmonic to the receiver input and to reduce the level of the lower fundamental fre- quency. If the receiver has a high input impedance with unshielded lead-in wire from the

antenna, the harmonic signal can be loosely

coupled to the receiver input by wrapping a few turns of an insulated wire around the an- tenna lead-in and connecting it directly to the output of the oscillator. For receivers

with built-in or whip antennas, you must experiment to find a way to inject the

osci llator signal

Using harmonics of the oscillator being

calibrated makes it necessary to know the re- lationship between the oscillator error and

the beat note that is measured during calibration. Let the oscillator output be designated

as fp. This is made up of two components, the

correct frequency f 0 , plus an error that we

can designate as Af. So:

f

F = fQ + Af

A harmonic of this oscillator signal (written

Nfp) will beat against the carrier, f^,. The

resulting beat note fg is the difference

between the two, written as:

Nfr

INPUT

(SINE WAVE)

OUTPUT

SQUARE WAVE)

CR1 , CR2 = IN270, 1 N34

FIGURE 5.28. DIODE CLIPPING CIRCUIT TO PRODUCE HARMONICS FROM A PURE SINE WAVE SIGNAL.

97
The bars mean that a negative sign in the answer is ignored; that is, the answer is always a positive number.

Now substitute the first equation into

the second:

fB = f

c

- Nf

Q

- NAf p .

But Nf0 equals f p. That's why we multiplied

by N. So:

f

B = NAf, or

Af =

which says that the error equals the beat that

we hear divided by N. For example, if a beat

frequency of 100 Hz were measured between the WWV 5 MHz signal and the 50th harmonic of a 100 kHz oscillator signal, the frequency error

of the 100 kHz signal would be:

Af _ 100 Hz _ „ u Af - -gg- - 2 Hz.

The oscillator frequency is in error by 2 Hz. To determine whether the oscillator is high or low in frequency, the oscillator

frequency has to be changed to note which way

the beat frequency decreases. If increasing

the oscillator frequency decreases the beat

note, it indicates that the oscillator frequency is lower than the WWV/WWVH frequency.

In a receiver with no tuned RF amplifier

between the mixer stage and the antenna input,

a low frequency sine wave signal can enter the

mixer stage directly and generate unwanted

harmonics and confusing beat notes due to the

nonlinear characteristics of a mixer circuit.

However, a good communications receiver or a special -purpose WWV receiver generally has

tuned RF amplifier stages or preselectors

before its first mixer stage. Then, only the

desired beat note from two input signals is produced.

If the beat note is above 50 Hz, headphones, a speaker, or a counter can be used. Below that frequency, a dc oscilloscope can be

connected to the receiver detector. A signal

strength meter can be used and the beats

counted visually. The automatic gain control

(AGC) should be disabled, if possible, for the meter fluctuations to be more noticeable. The

manual RF gain can be adjusted to compensate

for loss of the AGC.

To correct the oscillator frequency, the

adjustment knob is turned in the direction

which lowers the frequency of the beat note. Between 50 Hz and about 1 Hz, the beat note cannot be heard and the signal strength meter

will begin to respond to the beat note as it

approaches 1 Hz. As zero beat is approached,

a very slow rise and fall of the background

noise or the WWV audio tone can also be heard

on the speaker. The meter effect is much

easier to follow. As it approaches zero beat,

the very slow rise and fall of the signal

strength may sometimes become difficult to distinguish from signal fading due to propa- gation effects.

To overcome fading effects, the oscillator adjustment can be interpolated. First,

adjust the oscillator to a minimum beat frequency that can be measured without interference. For accuracy, count the number of deflections of the signal strength meter in 10 seconds. The setting of the frequency adjust- ment is then marked. The adjustment is then

made to pass zero beat until the beat is again

visible on the meter. By obtaining the same number of meter deflections as the previous

beat note, the frequency can be set midway

between the two adjustments.

The frequency of crystal oscillators

changes with time. This is commonly referred

to as the aging or drift rate. Therefore, all crystal oscillators must be recalibrated periodically if maximum accuracy is desired.

5.5.2 OSCILLOSCOPE LISSAJ0US PATTERN METHOD

Audio oscillators can be calibrated by

using WWV/WWVH audio tones to produce Lissajous patterns on an oscilloscope. Lissajous

patterns are fully discussed in Chapter 3. The WWV audio signal is applied to the vertical input of the scope and the oscillator

signal to be calibrated is used to drive the

horizontal amplifier. The resultant pattern

tells the user two things: (1) the frequency

ratio between his oscillator setting and the

received tone, and (2) movement of the oscillator relative to WWV.

The tones available on WWV nd WWVH are mainly 500 and 600 Hz. CHU transmits 1 kHz ticks that last for about one-third second.

If these are used, they will produce an oscilloscope pattern at the tick rate; i.e., once

per second. Even the WWV/WWVH ticks can be

used this way. Most important, the user should be aware of the exact format being

broadcast to avoid making errors. This can be

a problem because WWV and WWVH switch between

500 and 600 Hz in alternate minutes and the

440 Hz is used for only one minute every hour.

In a typical application, the user will

be able to check the accuracy of the dial

98
operation. First, he picks a dial setting

giving a frequency ratio to a WWV tone that is an integer. Then he turns the dial slowly

until the pattern is stationary. By reading

the dial setting, a calibration can be made

and the dial reset to another frequency that

is an integer ratio. The same technique could

be applied to fixed frequency sources if they

are in correct ratio to the tones on WWV and

WWVH. The ratio of the two frequencies is equal to the ratio of the number of loops

along two adjacent edges. If the vertical

input frequency is known, the horizontal input

frequency can be expressed by the equation:

N

v

f

h = N^

f

v

'

Where: f. = horizontal input frequency

f = vertical input frequency

N = number of loops on the vertical

edge

= number of loops on the horizontal edge

:

'•• • m\mmmm

VERTICAL INPUT: WWVH 500 Hz TONE

HORIZONTAL INPUT: AUDIO OSCILLATOR

600 Hz TONE

FIGURE 5.29. OSCILLOSCOPE DISPLAY SHOWING

LISSAJOUS PATTERN.

The pattern shown in figure 5.29 has five

loops on the horizontal edge and six loops on the vertical edge. The vertical input frequency is the WWVH 500 Hz tone. To determine

the horizontal input frequency, the known

values are substituted in the above equation

and the result is:

f. = | x 500 = 600 Hz.

Therefore, the horizontal input frequency is 600 Hz.

It is possible to calibrate over a ten- to-one range in frequency both upwards and

downwards from the 500 and 600 Hz audio tones transmitted by WWV and WWVH; that is, from 50 Hz to 6 kHz. However, not all frequencies

between them can be calibrated with the 500

and 600 Hz tones because not all frequencies

have a ratio of integers with less than the

number ten in the numerator and denominator.

For example, a frequency of 130 Hz compared

with the 500 Hz tone would give a ratio of

50:13. It would be impossible to count 50 loops on the horizontal edge. But a frequency

ratio of 500 Hz to 125 Hz is possible because

there will be four loops on the horizontal

edge and only one loop on the vertical edge or

a ratio of 4:1. A frequency ratio of 1 : 1 pro- duces a less complex pattern of a tilted line,

circle, or el 1 ipse.

If the frequencies are exactly equal, the

figure will remain stationary. If one frequency is different from the other, the figure

will not remain stationary but will "rotate." Because one complete "rotation" of the figure

is equal to one cycle, the number of cycles

per unit of time is the frequency error.

For example : If a Lissajous figure takes

ten seconds to "rotate" through one cycle and

the frequency being compared is 600 Hz, the

frequency error is:

Af = - = —,= 0.1 Hz

T 10

c _ Relative _ Af _ 0.1 _ , 7 in -4

F = c = — = = 1-7 x 10 Frequency

f 60Q

Since the error is inversely proportional to the time it takes to complete one cycle, it is obvious that the longer it takes to complete a cycle, the smaller the error will be.

The measurement accuracy of a Lissajous

pattern by the comparison method described

above is inversely proportional to the fre- quency. For example, if the time required for

a Lissajous figure to "rotate" through one cycle is ten seconds at a frequency of 1 MHz:

F = Relative Frequency = —

f

0.1 Hz _ , 1n-7

= z = 1 X 10

1 x 10

b Hz

If, instead of measuring at 1 MHz, we observe

the Lissajous pattern at 1 kHz:

Af = f x F = 1 x 10

3 x 1 x 10" 7 = 10" 4 Hz

99
and the observation time would be:

Af 10' 4 = 10,000 seconds.

Thus, it would take too long to measure signals with relative frequencies of less than

1 x 10- 5 at 1 kHz. A more accurate method

which measures the phase shift on an oscilloscope will be discussed next.

5.5.3 OSCILLOSCOPE PATTERN DRIFT METHOD

The oscilloscope pattern drift method is good for comparing two frequencies if you have

an oscilloscope with external triggering. It can detect smaller frequency differences than

the Lissajous pattern at audio frequencies.

The method consists of an oscilloscope

with an accurately calibrated sweep time base. External triggering is obtained from the signal to be calibrated. This signal can be any integer submultiple of the tone being received

from WWV/WWVH. The receiver (tuned to WW) has

its audio output connected to the vertical

input of the oscilloscope. With the sweep

rate set at 1 millisecond/division, the

trigger level is adjusted so that a zero crossover of the corresponding 600 or 500 Hz signal is near midscale on the scope.

By measuring the phase drift during a given time interval, the frequency error is determined. If the zero crossover moves to the right, the audio signal frequency is higher than the WWV signal, and if it moves to the left, the signal is lower in frequency

(fig. 5.30).

For example, if during a count of 10 seconds at 500 Hz, the zero crossover advanced

from left to right by 0.1 millisecond, the

relative frequency is:

c _ phase drift _ +0.1 x 10

3 ^, ln-5

F c = = +1x10

time interval 10

and the frequency difference is

Af = f x 1 x 10" 5 = 500 x 10

5 = +0.005 Hz.

Therefore, the 500 Hz signal has an error of

+0.005 Hz. Under ideal conditions, a relative

frequency of 1 part in 10 6 can be determined

by increasing the observation time.

mmmmw^Ammm

INITIAL FINAL

VERTICAL SIGNAL: WWVH 600 Hz TONE

TRIGGER: AUDIO OSCILLATOR

600 Hz TONE SWEEP RATE: 100 MICROSECONDS/DIVISION

INITIAL ZERO CROSSOVER: 530 MICROSECONDS

FINAL ZERO CROSSOVER: 680 MICROSECONDS

PERIOD OF MEASUREMENT: 10 SECONDS

TOTAL PHASE SHIFT: +150 MICROSECONDS

r +150 X 10 r ln -5

F = = +1.5 x 10

10

Af = 600 x 1 .5 x 10 -5

900 x 10

J = +0.009 Hz

FIGURE 5.30. OSCILLOSCOPE DISPLAY USING

PATTERN DRIFT METHOD.

5.5.4 FREQUENCY CALIBRATIONS BY TIME COMPARISON OF CLOCKS

For those users who are set up to recover

and maintain time from WWV or WWVH, there is an alternate way to calibrate the frequency of

the oscillator driving the clock. This is simply to record the time differences from day

to day and then calculate the frequency rate. The operation is analogous to telling your

jeweler that your watch gains or loses so many seconds a day. He then adjusts the rate to compensate.

The operation proceeds something like

this. Each day, note the amount the clock

output differs from WWV. Then keep an accurate record of the history of the oscillator

in terms of the time gained or lost with re- spect to WWV ticks. To get accurate data,

readings from several days should be averaged.

For example: If the time error averaged

out to be 1 millisecond per day (say for a 5- day period), then you could compute the fre- quency offset by noting that (see Chapter 3):

Af

f

At

T

100
which simply says that the time lost or gained

by a clock whose oscillator is running at frequency f is proportional to the frequency

error. In our example, putting all the terms

in the same units (in this case, seconds) we

get:

-3 At _ 5 x 10 s time error

T 5 x 86,400 s

=1.2xl0-8 =-^

f

F, the relative frequency of this oscillator

would be -1.2 x 10- 8. So an oscillator that

is off frequency by about a part in 10 8 would

gain or lose about a millisecond a day. Using

the techniques outlined in the section on time

checking, this millisecond error would be

quite visible to a user.

This example illustrates that this technique could be extended to other levels of accuracy. In fact, data have been obtained

that allow WWV signals to be used for calibrations approaching a part in 10 10

.

We can use our example to illustrate

another side of the time/frequency problem;

namely, how to specify frequency error. Assuming that our answer was one part in 10 8

, we

can write it mathematically as 1 x 10- 8

, and

(if the oscillator were 100 kHz and high in frequency) our actual numerical frequency

would be 100,000.001 hertz. This can also be

written as 0.000001% frequency error.

In summary then, frequency calibrations

can be made by recording only the time error produced by an oscillator. This technique is recommended for lower quality oscillators and

for situations where the user is already mea- suring time of day from WWV for some other

reason.

5.6 FINDING THE PROPAGATION PATH DELAY

The problem of finding the radio propagation path delay is twofold. First, the great

circle (curved) distance must be found between

the transmitter and receiver. Second, the

number of times the signal bounces from iono-

| sphere to earth (hops) must be known. With

these two facts, we can solve for the total

path delay. The accuracy we can expect will

be somewhere around a millisecond. We would

be hard put to improve that figure very much.

I For groundwave signals at very low frequeni cies, we could carry a clock and calibrate the

path, but for HF waves of the type transmitted

by WWV and CHU, it probably isn't worth the

effort. The reason for this is that at any particular time, the path may not be at its

average value. Since the path is constantly

changing by small amounts, it would not pay to try to measure it with great exactness.

For those users who want time with the

best possible accuracy and who need a method

for calculating the path delay, the following

methods will work. The new hand-held calculators that are available will greatly assist in the computations. However, the math can be

done with a slide rule or a book of tables. Some examples are given to help in the

sol ution.

5.6.1 GREAT CIRCLE DISTANCE CALCULATIONS

Using figure 5.31 as a reference:

A and B are two points on the earth.

Pole is one of the two poles, North or

South.

LA = Latitude at point A.

LB = Latitude at point B.

L0A

= Longitude at point A

L0B

= Longitude at point B

P = L 0A " L0B"

dg

FIGURE 5.31. DIAGRAM FOR GREAT CIRCLE

DISTANCE CALCULATIONS.

101
PROBLEM : Using the above information, find

the great circle distance (dg) in nautical miles between the two points A and B. All angles are given in degrees.

For points A and B on the same side

of the equator :

dg = 60 cos- 1 (cos cos Lg cos P

+ sin sin Lg) nautical miles.

For points A and B on opposite sides

of the equator :

dg = 60 cos- 1 (cos L^ cos Lg cos P

- sin L^ sin Lg) nautical miles.

CONVERSION TO STATUTE MILES AND KILOMETERS :

1.151 x nautical miles = statute miles.

1.8522 x nautical miles = kilometers.

EXAMPLE : Find the great circle distance from

radio station WWVH, Kauai, Hawaii

(point A) to WWV, Fort Collins,

Colorado (point B) in statute miles

and kilometers.

The coordinates of the two stations are:

Latitude Longitude

LA = 21°59'26" N LQA

= 1 59°46 1 00" W

Lg = 40°40'49" N LQB

= 105°02'27" W

Then, P= 159°46'00" - 105°02'27" = 54°43'33"

SOLUTION : Taking sines and cosines:

sin LA = 0.37445 cos L

ft = 0. 92725

sin Lg = 0.65184 cos Lg = 0.75836

cos P = 0.57749

dg = 60 cos"

1 [(0.92725 x 0.75836 x 0.57749)

+ (0.37445 x 0.65184)]

= 60 cos"

1 [(0.40608) + (0.24408)]

= 60 cos"

1 0.65017 = 60 x 49.44586

= 2966.75 nautical miles

= 3414.73 statute miles

= 5495.00 kilometers

5.6.2 PROPAGATION DELAYS

The delay of high frequency radio waves

depends on many factors. The height of the

ionosphere and the distance between transmit- ter and receiver are important. Knowing the exact number of hops is important; otherwise, errors of from 0.5 to 1 millisecond can occur.

The ionosphere is made up of a number of

layers whose actual height varies both daily

and seasonally. This changes the path delay a lot if the receiver and transmitter are close

to each other. Much of the time, such changes

are not important to the person who wants to get time information, but if you are concerned

with errors of about 300 microseconds, you must take care to account for all delays.

Since the earth is round, there is a maximum ground distance a single hop can span

(fig. 5.33). This is about 4000 kilometers or 2200 miles. For greater distances, the radio waves must obviously be reflected a number of

times. Any error in the wave path computation

for one hop must be multiplied by the total

number of hops.

For minimum error, the frequency used for

reception should be selected for the least

number of hops. This also results in a stronger signal due to smaller losses. At any particular distance, the user must select the

proper shortwave frequency from WWV/WWVH so the signal can be received. Some frequencies

will penetrate the ionosphere and not reflect.

Other frequencies will skip over the user. He

is then said to be located in the "skip dis- tance" for that frequency. A lower frequency must be selected. Use of the maximum frequency that is receivable assures the least

number of hops. This maximum usable frequency

is called the MUF. A higher frequency will

skip over the receiving station. A frequency

about 10% below the MUF will provide the best

reception.

For distances under one thousand miles,

the single-hop mode of transmission dominates. For short distances, the height must be

102
FIGURE 5.32. REFLECTION OF RADIO WAVES AT DIFFERENT IONIZED LAYERS.

estimated carefully. For distances of over

1600 kilometers (1000 miles), where only a single-hop mode of transmission occurs, a much

wider range of error in the height can be

tolerated.

By estimating height according to time

of day, season, latitude, and sunspot cycle, errors can be reduced. For F 2 -layer reflections, which is the principal layer used,

there are several "averages" in height that

can be used. An average height of 250 km is reasonable for the winter months when propagation conditions are good at 10 MHz and

above. In summer, the average height can be

increased to 350 km. As reception again

begins to improve in the fall, the estimated

height can be decreased again. Using this

approximation method, the error in path length

computation can be reduced considerably.

For distances from one to two thousand

miles (3220 km), where multiple-hop transmission occurs, the method above produces errors. For best results, the height must be determined with greater accuracy. Heights for a particular latitude, longitude, and time of

day are available from the World Data Center,

N0AA, Department of Commerce, Boulder, CO 80303.

By performing a single time comparison

with a station, the probability of error in the hundreds of microseconds becomes very

great due to the changes in propagation. A high degree of accuracy for a single measure- ment is remote. But, by conducting measure- ments at several times over a period of a week

and by averaging with methods such as the

moving average method, errors can be reduced.

The following equations for determining

wave angle and propagation time delay have been

simplified and are approximate, giving errors

of 2% or less.

103
DEFINITIONS OF SYMBOLS PROPAGATION TIME DELAY

wave angle or angle of radiation

in degrees.

$ = angle of incidence in degrees.

TD =

2R sin 0 _d£L p 299.8 sin $ 299.8 sin

(R and dg in km)

9 = 1/2 central angle subtended by

radius R from A to B.

Where:

dg = great circle distance from trans- mitter to receiver in kilometers. For S3 < 4000 km, 9 < 18 degrees:

N = number of hops to cover distance

dg.

4> = 90° - tan- 1 + (approximately)

dg _ great circle distance per hop

N (km/hop). SAMPLE CALCULATIONS

h

1 = virtual height of ionized layer

(km).

R = 6,370 km, mean radius of earth.

EXAMPLE: Given: dg = 3220 km

h

1 =250 km

N = 2

total propagation distance (km). Determine wave angle A and propagation time

delay TD.

TD = propagation time delay

p (milliseconds).

A=tan-* (lN^ + |a_) - o.0045 ^= 13.25°,

WAVE ANGLE EQUATIONS

Or, by the approximate method:

A = tan"

1

( h FTTTrn)

+ tan 5

" e

9(degrees) =

fjjjj (^ ) = 0.0045 &

a a

(2NIV _ dg_) A 57 3 = 14 2°

" 1 dg 4RN ;3/ --i

TD_ =

p 299.8 sin 0

For ^ < 4000 km or 9 < 18 degrees:

But: * = 90° - tan"

1 (2^. + d|_)

= 69.

C

A = tan"

1 (^ + ^_) - A zan 0 0045 4fl

( dg 4RN ; N So: TD

p = 299.8

3 s?n 69.5

=11 - 5ms

For 9 < 20 degrees:

r 2Nh' A S O dg

^-) 57 3 4RN ;

EXAMPLE : Given : dg = 3220 km

h

1 = 334 km

N = 1

104
TD = 0QQ " W •—t= 11.4 ms

p 299.8 sin 0

Thus, the time delay of 2 hops at 250 km virtual height and 1 hop at 334 km virtual height

will differ by only 100 microseconds.

5.7 THE NBS TELEPHONE HME-OF-DAY SERVICE

For those users who cannnot set up a shortwave receiver for WWV or who have other

difficulties, NBS operates a telephone timeof-day service. Anyone who dials 303-499-7111

will hear the very same signals that are broadcast over WWV. The machines at Fort

Collins, Colorado, which generate the WWV

format, are connected via telephone lines to a room in the Boulder Laboratories where the

telephone equipment is maintained.

This telephone service is widely used,

both in the local Denver calling area and via

long distance. The telephone service can

handle eight incoming calls simultaneously.

In addition, four lines are available through

the government switchboard at Boulder for use

by other government agencies. These are on

FTS number 323-4245.

The actual time you hear on the telephone

will differ by a few milliseconds from that

heard over WWV via radio. This is because of

the delay in the telephone lines. In addition,

if you are calling from any great distance,

the way in which the telephone company handles

your call limits the total accuracy of the WWV

tone to a few hertz. This means that if you

are trying to make a calibration at 1000 Hz,

you could have an error as great as three

cycles per second. This accuracy is not as good as that which can be achieved using radio

methods. Users are warned about this limit of

accuracy over the telephone.

Because the telephone service is very

often fully utilized, you may get a busy signal. If this happens, wait a few minutes and

redial. After your call is connected, the

system will automatically hang up in three

minutes unless you hang up first.

5.8 SUMMARY

As stated early in this chapter, HF radio

broadcasts are commonly used for time and frequency calibrations. Accuracies ranging from

1 ms for time-of-day to 1 part in 10 7 for frequency can be obtained. The NBS telephone

time-of-day service can also be used when

accuracy requirements are not critical. How- ever, if your requirements are higher, you

should probably use one of the other methods

discussed in the following chapters.

105

CHAPTER 6. CALIBRATIONS USING LF AND VLF RADIO TRANSMISSIONS

Along with the widely used high frequency

radio transmissions of WWV and WWVH, the

National Bureau of Standards operates a frequency and time service on WWVB, a low frequency (LF) radio station. WWVB operates at

60 kHz to take advantage of the stable radio

paths in that frequency range. Both frequency

and time signals are provided, but no voice

transmissions are made due to the narrow bandwidth of the transmitter/antenna combination.

Many countries have services in this band,

which ranges from 30 to 300 kHz, as well as in

the VLF (very low frequency) band from 3 to 30

kHz. In addition to discussing the use of

WWVB, this chapter also covers other VLF sta- tions throughout the world. The Loran-C ser- vice at 100 kHz is discussed in a Chapter 9.

It may seem unusual to send signals in a frequency band that is almost in the audio

range, and indeed, these signals do pose some

special problems for the transmitter and

receiver-design engineers. However, low

frequencies such as the 60 kHz of WWVB are used because of their remarkable stability.

Radio waves at low frequencies use the earth

and the ionosphere as a waveguide and follow

the earth's curvature for long distances.

Accuracies of a part in 10 11 or better for

frequency and 500 microseconds for time can

be achieved by using LF or VLF broadcasts.

6.1 ANTENNAS FOR USE AT VLF-LF

At the transmitters, it is difficult to radiate high power levels. Special large antenna arrays are used, but even they are inefficient. Resonance of receiving antennas

is hard to achieve because of the low frequency and resulting long wavelength. A channel 9 TV antenna is about 2 feet long. At 60 kHz, an antenna has to be almost 2 miles long (a

quarter-wavelength) to be resonant. This is obviously impossible so a compromise must be

made.

Since a quarter-wavelength antenna is out

of the question, antennas that are electrically "short" are used with tuning boxes and

special couplers. On vertical antennas, top

loading is often used. This consists of

radial wires extending from the top of the antenna to the ground. Often, they serve as guys. Of course, as with any antenna system, a good ground is essential. The physical location of antennas is important. Placement will

affect signal strength and noise. Keep the antenna away from metal objects. Longwire antennas should be at least 15 to 20 feet

above ground. Several commercial antennas

use preamplifiers so the connecting coaxial

cable supplies power to the amplifier. In those cases, care must be taken to avoid

shorting the cable.

Manufacturers of VLF-LF radio receivers

use a variety of antenna types. Long-wire antennas up to several hundred feet are available. Whip antennas 8 to 10 feet long are used where space is a problem. On the other

hand, air loop antennas are able to reject

interference but do not have as much gain as whips. This is done by turning the loop antenna so that the null of its figure 8 pattern is pointed at the source of interference. Ferrite loop antennas are becoming

more popular since they are very small com- pared to an air loop, but they can cause some unwanted phase shift due to temperature. If you want the very best phase record, the kind

of antenna and how it is used are important

considerations.

All of the above antennas benefit from

having couplers and/or amplifiers incorporated

in the antenna structure to allow a match to be made with the shielded antenna cable. The

usual precautions about placing antennas near

metal objects or buildings should be observed.

Here, the manufacturers' instructions should

be followed. Many engineering handbooks and antenna manuals contain details about antenna

construction.

6.2 SIGNAL FORMATS

Even after the antenna problems are solved, sanding information on low-frequency

carriers leaves much to be desired. WWVB

sends a carrier for frequency information and

changes the level (-10 decibels (dB)) of that

carrier to transmit a time code in the form of

binary "ones" and "zeros." The Navy VLF sta- tions use on/off keying to send code and some- times use frequency-shift or minimum-shift

keying. The Omega Navigation System sends

only the carrier. Receivers for these VLF signals usually "lock" onto the carrier and

thus recover frequency information. In addition, some means is provided to hear the

station. This is usually just a tone output. For example, if you were to listen to WWVB,

you would hear the one-second code segments

as a tone that changes from loud to soft. There are no voice signals on any of the LF or VLF stations. The bandwidth used cannot transmit voice signals. Almost all VLF-LF stations are controlled by atomic oscillators.

107
TIME

(UTC)

FIGURE 6.1. PHASE OF WWVB AS RECEIVED IN MARYLAND.

6.3 PROPAGATION CHARACTERISTICS AND OTHER

PHASE CHANGES

Phase records made of VLF and LF stations

show changes caused by the daily and seasonal

changes in the propagation path. Among other

things, LF and VLF signals have regular phase

shifts. These shifts occur when sunrise or sunset occurs on the path from the transmitter

to receiver. For instance, as the path is changing from all darkness to all daylight,

the ionosphere lowers. This shortens the path

from transmitter to receiver. This shortened

path causes the received phase to advance.

This phase advance continues until the entire

path is in sunlight. The phase then stabilizes until either the transmitter or receiver

enters darkness. When this happens, the

ionosphere begins to rise, causing a phase

retardation. Figure 6.1 shows a strip chart

recording of the phase of WWVB, Ft. Collins,

Colorado, as received in Maryland. The day/

night changes are easily seen. The magnitude

of the change is a function of the path

length, and the rate of the change is a function of path direction.

A phase recording from a stable VLF or LF radio station contains a great deal of information. The user's job is to sort out this

information so that he can understand what is happening to the frequency source that is being calibrated. His first assumption is usually that the signal from the station is almost perfectly controlled. That is, it starts on frequency and stays there. This is not always true. Errors in control at the

transmitter can cause large phase shifts in the received signal that would make it appear

as though the local oscillator were changing. Most stations operate with a near perfect

record, but they do make mistakes. This hap- pens just often enough so that the user needs

additional information to help him. This

information takes the form of monthly or weekly notices of the actual phase of the

signal. This is usually in terms of microseconds advance or retard. A sample of NBS data for WWVB and other monitored stations is shown in figure 6.2. U. S. Naval Observatory

(USNO) data for its stations are shown in fig- ure 6.3. These data can be obtained free upon

request.

The implication of possible transmitting

errors to the user is that he cannot hurry

through calibrations! Certain practices with

respect to LF radio calibrations are highly

recommended. These include careful attention

to such details as receiver adjustments--do

not change knob settings or cabling. Always

operate the equipment continuously if possible. This gives you a continuous chart record

so that you know when to expect sunrise/sunset

phase shifts. You can also detect local

interference and noise conditions. If your

receiver has a mechanical phase counter, jot

down the readings on the chart. This is a great help in trying to reconstruct events that happen on weekends or at night.

A typical occurrence for a tracking

receiver is the cycle phase shift. Since the

receiver is faithfully following a zero cros- sing of the received phase, it doesn't know

which of the thousands of crossings it is locked to. If it loses lock, it will simply

go to the next crossing that comes along. On

a phase chart this will show up as a shift in phase equal to one cycle of the carrier.

For WWVB at 60 kHz, a cycle of phase

equals 16.67 microseconds. If the chart

recorder/receiver combination is producing a chart that is 50 microseconds across, the pen

will move about 1/3 of the way across the

chart for each cycle change. Such changes can

be identified and must be ignored during

calibration runs. This is due to the fact

that phase charts are always ambiguous by an amount equal to the period of a carrier cycle.

How does the user tell whether the re- corded phase difference indicates a change in his oscillator, the path, or the transmitted

108
3. PHASE DEVIATIONS FOR WWVB AND OTHER NBS-MON ITORED BROADCASTS

WWVB (60 kHz)

Values given for WWVB are the time difference between the time markers of the UTC(NBS) time scale and

the first positive-going zero voltage crossover measured at the transmitting antenna. The uncertainty

of individual measurements is +0.5 microsecond. Values listed are for 1500 UTC.

Omega, North Dakota (13-1 kHz) and Omega, Hawaii (11.8 kHz)

Relative phase values are given for VLF stations and only the change from the previous available day's

reading is published. Days when the data were satisfactory but readings were not taken (for example,

on weekends or station maintenance days) are marked (-). If data are lost, continuity is also lost and the indication is (*) , which means that reading cannot be compared to the previous day.

LORAN-C (DANA), Dana, Indiana (100 kHz) and LORAN-C (FALLON), Fallon, Nevada (100 kHz)

Values for Loran-C (Dana) and Loran-C (Fallon) are the time difference between the UTC(NBS) time pulses

and the 1 pps output of the Loran-C receiver. Uncertainty in the measurements is +_ 0.1 microsecond.

JULY

MJD

UTC(NBS) - WWVB ANTENNA PHASE

(60 kHz)

(in us)

UTC(NBS) - RECEIVED PHASE (in us) OMEGA

(11.8 kHz) OMEGA

(13.1 kHz) LORAN-C (DANA)(100 kHz) LORAN-C (FALLON)(100 kHz)

1 43690 5.90 (-) (-) (-) (-)

2 43691 5-99 (-) (-) (-) (-)

3 43692 6.08 + 2.5 + 0.1 73684.84 3949.27

4 43693 5.98 (-). (-) (-)

5 43694 5.88 - 1.5 - 0.1 73685.21 3949.42

6 43695 5.83 + 2.3 + 0.1 73685.22 3949.47

7 43696 5.54 - 3.2 - 0.8 73685.27 3949. 42

8 43697 5.61 - 8.8 (-) (-) (-)

9 43698 5. 68 (-) (") (-) (-) 10 43699 5.75 - 9-6 + 0.3 73685.41 3949.60

1 1 43700 5.93 + 6.4 -0.1 73685.40 3949.59

1 2 43701 5.66 + 4.5 - o.2 73685.41 3949 55

13 43702 5.81 + .4.5 + 0.5 73685.56 3949.67

1* 43703 5.67 + 0.6 0.0 73685.59 3949.69

15 43704 5.70 (-) (-) (-) (-) 16 43705 5.72 (-) (-) (-) (-)

17 43706 5.75 + 0.2 + 0.3 73685.69 3949.77

18 43707 5.66 + 1.5 - 0.2 73685.75 3949.83

19 43708 5.74 - 3-1 + 0.1 73685.80 3949.85

20 43709 5.79 - 0.5 - 0.1 73685.89 3949.81

21 43710 5.72 - 1.0 - 0.1 73685.92 3949.79

22 43711 5.78 (-) (-) (-) (-) 23 43712 5.84 (-) (-) (-) (-) 24 43713 5.90 + 3-1 - 0.1 73685.93 3949.91

25 43714 5.86 - 0.8 - 0.1 73686.00 3949.96

26 43715 5.57 + 1.0 + 0.4 73685.96 3950.06

27 4371.6 5.71 + 0.3 + 0.2 73686.1 1^- 2950. 15 ^^J8 ^ —<i^^^7J^ "

FIGURE 6.2. NBS DATA PUBLISHED IN NBS TIME AND FREQUENCY BULLETIN.

109
U. S. NAVAL OBSERVATORY

WASHINGTON, O.C. 20390

26 JUL. 1979

DAILY PHASE VALUES AND TIME DIFFERENCES SERIES 4 THE TABLE GIVES: UTC(USNO MC ) - TRANSMITTING STATION

NO. 651

UNIT = ONE MICROSECOND

JUL .

FREQUENCY NV KHZ (UTC)

MJD

16 44070

17 44071

18 44072

19 44073

20 44074

21 44075

22 4". 076

2 3 £ \ C " ? 24 4407c'- 25 44079

FREQUENCY

KHZ (UTC)

/9970

PAC.

LC/4990

CEN. PAC. LC/9930

E. COAST

LC/7970

NOR. SEA LC/7990

MED. SEA

LC/7930

N. ATL. LC/9990

N < PAC.

JUL . MJD

19 44073

20 44074

21 44075

22 44076

23 44077

24 44078

25 44079

1 00 100 1 00 1 00 1 00 100 10 0 -1.2 C .7 2 .93 — 2 .2 0.2 — 1 . 8 _ 1 ^ c 1 • o -1.3 0.5 2.93 -2 .4 0.1 -1 .7 -1.5

-1.3 0.5 2 .88 -2 .4 0.1 -1 .9 -1.3

-1.4 0.5 2.86 -2 .4 0.1 -1 .8 -1.4 -1.4 0.5 2.85 -2 .5 0.0 -1 .9 -1.5

-1.3 0.6 2.87 -2 .3 0.2 -2 .0 -1.8 -1.4 0.6 2.83 - -2 .1 -1.8

-1.3 0.6 2.90 -2 .2 0.3 -2 .2 -1.8

-1.4 0.6 2.88 -2 .2 0.4 -2 .0 -1.9 -1.3 2.88 -1 .9

3 1 7 5 6 4 2 LC/5990 LC/7960 O/H O/ND Q/ND O/A O/L O/T GBR W.C.CAN GULF AK 11.8 13.1 13.6 13.6 13.6 13.6 1 6. 0 100 1 00 2 5MS + 6MS+ 6MS + 30MS + 25MS + 1 1MS+ 1 9MS+

2.2 1.8 954 425 421 576 141 622 515

2.2 1.0 054 425 421 576 142 622 514

2.1 1.5 o ; i 424 421 576 143 622 514

1 .6 95j 425 421 574 143 622 514

2.1 1.7 953 425 421 575 141 622 515

1.7 953 426 421 575 141 622 514 954 425 421 575 141 622 514

LC/9940

W.C.USA

100 -5.3

-5.4

-5. 4 -5.4 -5.3

-5.3

-5.3

-5.3 -5.3 -5.3

8 NLK 18.6

12MS +

335 341(1)

340

340

340

340

339

(1) NLK 19 JUL. AFTER MAINTENANCE PERIOD STEP ABOUT PLUS 6

(2) PROPAGATION DISTURBANCES WERE OBSERVED NEAR THE FOLLOWING TIMES:

20 JUL. 1940/5

21 JUL. 1110/4. 1340/6

22 JUL. 1310/5, 1440/5, 1735/4, 1925/5, 2 110/6

23 JUL. 1435/4, 1645/5, 2055/5

24 JUL. I'z I >/S

25 JUL. 1600/4, 1735/3.

(3) OMEGA STATIONS OFF-AIR TIMES (FROM COAST GUARD ONSOD, WASHINGTON, DC):

JAPAN 1 7 JUL. 0039 TO 0041 UT , 0214 TO 0217 UT, 0232 TO 0234 UT. 0918 TO 0920 UT LIBERIA 1 7 JUL. 0240 TO 0242 UT , 0305 TO 0403 UT

21 JUL. 1 610 TO 1 622 UT LA REUNION 1 7 JUL. 0423 TO 0425 UT , 071 7 TO 0 73 1 UT, 0735 TO 0737 UT ,

1 1 36 TO 1 1 59 UT, 1 247 TO 1306 UT, 1412 TO 1415 UT

1 8 JUL. 0030 TO 0032 UT , 06 32 TO 0645 UT, 1036 TO 1045 UT

19 JUL. 0509 TO 0512 UT, 1 059 TO 1 105 UT, 1439 TO 1441 UT

FIGURE 6.3. USNO PHASE DATA, PUBLISHED WEEKLY.

110
signal? The answer is that he knows from

experience! If you plan to use VLF or LF signals for frequency calibrations, you must

become very familiar with the characteristics

of the signals you are using. For accurate

calibrations, you must obtain the station data

from NBS or the USNO. Battery backup for

receivers is highly desirable; in fact, almost

necessary if you plan to calibrate precise

oscillators over a period of many days. All of this may sound like a hopeless task, but it

isn't. The results are worth the effort when

you consider that you can perform calibrations

up to 1 part in 10 11 in 24 hours.

6.4 FIELD STRENGTHS OF VLF-LF STATIONS

The field strength of the WWVB signal has

been measured along nine radial paths from the

station. These measurements are summarized on the field contour map shown in figure 6.4. Not shown is the nonlinear field gradient

between the 500 microvolt per meter contour

and the 100 microvolt per meter contour.

Destructive interference occurs between

the first hop skywave and the groundwave at

approximately 1200 km (750 miles) from the

station. On some radial paths, this dip in

field intensity is quite severe and has proven

to be the cause of signal loss at certain

times. The distance between the station and

this null varies from day to night. It is also seasonal. The sharpness of the null is much less pronounced in the winter. The field

intensity, in general, is slightly higher

during the winter months. Shown in figures

6.5 and 6.6 are the field strengths along

radial s to Brownsville, Texas and Nantucket,

Massachusetts, both measured in September.

Other radial plots are available to interested

users.

The signal strength of the L). S. Navy

stations is usually very high unless they

are undergoing repairs. The Omega navigation stations have a high power output also. But Omega is time shared; that is, the eight

transmitters take turns broadcasting on the same frequencies. This has the effect of

reducing the available signal power and

commutation is required; that is, you must

have a means of turning the receiver on only

when the station you want to track is transmitting.

FIGURE 6.J». MEASURED FIELD INTENSITY COUNTOURS : WWVB @ 13 kW ERP.

Ill
ioo r

MILES FROM ANTENNA

FIGURE 6.5. MEASURED FIELD INTENSITY, NANTUCKET, MASSACHUSETTS RADIAL, WWVB 60 kHz.

112
6.5 INTERFERENCE receive WWVB. Another method is to produce a unidirectional antenna by combining the volt- ages induced in a loop and a whip. If these

Some WWVB users along the U. S. Atlantic two voltages are equal and in phase, the

coast have experienced interference from a resultant pattern is a cardioid. Since these

British standard frequency radio station, MSF. voltages are induced into the whip by the

Some years ago, MSF was not only lower pow- electric field, and into the loop by the

ered, but it was also offset in frequency from magnetic field, the phase of the whip voltage

WWVB. However, it now radiates 25 kW of power must be shifted by 90° before combination,

on 60 kHz. There are various ways to tell if Interested readers are advised to consult a one is receiving MSF instead of WWVB. The 45° suitable antenna manual or to discuss their

phase shift (discussed in Section 6.6.1, fig. problems with the receiver manufacturer.

6.9) should be quite apparent if WWVB is being

received and phase-tracked.

6.6 USING WWVB FOR FREQUENCY CALIBRATIONS

One solution is to use a directional

antenna. If a loop is being used, the MSF

signal can be nulled. Eastern U. S. receiving Our discussion of calibrations using lowlocations from Boston south that are using frequency signals begins with WWVB (60 kHz),

loops should be able to null MSF and still Frequency calibrations usually involve phase

FIGURE 6.7. WWVB PHASE RECORDING USING A FIGURE 6.8. TYPICAL WWVB PHASE RECORDING.

HIGH-QUALITY OSCILLATOR.

113
recordings of the received signals. Typically, a special tracking receiver is used to compare a local oscillator with the received WWVB signals. The output of the receiver can

be displayed on its front panel digitally or

as a meter reading. However, this discussion

is limited to making a phase record on a chart

recorder. Often, these recorders are built

into the receivers. Since we are looking at a phase recording of the received signal, we will see the effect of any path changes. In addition, we will see our oscillator phase

change relative to the WWVB signal.

The amount of phase change depends on the

relative difference between our oscillator and

the cesium oscillator controlling the WWVB

signal at the transmitter. The sample phase

chart shown in figure 6.7 has very little

oscillator drift. This is because a quality

oscillator was being measured. A more typical

record would have a whole series of lines

crossing the chart as in figure 6.8. This

poses a dilemma for the user. If the oscillator being calibrated is far off frequency

from WWVB, he needs either a fast chart or one that displays many microseconds of phase across the width of paper. For high-quality

frequency sources, the paper can be slowed

down or the chart width, in microseconds, can

be reduced. The user is encouraged to study

the manuals for the particular receiver he is using. If the user is planning to buy a receiver, he should consider the problem of

what the record is going to look like before

selecting a particular receiver/recorder

combination.

Measurements made with VLF-LF signals

should be done when the path from transmitter

to receiver is all in sunlight. An all dark

path would be second choice. The sunrise/

sunset phase shifts (called diurnals) will

affect the accuracy of calibrations and mea- surements should not be made during these

times. There are other sudden phase shifts

due to particles entering the ionosphere that

can affect the signal. These usually last

only ten minutes or a half-hour. This ex- plains the advantage of having a chart that

runs at least several hours for a calibration.

During sudden phase shifts, the signal level may drop. Most receivers have a meter for

signal strength, but the more elaborate re- ceivers are able to chart record the signal

amplitude. This is a very useful record if

the calibrations being made are to be within

parts in 10 10 or better. In other words, you

need all the help you can get if you are to make the very best measurements.

As mentioned earlier, some receivers use front panel displays to indicate frequency

offset. Later in this chapter, we mention

methods that use oscilloscopes for frequency

calibration. Even a zero-beat or Lissajous

method could be used with the WWVB signal.

Remember that what you are getting from WWVB

is a standard frequency signal that started

out from a nearly perfect frequency source. When it arrives, it has been contaminated by

path variations and noise. A chart recorder

giving a history of the path and oscillator

changes is very handy to have. To get the

best resolution from the WWVB signals, a chart

record is mandatory. The results are worth

the effort. As with any measurement, extreme

care must be exercised to realize the best

results.

6.6.1 PHASE-SHIFT IDENTIFICATION

Many WWVB users have phase-tracking

receivers which make use of the stabilized

FIGURE 6.9. WWVB PHASE PLOT SHOWING

PHASE-SHIFT IDENTIFICATION.

114
carrier to provide a frequency reference for

calibration purposes. In order for the user

to identify WWVB by its phase track alone, a 45° phase shift is programmed into the trans- mitted phase. This shift occurs at 10 minutes

past the hour and returns to nominal value at

15 minutes past the hour. Three hours of

strip chart at a typical user site might look

something like figure 6.9.

6.6.2 METHODS OF FREQUENCY COMPARISON

Having listed some of the problems in LF propagation, we can now turn our attention to

actual calibration methods. There are two ways to compute relative frequency from the

plot in figure 6.1. One can work with the

slope of the daytime plot--the straight

portion—or one can take the single values at discrete points one day apart. On the figure,

the 2200 UTC values for June 3 and June 4 are subtracted to give a value of 1 microsecond

change for the 24-hour period. The relative

frequency, Af/f , is the same as the time

offset, At/T . In this case:

Af _ At _ 1 us _ 1 us

f T 1 day 86,400 x 10

6 us

= 1.16 x 10

When the relative frequency is large, a useable value can be computed using only a few

hours of the daytime slope. If the relative

frequency is very small, many days may have to

be used in the computation. This is especially true when the radio propagation stability

is poor or the signal is noisy. If many days

are involved in the measurement, cycle jumps

and transmitter phase shifts entered into the

phase plot must be carefully taken into account.

If the receiving site has no phasetracking receiver available, other schemes may

be used. If a tuned radio frequency receiver

is available, the actual 60 kHz wave may be

observed on an oscilloscope. If the oscilloscope is triggered by a local clock, the

position on the oscilloscope face of a par- ticular cycle will change with time. The amount of the change in any given time will

provide a value of At/T from which the relative frequency may be computed as before. A standard reference 60 kHz or submultiple, if

available, may be used to trigger the

oscilloscope.

Electronic counter techniques can also be

used for frequency calibrations. Without

going into detail, these techniques involve

the use of the received signal as one input to

a counter while the oscillator being calibrated is used as the other. Manufacturers of

receivers and counters discuss these techniques in their manuals and application notes. You may also refer to chapters 1 and 4 of this

book, which talk about counters and their

uses. Keep in mind that the accuracies you

can get with oscilloscopes and counters will

generally be much less than those obtained by

using tracking receivers. The cost will be

less too. Also, a quick calibration on a counter cannot identify sudden phase changes,

and you lose the big advantage of having a chart record of the station performance.

6.7 USING OTHER LF AND VLF STATIONS FOR TIME AND FREQUENCY CALIBRATIONS

6.7.1 THE OMEGA NAVIGATION SYSTEM

The Omega Navigation System is composed

of a group of VLF radio stations operating in

the 10 to 15 kHz range. Each station timeshares common frequencies used for navigation.

In addition, each station may transmit some frequencies unique to that particular station.

See Table 6.1 for details.

If one wishes to use an Omega station for

frequency calibration, a phase-tracking

receiver is highly recommended. If one of the

navigation frequencies is to be used, then an Omega commutator must also be used. This is a device that turns the phase-tracking receiver

on and off at the proper times to receive only

the desired Omega station.

The frequencies and the format segments

of the Omega stations are derived from cesium

beam oscillators. The USN0 monitors and

reports the Omega stations' phase values.

These stations radiate a nominal 10 kW of

power. This power level should be sufficient

to allow the user to receive at least three

stations no matter where he is located.

A. Operating Characteristics of Omega

Omega transmitting stations operate in the internationally allocated VLF navigational

band between 10 and 14 kHz. This very low

transmitting frequency enables Omega to pro- vide adequate navigation signals at much

longer ranges than other ground-based navigation systems.

The operating characteristics of the

system can be categorized as follows: signal

format and control, the requirement for lane

identification typical of phase comparison

115
TABLE 6.1. CHARACTERISTICS OF THE OMEGA NAVIGATION SYSTEM STATIONS

STATION LOCATION

LATITUDE,

LONGITUDE

POWER

(kW) ANTENNA

CARRIER

FREQUENCIES* ACCURACY

OMEGA

fi/N

ALDA,

NORWAY

66° 25' N 13° 09' E 10 OMNI- DIRECTIONAL

10.2 A 11-1/3 C 13.6 B

-12

5 x 10

OMEGA

Q/L

MONROVIA, LIBERIA

06° 18' N 10° 40' W

10 OMNI- DIRECTIONAL

10.2 B 11-1/3 D 13.6 C

-12

1 x 10

1

OMEGA

n/H

HAIKU, OAHU, HAWAII

21° 24' N 157° 50' W

10 OMNI- DIRECTIONAL

10.2 C 11-1/3 E 13.6 D

1 x 10' 12

OMEGA

n/ND

LA MOURE, NORTH DAKOTA

46° 22' N 98° 20' W

10 OMNI- DIRECTIONAL

10.2 D 11-1/3 F 13.6 E

1 x 10" 12

OMEGA

fi/LR LA REUNION

20° 58' S 55° 17' E 10 OMNI- DIRECTIONAL

10.2 E 11-1/3 G 13.6 F

1 x 10" 12

OMEGA

fi/A

GOLFO NUEVO,

ARGENTINA

43° 03' S 65° IT W

10 OMNI- DIRECTIONAL

10.2 F 11-1/3 H 13.6 G

1 x 10" 12

OMEGA

n/T

TRINIDAD**

10° 42' N 31° 38' W

1 OMNI- DIRECTIONAL

10.2 B 11-1/3 D 13.6 C

1 x 10" 12

OMEGA

Q/J

TSUSHIMA IS.

JAPAN

34° 37' N 129° 27' E 10 OMNI- DIRECTIONAL

10.2 H 11-1/3 B 13.6 A

1 x 10" 12

*SEE FIGURE 6.10 FOR TRANSMISSION SCHEDULE OF VARIOUS FREQUENCIES.

**TRINIDAD IS A TEMPORARY STATION. PERMANENT STATION WILL BE IN AUSTRALIA.

systems and the process of handling errors attributable to signal propagation.

All stations now transmit three basic

navigational frequencies (10.2 kHz, 11-1/3

kHz, 13.6 kHz) more or less omnidirectionally.

In order to prevent interference, transmissions from each station are time-sequenced as shown in figure 6. 10.

This pattern is arranged so that during

each transmission interval (approximately 1 second), only three stations are radiating,

each at a different frequency. The duration

of each transmission varies from 0.9 to 1.2 seconds, depending on the station's assigned

location within the signal pattern. With

eight stations in the implemented system and a silent interval of 0.2 second between each

transmission, the entire cycle of the signal

pattern repeats every 10 seconds.

Besides the three basic navigational frequencies, other frequencies have been added to the Omega signal format. Original plans were

made to transmit two unique frequencies at each station for the purpose of inter-station

time synchronization but this requirement has been removed through use of highly stable

cesium frequency standards.

Present plans call for the incorporation

of a fourth navigation frequency, 11.050 kHz,

which will allow for lane resolution capa- bility as great as 288 nautical miles. In addition, a unique frequency transmission for

each station can be added which will aid in time dissemination by providing a beat frequency and a high duty cycle at that frequency. The changes depicted in figure 6.10,

which are not yet implemented, should be made

by 1979.

116
—__S>egment

Station -

A B C D E F G H

Norway (A) 10.2 [J 13.6 |_j 11 1/3 [J 12. T [J 12.1

+ |J H.05 |J 1 2 . 1 + [j 12.1

+ |J

Liberia (B)

Hawaii (C)

N. Dakota (D)

La Reunion (E)

Argentina (F)

Australia (G)* Area

Japan (H)

Tj\_\ 10 - 2 U

136 U111/3 U 12.0

+ U 12.0+ U

1105 U 12.0

+ U

n. sU ii. sU 10.2 (J

ile [J 11 1/3 [J ii .8

+ U

ii .8

+ (J n-os [J

11.05 [J 13.1

+ L| 13.

+ [J 102 [J 13.6 [J 11 1/3 [J 1 3 .1 + |j 13. I +

|_]

12. 3^] 11-05 [J 12.

+ jj 12.3

+ U

102 U

136 U11 1/3 U 1 2 .

+ |J

2.9"jJ 1 2.9

+ [J

11.05 [ 12.9

+ U

12.

+ |J 102 |J ll6 |J

11 1/3 [J

11 1/3 [J 13.

+ [J 13.

+ [J 11 05 U 13.0 + [J H-O^IJ 10.2 13.6 |J

13.6 [J11 1/3 [J 12.

+ [J 12.8+ [J 11.05 |J 1 2 . S " |_| 12. 8

+ |J 10.2 |J p- 0.9-1 p-1.0-H p-l.l-H 1.2 -—I

J*—

1.1 —H p-Q.9-H p—1.2 —H p- 1.0 -H

0.0

Trinidad temporarily

filling G slot

2.3 3.6 5 0 6.3 Time (seconds) Proposed full format is shown: — + is unique frequency at each

station — 11.05 is fourth navigation frequency

7.4 10.0

FIGURE 6.10. OMEGA SIGNAL TRANSMISSION FORMAT.

B. Synchronization Contro l

The Omega signal format is designed so that each station within the network can be

identified by the transmission of a particular

frequency at a prescribed time. In addition,

the synchronization of all transmissions is tightly controlled and the phase relationships

between all signals are maintained to within a few centicycles. With this high phase stability in the transmissions, the accuracy of a navigational fix is then primarily limited to the receiver and the accuracy of the navigator's propagation correction tables.

All Omega transmitting stations are syn- chronized by means of very stable cesium beam

frequency standards. These standards or clocks are referenced to the atomic time scale

which differs from Coordinated Universal Time

(UTC) more commonly in use. Thus, in 1978,

the Omega epoch or time reference is seven

seconds ahead of UTC since the yearly adjustments for earth motion have not been made to make Omega Epoch in agreement with UTC.

C. Propagation Characteristics

The propagation characteristics that

permit the use of Omega at great range also

introduce certain limitations. Two areas that

require special attention are normal time

variations and modal interference. Since

signals are propagated within the waveguide

formed by the earth and ionosphere, changes in propagation parameters such as velocity may be

expected as a result of changes in the ionosphere or ground.

The most obvious navigational ly undesirable variation is a daily or diurnal phase

change. Normal changes in illumination of the

ionosphere by the sun throughout the day may cause an uncorrected phase measurement to vary

as much as one complete cycle. Since these

variations are highly repeatable, prediction

and correction are possible.

Unpredictable short-term variations may

also occur. Ninety-five percent of the time,

these are small variations related to random

propagational variations whcih will not degrade normal navigational accuracy. Occasionally, however, large disturbances can occur as

a result of solar emission of X-ray or particle bursts. The emission of X-rays from the

sun occasionally cause a short-term disruption

of Omega signals which is referred to as a Sudden Phase Anomaly (SPA). The duration of a SPA is generally not greater than one hour but

an LOP may experience a shift of several

miles. SPA's occur with an average frequency

117
of about 7-10 per month. They usually affect

signals from only a few stations at a time

since X-rays from the sun tend to enter a limited illuminated portion of the earth's

surface.

The release of a large quantity of pro- tons from the sun, although an infrequent

occurrence, produces what is known as a Polar

Cap Disturbance (PCD). The effect of a PCD may be to shift an LOP 6-8 miles for a period

of several days. This disturbance generally

lasts for several days and varies in magnitude

during the period. PCD's affect only those

transmissions involving arctic propagation

paths. Because of its possible long duration

and large LOP shift, PCD notices are broadcast

as navigational warning messages.

Modal interference is a special form of

signal interference wherein the various waveguide modes of signal propagation interfere

with each other and irregularities appear in the phase pattern. Ideally, one mode would be completely dominant at all times and the

resultant phase grid would be regular. In practice, competing modes do not completely

disappear and three situations are recognizable:

1. If the competing mode is very small,

then the dominant mode will establish a nearly regular phase pattern

as is intended, and usually this is what happens during the day.

2. A second possibility is that the

competing mode may be almost equal

to the dominant mode.

3. The potentially serious case is that

in which modal dominance can change.

This may occur, for example, if one mode is dominant during the day and

a second mode at night. Clearly,

somewhere during sunset and sunrise,

the transitional period, the two modes must be equal. Depending upon phasing of the modes at equality,

abnormal transitions may occur in which cycles are "slipped" or lost. Positional errors of a full wavelength are possible under such

conditions and use of a station so affected should be avoided. If this

is not possible, particular atten- tion must be given to proper lane

identification.

Propagation Corrections (PPC's) must be

applied to each Omega receiver reading to compensate for ionospherical ly induced signal

variations and thereby improve position fixing

accuracy. Omega propagation correction tables

for each transmitting station (A through H)

contain necessary data for correcting Omega

receiver readouts affected by prevailing

propagation conditions relative to the nominal

conditions on which all charts and tables are

based. A brief introduction, which also

describes the arrangement and application of

the corrections together will illustrative

examples, precedes the tabular data within

each PPC table. For more detailed information

on the Omega Charts and Tables as well as information on obtaining these charts, write

to: Defense Mapping Agency, Hydrographic

Center, Attn: Code DSI-2, Washington, DC. 20390.

D. Omega Notices and Navigational Warnings

As with other navigational aids, information is disseminated concerning station offthe-air periods, VLF propagation disturbances,

and other pertinent data affecting usage of

the system. Status information on Omega sta- tions can be obtained on the telephone by calling (202) 245-0298 in Washington, D. C.

Omega status messages are also broadcast

by various time service stations. NBS sta- tions WWV and WWVH transmit a 40-second mes- sage at 16 minutes and 47 minutes after the

hour, respectively. Norway Radio Station

Rogaland broadcasts notices in international morse code on HF 4 times daily. Other sta- tions in the global time service and maritime

information network are expected to be added

in the future.

In addition to the regular issued notices

on station off-air periods, major planned

maintenance may take place in the months

listed for each station:

ARGENTINA MARCH

LIBERIA APRIL

HAWAII MAY

LA REUNION JUNE NORWAY JULY TRINIDAD FEBRUARY

NORTH DAKOTA - SEPTEMBER

JAPAN OCTOBER

The actual off-air times are disseminated

as noted above sufficiently in advance and may vary from a few days to several weeks depending on the maintenance or repairs required.

6.7.2 DCF 77, WEST GERMANY (77.5 kHz)

DCF 77 is located at Mainflingen, about

25 kilometers southeast of Frankfurt/Main. It

is operated by the Physikalisch-Technische

Bundesanstalt (PTB) in Braunschweig.

118
DCF 77 transmits Central European Time

(CET) or Central European Summer Time (CEST),

which is equal to UTC(PTB) plus 1 or 2 hours

respectively. The carrier phase is controlled

with respect to UTC(PTB); phase time changes

are kept smaller than 0.5 us. The station

transmits continuously except for short interruptions because of technical faults or maintenance. Longer breaks may be experienced

during thunderstorms.

A. Time Signals

The carrier is modulated by means of

seconds markers with the exception of second

number 59 of each minute which signifies that

the next marker will be the minute marker. At

the beginning of each second (with the exception of the 59th) the carrier amplitude is reduced to about 25% for a duration of 100 or

200 ms. The beginning of the decrease of . the

carrier amplitude characterizes the exact beginning of the corresponding second.

The seconds markers are phase-synchronous

with the carrier. In general, the uncertainty

of the received DCF 77 time signals is large

compared to that of the emitted time signals.

This results from the limited bandwidth of the

transmitter antenna, skywave influence and

possible interference. At a distance of some hundred kilometers, a time signal uncertainty

of less than 0.1 ms is achievable.

B. Time Code

The seconds marker durations of 100 and

200 ms correspond to binary 0 or 1

, respectively, in a BCD code used for the coded

transmission of time and date.

S - (second marker 20), a 200 ms marker,

designates the start of the time

information.

SZ - Summer Time bit.

A - Announcement of an approaching change

from CET.

R - Second marker 15 has a duration of 200 ms

if the reserve antenna is used. (This may

result in a small phase time change of the

carrier due to the different location of

the antenna.

PI, P2, P3 - Parity check bits.

FIGURE 6.11. DCF 77 TIME CODE FORMAT.

119
There are three groups of time information, each followed by a parity check bit P:

PI = NUMBER OF THE MINUTE

P2 = NUMBER OF THE HOUR

P3 = NUMBERS OF THE CALENDAR DAY, THE DAY OF THE WEEK, THE MONTH AND THE YEAR

Counting the binary "ones" of the information

group concerned and of the corresponding parity check bit yields an even number.

In the case of the transmission of Legal

Time in the form of CEST, seconds marker num- ber 17 has a duration of 200 ms. During one hour before the change from CET to CEST or from CEST to CET, seconds marker 16 has a duration of 200 ms, thus announcing the

approaching change. The coding is shown in figure 6.11.

General information about DCF 77 is shown

in Table 6.2.

6.7.3 HBG, SWITZERLAND (75 kHz)

HBG in Prangins, Switzerland is operated

by the Observatoire Cantonal in Neuchatel

The useful range for clock synchronization is 3000 kilometers.

A. Signal Format

The carrier is interrupted for 0.1 second

at the beginning of each second. An additional interruption occurs from 0.2 to 0.3 second

at the beginning of each minute.

B. Time Code

No continuous time code is transmitted.

During the weekends of changes to daylight

saving time or back to standard time, a special code is transmitted, including a separate

4-bit address for each European country.

Additional details about HBG are shown in Table 6.2.

6.7.4 JG2AS/JJF-2, JAPAN (40 kHz)

The Radio Research Laboratories in Tokyo

operate JG2AS and JJF-2. JJF-2 consists of

telegraph signals; JG2AS broadcasts in the

absence of these telegraph signals. The UTC time scale is broadcast. The call sign is transmitted twice each hour (at 15 and 45 minutes) in Morse code.

(1) Hourly modulation schedule

15 45 O min.

second pulses call sign

Call sign Three times by the Morse code

(2) Wave form of second pulses

mark

space

f- 500 ms

exact time (second)

500 ms — 4" 500 ms ~

exact time (second)

(3) Identification of minute signal

ESI

^ r222_ n a ezl

i 58s

I 59s

I (100 ms duration)

FIGURE 6.12. JG2AS EMISSION SCHEDULE,

120
The 40 kHz carrier frequency (of 500 ms duration) is broadcast at the beginning of each

second except for the 59th pulse in each minute, which is 100 ms long.

The JG2AS emission schedule is shown in figure 6.12. Other details are shown in Table

6.2.

6.7.5 MSF, ENGLAND (60 kHz)

MSF is located at Rugby, United Kingdom.

It is operated by the National Physical Laboratory in Teddington, Middlesex. Transmissions are continuous except for a maintenance

period from 1000-1400 UTC on the first Tuesday

of each month. MSF transmits both a fast code

and a slow code.

A. Fast Code

Calendar information; i.e., month, day,

hour, and minute are given, the complete data

burst occupying a total of 330 ms at the fast

bit rate of 100 bits/second in the 500 ms interval following the minute marker. The

full format of the code is shown in figure

6.13. It includes a control pulse and marker

bit to initiate recognition systems in the

receiver, 24 data bits, 2 parity checks (odd

parity) and a British Standard Time (BST)

indicator (UTC + 1 hour) so transmissions are advanced when BST is in effect.

B. Slow Code

The slow code operates at a 1 bit/second

rate. The sequence of signals in each minute

of the transmission is shown in figure 6.14. Pulse length modulation is used, the carrier

being interrupted for 100 ms for a logic "0" and for 200 ms for a logic "1." Full calendar

and time information is provided; i.e., year,

month, day-of-month , day-of-week, hour, and

minute referred to the minute marker immediately following the code. The minute itself

is identified by an 8-bit precursor transmitted in seconds 52 to 59 in the form of the

sequence 0111 1110. Exhaustive trials by

computer simulation have confirmed that this

sequence is unique and will not occur as part

of the code proper at any time; the identifier

is also well protected against a code with one

or two errors present.

The introduction of a minute identifier

enables a very simple form of receiver/decoder

to be developed making use of a shift register

whose length can be adjusted to acquire as little or as much of the code as is desired. For example, if the register is only 8 bits

long, this will be sufficient to identify each

minute but will exclude all the other data,

which will pass through the shift register but

will not be recognized since the register

responds only to the combination 0111 1110. When this is received, the next carrier interruption is identified as the minute marker.

Similarly a register 21 bits long will extract

hour and minute information while the maximum

length of 43 bits is necessary for a full

decoding of calendar and time-of-day information.

The code described so far contains the

basic, primary information at what may be

called the first level of coding. However,

further subsidiary information is also encoded

at a second level where logic "1" is indicated

by a carrier interruption in the interval 200

to 300 ms following a seconds marker. This

form of coding, as shown in figure 6.14, is applied within the identifier at seconds 54 - 58, inclusive, and includes four parity checks

and also a BST indicator. The latter indi- cates the operation of BST, as in the fast

code, but with the difference that the clock

generating the slow code will be set 1 hour

ahead of UTC when British Summer Time is in effect. The second level of coding also

embraces the DUT1 indication in seconds 1-16,

inclusive, and brings this information into

the same format as the other signals.

The adoption of a minute identifier

immediately preceding the minute creates difficulties in introducing a leap second

since a positive leap second would be identified as the following minute marker while a negative leap second would remove the last bit

in the identifier itself. When necessary,

therefore, the leap second is inserted following second 16 where it can be accommodated

without disturbance to the coded sequence and

identifier.

Additional details about MSF can be found

in Table 6.2.

6.7.6 U.S. NAVY COMMUNICATION STATIONS

Some of the Navy communication stations

are commonly used to steer frequency sources:

NAA, NDT, NLK, NPM, NSS, and NWC. However,

use of these stations for frequency calibration is strictly secondary to their primary

function of military communications. These

stations are subject to frequency changes,

outages, and changes in schedule. Changes

are announced in advance to interested

users by the USN0. All stations operate

121
60kHz

C

(— H —^ ( M ^ 201084214020108421 20108421 108421

- OP

RB

0 25 50

M

R

100

CARRIER

HOUR MINUTE

BST

PB

200

D

NR, B

300 330

DAY MONTH MARKER BIT

60kHz + C

P, B 0, P M, R

500 ms

PARITY BIT CONTROL PULSE MINUTE MARKER

FIGURE 6.13. MSF 60 kHz FAST CODE FORMAT.

_L I I I I I I I I I 40 20 10 8 4

I I I I I I

2 1 10 J L

4 2 1 _L

0 1 2 3 4 5 6 7 8 9 10 1 1 1 2 1 3 14 1 5 16 1 7 1 8 1 9 20 21 22 23 24 25 26 27 28 29

k DUTI J 4D

DOW

DOM r ~\ r Nl

20 108 4 2 1 4 2 1 20 1034 2 1 40 20 1034 2 1 0 1 1 1 1 1 1 0

' JL_J I I I I I I I I I I I I I I I I I L_l I I L

'

30 31 32 33 34 35 36 37 33 39 40 41 42 43 44 45 46 47 43 49 50 51 52 53 54 55 56 57 58 59

t —| p- 100 ms G T

1 - ~U o ~L

200 ms

300 ms

1 - V

2 - V

(DUTI)

BST

H + N DOW L DOM —

1 - V

2 - V

1 - V

2 - V

S: SECONDS Y: YEAR G: LOGIC N, I: N: MINUTE T: DATA L: LEAP SECOND INSERTION S, R:

M: MONTH V: LEVEL P: PARITY CHECK BITS DoM : DoW :

FIGURE 6 14. MSF 60 kHz SLOW CODE FORMAT

MINUTE IDENTIFIER

SECONDS MARK DAY-OF-MONTH

DAY-OF-WEEK

122
continuously except for maintenance periods,

which are shown below:

STATION MAINTENANCE SCHEDULE

NAA 1400-1800 UTC each Monday; rescheduled for preceding Friday if Monday

is a holiday. Half-power operations

on Wednesday and Thursday from 1200-

2000 UTC. Scheduled/unscheduled

half-power operations as required.

NDT 2200-0600 UTC Thursday/Friday except

first Thursday/Friday of each month,

then 2200-0800 UTC.

NLK 1700-2200 UTC first & third Thursday

of each month.

NPM 2200-0200 UTC each Thursday and

Friday.

NSS 1200-2000 UTC every Tuesday.

NWC C001-0400 UTC every Wednesday.

Other details of these stations are shown in Table 6. 2.

6.7.7 OMA, CZECHOSLOVAKIA (50 kHz)

0MA in Liblice, Czechoslovakia, transmits

the UTC time scale. The carrier is phaselocked over a distance of 30 kilometers to the

phase of UTC (TP), the Czechoslovakian. time

scale. As a result, no long-term drift with

respect to UTC(TP) can occur. The rms phase

deviation is of approximately 100 ns; however,

during strong winds the phase excursions may

reach 500 ns since the antenna is not servocontrol led.

A. Time Code

The format of the OMA time code is shown

in figure 6.15. The signal is both amplitude

and phase keyed. Al seconds ticks of 0.1 sec- ond duration are prolonged to 0.5 second to identify minutes. A time code representing

the time of day of CET (Central European Time = UTC + 1 hour) is transmitted by means of

phase-reversing the carrier for 0.1 second in

corresponding seconds of the current minute.

Thus, in each minute a series of 4 pulses (PI to P4) is transmitted.

The pulses, together with the Al minute

pulse (M) form 4 time intervals which, mea- sured in seconds, represent 4 numbers: units

of minutes (UM), tens of minutes (TM), units

of hours (UH), and tens of hours (TH) of CET. As shown in figure 6.15, the duration of each

interval is prolonged by 1 second so that the

number 0 may be transmitted as a 1 second

interval. In the illustrated case, 10 hours

01 minute of CET is transmitted which corre- sponds to the following intervals:

10 h 00 m TIME OF DAY (CET)

-I r

10 h 01 m

0 I

1

l

I

I

I I

1 0. 5s > 0.1s

OMA

^' J""

AM SECOND MARK

AM MINUTE MARK

54 55 56 57 58 59 0

0.1s

"pi

1

P2X P3 P4

(UM+ls)=2s

Mil « /—*- TM+ lsHs( UH+ls > = 15 (TH + ls)=2s

HI56s DETAIL P2

JM = UNITS OF MINUTES

TM = TENS OF MINUTES

JH = UNITS OF HOURS

"H = TENS OF HOURS

100 150 f 250 ms

PHASE REVERSAL

FIGURE 6.15. TIME CODE FORMAT OF OMA - 50 kHz.

123
P2 - PI (in seconds) = UM + 1

P3 - P2 = TM + 1

P4 - P3 = UH + 1

M - P4 = TH + 1

Completion of the code by the date is planned

for the near future.

Further details about OMA are shown in Table 6.2.

6.7.8 RBU (66-2/3 kHz) AND RTZ (50 kHz), USSR

Table 6.2 gives details about these sta- tions. However, the contents of the UT1 cor- rections are described below:

The information about the value and the

sign of the DUT1 + dUTl difference is trans- mitted after each minute signal by the marking

of the corresponding second signals by additional impulses. In addition, it is trans- mitted in accordance with CCIR code. Additional information on dUTl is given, specifying more precisely the difference UT1 - UTC down to multiples of 0.02 second, the total

value of the correction being DUTl + dUTl

Positive values of dUTl are transmitted by

the marking of "p" seconds markers within the

range between the 20th and 25th seconds so

that dUTl = +0.02 second x "p." Negative

values of DUTl are transmitted by the marking

of "q" seconds markers within the range be- tween the 35th and 40th seconds so that dUTl = -0.02 second x "q."

6.7.9 VGC3 AND VTR3, USSR

These stations operate at 25.0, 25.1,

25.5, 23.0, and 20.5 kHz. Each station

transmits 40 minutes 4 times/day:

VGC3 from 0036 to 0117, 0336 to 0417, 0636

to 0717, and 1736 to 1817 hours UTC;

VTR3 from 0536 to 0617, 1436 to 1517, and

1836 to 1917 hours UTC.

Two types of modulation signal are trans- mitted during a duty period: (1) Al signals

with carrier frequency 25 kHz, duration

0.0125, 0.025, 0.1, 1, and 10 seconds with

repetition periods of 0.025, 0.1, 1, 10, and

60 seconds respectively; and (2) AO signals

with carrier frequencies 25.0, 25.1, 25.5,

23.0, and 20.5 kHz. The phases of these signals are matched with the time markers of the

transmitted scale.

No UT1 information is transmitted. See Table 6.2 for further details about these

stations.

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-> > -J Z - - 3 _i - - O - - O <C i—i inoi o -=t en <C CD O "_l st OO 3" CTl ~r- 00 CO co or O 1— 1— 00 LU OO CD LO O '— st <c < >- 1— LU 1— CJ O <_> Or or CO or or <C 3 o o Z Q£ 0 O OT Q_ OO O 0 i—i u: o o 00 00 0 0 00 0 0 =t 00 0 0 or 00 0 0

=) «£ r— 00 z <: oo lo O <C =3 i— st- —1 C_) o O 00 LO 00 or 00 C\J St nz 00 o<t LU OO LO 00

1—

N

00 00

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s: CO 1— 1— z z z o or or > >

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lu n: a o

1-1 o o or a x: <c 1—

LU LO LO O O CJ ID

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st C

126
6.8 MONITORING DATA AVAILABILITY

No matter which station is chosen as a oasis for cal ibration--WWVB, Omega, or VLF--

the user must be on a mailing list to receive

notices of changes in operation schedule.

«/WVB currently operates 24-hours a day every

jay. No changes in operating format are made

without advance notification. The Omega

stations do schedule maintenance outages and

some low-power operations. As the system

oecomes fully operational, these should

diminish. Announcements regarding the status

if the various Omega stations in the Omega

Navigation System are given hourly on both WWV

and WWVH. See figure 12.1 in Chapter 12 for announcement times. The Navy VLF stations are

the ones most likely to change their mode of

operation without notification.

Currently, NBS and USNO are monitoring

and reporting the phase differences of NLK,

Dmega, and some of the U. S. Loran-C stations.

In addition, NBS monitors and reports phase

differences for WWVB and the USNO offers data

on NAA. These data are available to interested users by contacting the Time and Fre- quency Division of NBS or the USNO.

6.9 USING THE WWVB TIME CODE

WWVB transmits a special binary-codeddecimal time code that provides time information. This service is used by seismologists,

standards labs, commercial power companies,

and others interested in synchronizations with

accuracies of the order of 500 microseconds.

The code provides information of days, hours,

and minutes. Seconds are resolved by counting

aits. A correction, applied to the transnitted time to obtain earth time, UT1 , is also

orovided for use by some astronomers and

navigators.

Unlike a code that is designed primarily

for machine decoding, the WWVB code was originally designed to be manually decoded

from strip chart recordings or tapes. To simplify this process for a human decoder, the

bits are placed such that the most significant

bit occurs first in each code word. The

actual bit itself, whether a binary zero, binary one, or position identifier, is determined by the duration of the power reduction

of the transmitted carrier.

6.9.1 TIME CODE FORMAT

The WWVB time code is generated by changing the power level of the 60-kHz carrier.

The carrier power is reduced 10 dB at the

beginning of each second and restored to full

power 200 milliseconds later for a binary

zero, 500 milliseconds later for a binary

one, and 800 milliseconds later for a reference marker or position identifier. Certain

groups of pulses are encoded to represent

decimal numbers which identify the minute,

hour, and day of year. The binary-to-decimal

weighting scheme is 8-4-2-1 with the most significant binary digit transmitted first. Note that this weighting sequence is the reverse of the WWV/WWVH code. The BCD groups

and their basic decimal equivalents are tabulated below:

BINARY GROUP DECIMAL

WEIGHT: 8 4 2 1 EQUIVALENT

0 0 0 0 0 0 0 0 1 1 0 0 10 2 0 0 11 3

0 10 0 4

0 10 1 5 0 110 6

0 111 7

1 0 0 0 8 10 0 1 9

The decimal equivalent of each group is derived by multiplying the individual binary

digits by the weight factor of their respective columns and then adding the four products

together. For example, the binary sequence

1001 in 8-4-2-1 code is equivalent to (1 x 8)

+ (0 x 4) + (0 x 2) + (1 x 1) = 8 + 0 + 0 + 1 = 9, as shown in the table. If fewer than

nine decimal digits are required, one or more

of the high-order binary digits may be dispensed with.

Once every minute, in serial fashion, the

code format presents BCD numbers corresponding

to the current minute, hour, and day on the

UTC scale. Two BCD groups identify the minute

(00 through 59); two groups identify the hour

(00 through 23); and three groups identify the

day of year (001 through 366). When repre- senting units, tens, or hundreds, the basic

8-4-2-1 weights are multiplied by 1, 10, or

100 respectively. The coded information

refers to the time at the beginning of the

one-minute frame. Within each frame, the

seconds may be determined by counting pulses.

Every new minute commences with a frame

reference pulse which lasts for 0.8 second.

Also, every ten-second interval within the

minute is marked by a position identifier

pulse of 0.8-second duration.

UT1 corrections to the nearest 0.1 second

are transmitted at seconds 36 through 44 of

each frame. Coded pulses at 36, 37, and 38

127
seconds indicate the positive or negative

relationship of UT1 with respect to UTC. Pulses at 36 and 38 seconds are transmitted as binary ones only if UT1 is early with respect

to UTC, in which case a correction must be added to the UTC signals to obtain UT1. The

pulse transmitted at 37 seconds is a binary

one if UT1 is late with respect to UTC, in which case the required UT1 correction must

then be subtracted. The magnitude of the UT1 correction is transmitted as a BCD group at

40, 41, 42, and 43 seconds. Because UT1 corrections are expressed in tenths of sec- onds, the basic 8-4-2-1 weight of that particular binary group is multiplied by 0.1 to obtain its proper decimal equivalent.

Figure 6.16 shows a sample frame of the

time code in rectified or dc form. The

negative-going edge of each pulse coincides

with the beginning of a second. Position

identifiers are labeled P ls P 2 , P3, P4, P5,

and P 0 . Brackets show the demarcation of the

minutes, hours, days, and UT1 sets. The

applicable weight factor is printed beneath

the coded pulses in each BCD group. Except

for the position identifiers and the frame

reference marker, all uncoded pulses are binary zeros.

In figure 6.16, the most significant digit of the minutes set is (1 x 40) + (0 x 20)

+ (0 x 10) = 40; the least significant digit

of that set is (0 x 8) + (0 x 4) + (1 x 2)

+ (0 x 1) = 2. Thus, at the beginning of the

frame, UTC was precisely 42 minutes past the

hour. The sets for hours and days reveal further that it is the 18th hour of the 258th

day of the year. The UT1 correction is -0.7

second, so at the beginning of the frame the correct time on the UT1 scale was 258 days, 18 hours, 41 minutes, 59.3 seconds.

6.9.2 TIME TRANSFER USING THE TRANSMITTED ENVELOPE

There are three ways to use the code. The first way--also the most expensive—is to

use a WWVB receiver with an automatic decoder

and display unit. There are a number of

manufacturers that produce this type of

equipment. A second level of user equipment

consists of a receiver having internal logic

circuits that provide a level shift code

output. This code may be applied directly to

a strip chart or to an oscilloscope for manual

use. The simplest equipment might consist of

a tuned radio frequency receiver with an oscilloscope used to observe the amplified

signal

Figure 6.17 shows the signal envelope as seen on an oscilloscope at the transmitter.

The on-time points are easily seen. With the

horizontal scale of the oscilloscope expanded

to 200 microseconds/centimeter, the on-time

point can be determined to about + 2 cycles of

the 60 kHz carrier. Of course, the farther

the receiving site is from the transmitter,

the more the signal-to-noise ratio degrades

1

TIME FRAME 1 MINUTE

(INDEX COUNT 1 SECOND)

20 30 50

I I I I I I I I I I I I I I I I I I I I I I I I I I

ON TIME POINT A FRAME REFERENCE

MARKER

Po Pi Po

-| |- 0.8 SECOND —j

)-— 0 . 2 SECOND ~| w |— ~] u |- —

-( f-— 0 . 5 SECOND 0.8 SECOND -| (r ~UUli¥TmnTFLJllT¥l^^ Q I » <1 O O O

** MINUTES- - HOURS DAYS ' —I Li I T C

o cm —i

L,-UTr

RELAT I 0NSH I

BINARY CODED DECIMAL TIME -OF -YEAR CODE W0R0 (23 DIGITS) CONTROL FUNCTIONS (15 DIGITS) USED FOR UT, CORRECTIONS

5 PPM POSITION IDENTIFIER MARKERS AND PULSES (P 0 THRU P 5 ) (REDUCED CARRIER 0.8 SECOND DURATION PLUS 0.2 SECOND DURATION PULSE)

W - WEIGHTED CODE DIGIT (CARRIER RESTORED IN 0.5 SECOND - BINARY ONE)

U - UNWEIGHTED CODE DIGIT (CARRIER RESTORED IN 0.2 SECOND - BINARY ZERO)

UTC AT POINT A 258 DAYS

18 HOURS

42 MINUTES

UT1 AT POINT A 258 DAYS

18 HOURS

41 MINUTES

59.3 SECONDS

NOTE: BEGINNING OF PULSE IS REPRESENTED BY N EGAT IVE - GOING EDGE.

FIGURE 6.16. WWVB TIME CODE FORMAT

128
MAIN TRANSMITTER

J L J I I 3

ms/cm

J I I I I I

1 1 1 20 40 60 80 100

ms/cm

FIGURE 6.17. WWVB ENVELOPES AS TRANSMITTED.

until some averaging techniques must be

employed. Since the WWVB data rate is so low--one bit per second--vi sual integration is very difficult, especially when the horizontal

scale is expanded to allow greater time

resolution. Averaging or signal integration

using a signal averager or an oscilloscope

camera works quite well at remote receiving

locations. (See Chapters 4 and 5 for more information about these techniques.)

The actual process of time recovery

consists of viewing the signal envelope on an oscilloscope that is being triggered by a local clock at the receiving site. Having

previously determined the path delay, its

value can be subtracted from the total observed delay on the oscilloscope to obtain the

local clock error.

In very noisy locations, where the

on-time point is difficult to identify, a photograph of the waveform can be helpful.

Sometimes it is advantageous to allow a number

of oscilloscope traces to be exposed on the

same piece of film. This has an averaging

effect that simplifies the location of the

on-time point. However, a single exposure can

be used. Take the photo and draw one horizontal line through the average of the waveform

amplitude and another line along the average

slope of the dropout. The intersection of

these lines is the on-time point.

Receiving sites that are long distances

from the transmitter have an added problem

when high resolution timing is desired. If the delay to the receiver site is, say, 15 milliseconds and the sweep speed is set to 1 millisecond/centimeter, the on-time point of

the envelope will be off the oscilloscope

face. This problem can be resolved if a second clock is available. The auxiliary

clock can be adjusted late so that when used

for the oscilloscope trigger source, the

on-time point will be near the beginning of

the trace. The sweep speed can now be in- creased to obtain the desired resolution. The

time intervals to be accounted for are the

time between the beginning of the trace and

the on-time point, and the interval between

the local clock and the auxiliary clock. The

local clock error, At , will be

At = D - At - At

e c
where: D is the propagation delay

At is the interval between the two clocks

Atg is the trace delay on the

osci 1 loscope

If Ate is positive, then the local clock

pulse occurs after the transmitted time pulse.

If At is negative, the local clock pulse

occurs first.

6.10 SUMMARY

Although you can get better accuracies by

using LF and VLF broadcasts than you can from

HF broadcasts, use of these signals poses

special problems that are not encountered at

HF. But if you need frequency accuracies of a part in 10 11 or better, or time synchronization to 500 microseconds, VLF or LF broadcasts

may suit your needs very well. However, you

must be willing the spend the time, money,

and effort necessary to get maximum results.

You do have an alternative, though. The

frequency calibration service using network

television and the TV Line-10 method of

obtaining time and/or frequency may be less

expensive and easier to use.

130
CHAPTER 7. FREQUENCY CALIBRATIONS USING TELEVISION SIGNALS

One of the newer frequency calibration

services offered by NBS uses TV signals which

have some advantages over radio broadcasts:

First, the signals are readily available and

usually very strong. Second, television re- ceivers and antennas are simple to operate and

install. Radio signals are carefully con- trolled at the transmitter, but when received

by the user, they are sometimes degraded in quality and difficult to locate in the radio

bands. Calibration via television lets the user concentrate on the calibration and spend

less time fussing to get a good signal.

This frequency calibration service uses television signals in a transfer mode. If a user wants to make a calibration, he compares

a TV signal coming from the national networks

with his local oscillator. NBS monitors the

same signal, calibrates it, and tells the user what correction to use.

The network signals are controlled by

atomic oscillators and change their frequency

very slowly, so the user needs new correction

data at infrequent intervals. NBS checks the

networks daily but publishes the corrections

only once a month. This is often enough. The

results obtained by this method of frequency

calibration match or exceed any other method

available. Anyone desiring a frequency calibration can make one quickly and have confidence in the results. Accuracies obtainable

range from a part in 10 9 to a few parts in

10".

7. 1 HOW THIS FITS INTO OTHER NBS SERVICES

NBS operates radio stations WWV, WWVH,

and WWVB. It also has a TV Line-10 service

which is explained in the next chapter. This

service complements the radio broadcasts and

Line-10. Users who have the equipment and

skill necessary to use radio signals will

probably want to continue to use them. How- ever, if TV signals are available and the

radio services fall short in some respect,

then a switch is indicated. Many users can get a good TV signal in their area, but find

it difficult to erect a suitable antenna for

the NBS radio signals. This television frequency calibration service operates equally

well on cable (CATV) systems, the UHF channels, and in areas of Canada and Mexico that

receive U.S. network signals. But the re- ceiver must be a color set.

7.2 BASIC PRINCIPLES OF THE TV FREQUENCY CALIBRATION SERVICE

The major television networks in the U.S. use atomic oscillators to generate their ref- erence signals. These are either cesium or

NBS SERVICES

NETWORK

SERVICES

USER FUNCTIONS

A NBS FREQUENCY STANDARD

B NETWORK STANDARD

C USER'S OSCILLATOR

FREQUENCY STANDARDS

COMPARE A-C = CALIBRATION OF C

NBS FREQUENCY STANDARD

NETWORK NETWORK

FREQUENCY

COLOR TV BROADCASTS STANDARD +

USER'S OSCILLATOR

SIGNALS RECEIVED

BS MON

NET SIGM

\sK

S

I

EASY INEXPENSIVE

ACCESS / / /

V

PHASE COMPARATOR

(TWO METHODS

COMPARED

NBS STANDARD

VS NETWORKS

published data

[easy, inexpensive]

l access j

USER'S OSCILLATOR

VS NETWORK

USER'S OSCILLATOR

VS NBS STANDARD

ARE DESCRIBED BELOW)

CALIBRATION

USER'S OSCILLATOR

CALIBRATED

131
NETWORK

5 MHz

3.58 COLOR

SIGNALJ

V

TV RECEIVER

-

- o

1 o —

o

LOCAL 5 MHz

OSCILLATOR

PHASE

COMPARE

3.58

COLOR SUBCARRIER SYNTHESIZER

FIGURE 7.1. EQUIPMENT SETUP FOR COMPARING A 5 MHz OSCILLATOR WITH NETWORK 5 MHz OSCILLATOR.

rubidium oscillators that produce the necessary color signal at a frequency of 3.58 MHz. All home color television receivers "lock" onto the color subcarrier signal. So, when a color set is tuned to a network program, its

internal 3.58 MHz oscillator generates a rep- lica of the atomic oscillator signal back at the network studio. This allows everyone with

a color set to have almost direct access to a number of atomic oscillators for calibrations.

And not just any oscillators at that--these

are checked by the National Bureau of Stan- dards which publishes their exact frequency.

The 3.58 MHz signal from the color re- ceiver is not a substitute for your own oscillator. It is a calibrating signal that can be

used to set your oscillator very accurately.

In only 15 minutes, you can get results that

would take hours or days to get using NBS radio stations.

Let's look at the basic principle of the

service. The oscillators used by the networks

are 5 MHz units modified to include a synthesizer to generate a 3. 5795454545. .. MHz (rounded to 3.58 MHz) color subcarrier signal. The

3.58 MHz was synthesized by taking

££ x 5 MHz.

If one wishes to adjust the frequency of a 5 MHz oscillator to agree with the network 5 MHz, it can be done by utilizing the phase

detector scheme shown in figure 7.1. You might ask, why measure phase to calibrate

frequency? As shown by the meter in figure

7.1, two frequencies are compared by making a phase comparison. In this case, the signals

compared are at 3.58 MHz, so the phase meter

full scale deflection "reads" one cycle (360°)

of that frequency or, using the period of the

signal, about 279 nanoseconds full scale. If the local oscillator frequency changes relative to the network color signal, the meter

moves. How much does it move? If those two frequencies differed by one cycle per second,

the meter would deflect zero to full scale in one second and then start over again in the next second.

Notice, however, that if the oscillator

in the figure were of high quality, we could

expect a slow moving meter. Let's take an example: Assume the crystal oscillator frequency is already set to within one part in

10 10 of the network oscillator frequency. The

problem is to figure out how much the meter

will move; that is, how many degrees of phase

or nanoseconds will be accumulated in one second. Set up the problem like this:

Nanoseconds accumulated _

^ io~^

1 second

The right side of the equation is a number

without dimensions (a numeric) so the left

side must match. This means that we multiply

the denominator (seconds) by 10+9 to get

nanoseconds for our units. This will cancel

the nanoseconds in the numerator, giving us a numeric on the left side.

SOLVING : Number of nanoseconds accumulated in 10+9 nanoseconds (one second)

= 10- 10 x 10+9 = 10- 1

= one-tenth of a nanosecond in one second.

This means that our meter will take ten sec- onds to move one nanosecond. Or, we can say

132
LARGE

, PHASE JUMPS LOCAL STATION ORIGINATION

IDEAL PHASE PLOT

SLOW

CONTINUOUS NETWORK

PATH CHANGE

FIGURE 7.2. ILLUSTRATION OF PHASE INSTABILITIES INTRODUCED BY NETWORK PATH.

that our meter will move full scale (279

nanoseconds) in 2790 seconds. This is about

46 minutes.

So, measuring electrical phase i_s the way

to go--it is really just another way of saying

that we are measuring part of a cycle (called

the relative frequency-Chapter 3), written:

At

T

Af

f

where the relative frequency, Af/f, is our

small fraction, 1 x 10- 10. Both methods

described in the following pages are based on this principle of measuring phase to get frequency. The results for both methods give an answer as a relative frequency from the

oscillator's nominal value.

If we synthesize 3.58 MHz from our 5 MHz oscillator, the phase of the two 3.58 MHz

signals may be compared either on a meter or

on a chart recorder. If the meter stands

still or the chart recorder draws a straight

line, indicating a fixed phase relationship

between the two 3.58 signals, it follows that

the two 5 MHz oscillators agree in frequency.

Of course, when we are using off-the-air

TV signals, the meter will not stand still and

the chart recorder won't draw a straight line. Due to instabilities in the propagation path,

we will see small "bumps" or jumps in phase.

In addition, the oscillator itself will affect

the straightness of the line.

7.2.1 PHASE INSTABILITIES OF THE TV SIGNALS

If the 5 MHz local oscillator were per- fectly matched with the network oscillator so that the only instabilities were those introduced by the network path, a phase chart

recording of the two signals might look somethi ng 1 i ke figure 7.2.

There are basically four types of phase

instability illustrated: large and small

phase jumps, network path length changes, and

local station originations. All of these

instabilities tend to limit the precision of

phase measurement, and hence, the precision

with which you can calibrate the frequency of

the local oscillator to the network 3.58 MHz. The ideal phase plot is shown as a straight

dotted line at +30°, constant phase. The

ultimate resolution is limited primarily by

the slow continuous network path change illustrated from minutes 11 through 14. In most

cases, resolution is limited to about 10 nanoseconds in 15 minutes. This corresponds

to a resolution of:

Af

f

At

T

10 ns 10 ns

15 min 900 s

10 0.9 x 10

3 s = 1.1 x 10

11

For measurement times shorter than 15 minutes, the measurement resolution will be

reduced roughly in proportion to the reduction

in measurement time; i.e., 1 x 10- 10 in 1.5

minutes. Note that this resolution represents

about the best one can hope to achieve. If large and small phase jumps are not taken into

account, the result can be much worse.

The large phase jumps (up to ± 90° or ± 70 ns) are primarily the result of the way the

networks operate. Phase jumps are caused by

switching from one video tape machine or camera to another, with different lengths of

cable being placed in the path. Most large

phase jumps are caused by changes from a pro- gram to a commercial and back again. Using TV requires some attention to what is on the

133
screen—one should be alert to sudden phase

jumps coincident with commercials.

Small phase jumps are the result of phase

distortion in the microwave system used to carry the network programs and to multipath

signals between the local station transmitter

and the TV receiver. Differential phase distortion within the TV receiver also contributes. The magnitude of small phase jumps is on the order of 1 to 10 ns, depending on the

network and the degree of multipath at the

receiving location. For best results, you

should use an antenna system that minimizes

"ghosts."

During station breaks, the received 3.58

MHz often originates from the local television

station's 3.58 MHz oscillator. Unless the

local station is one of the few that is equipped with a rubidium or cesium oscillator, its

frequency will probably be no better than 1 x 10- 7

, and the phase will change more rapidly,

about one full cycle per second. No precision measurements can be made on local programming.

Many stations record network programs for re- broadcast at a different time. When network

programs are "tape delayed," the 3.58 MHz,

again, is referenced to the local station's

oscillator and is therefore invalid as a pre- cision reference. In any given area of the

U.S., a few days of experience will provide a user with a good idea of local program schedules.

7.2.2 TYPICAL VALUES FOR THE U.S. NETWORKS

oscillators. This is not what we want. For reasons that need not concern us here, the

frequency of the network oscillators differ

from the NBS frequency standard by about -3000

parts in 10 11. The exact difference is mea- sured by NBS and published in the monthly NBS Time and Frequency Bulletin. This publication

can be obtained, free of charge, by writing

the Time & Frequency Services Group, NBS,

Boulder, CO 80303. In performing a calibration, the oscillator versus TV should give a difference equal to the published values.

The average relative frequency for the

three commercial networks, with respect to the

NBS frequency standard, for the week June 24-

30, was:

NBC

EAST COAST

3013.9 x 10 11

CBS

ABC

= -3002.4 x 10-11

= -3000.0 x 10-11

NBC

CBS

ABC

WEST COAST

= -3015.2 x 10

: Not Monitored

= -3000.0 x 10

11

11

In all of the preceding discussions, it has been assumed that the goal is to calibrate

a local oscillator and to make it agree

exactly with the frequency of the network

The minus sign preceding the results indicates that the network subcarrier signal is lower in frequency than the MBS frequency

standard.

ATOM i a

OeClLLATOE

METWORK.

4TUDI0

DATA

IMfOQMATION

(10M4ULTATION DATA +

TV =

CALIBRATION

MFGr

DEVELOPS fiOMMEKCJAL EQUIPMENT

134
You might ask why there are two sets of

data. Network programs in the Eastern,

Central, and Mountain Time Zones originate

from New York City. However, in the Pacific

Time Zone, network programs originate from Los Angeles, so different oscillators are involved. The NBS monitors the networks on both

coasts. Therefore, if you live in the Eastern, Central, or Mountain Time Zone, you

should use the data published for the East

Coast. The West Coast data are only for those

users who live in the Pacific Time Zone.

one beat note to occur. Because of this, we simply time the period of the beat note to get

the information we need to make a calibration.

By measuring the time of the beat note period, we increase our measurement accuracy.

With some simple math, we can see how

time measurements can be used for frequency

calibrations. We will find the length (in

time) of our signals, notice how fast they are moving, and then solve for the period of the

beat note that results.

To summarize our discussion thus far, a person who wants to calibrate an oscillator

can use the television network color

subcarrier to do so. This is possible because

NBS checks the network frequencies and publishes the "offset" (or error) of the network

oscillators with respect to the NBS standard.

A user would know two things: (1) the difference between his oscillator and the network

oscillator (by measurement), and (2) the difference between the network and NBS (by publication). With this information, he can com- pute the difference between his oscillator and

NBS. Thus, his calibration is traceable to

NBS.

So far, we have talked mostly about

high-quality oscillators. TV signals are equally good for calibration of low-quality

oscillators. In fact, without even looking at the NBS calibration data, you can depend on the color signal from any TV station to be in error by less than 3 parts per million, even

if it is not showing a network program. The

FCC requires this accuracy from all stations.

Now, for our 3.58 MHz signal, the period

is:

At = — = = 27936 x 10 " seconds.

f 3.58 MHz

This tells us how far either of our signals

has to move in time or phase to get one cycle

of the beat note. If we know how long it

takes, we can solve for the offset. Recall

that relative frequency is simply the numerical ratio of two frequencies. It can be obtained by the next equation (see Chapter 3).

Af _ _ At _ HOW MUCH IT MOVES

f T HOW LONG IT TAKES

_ 1 PERIOD OF THE 3.58 MHz

1 PERIOD OF BEAT NOTE

7.3 HOW RELATIVE FREQUENCY IS MEASURED

The following sections will discuss how

instruments can be constructed to perform TV frequency calibrations. To have a basis for

discussing actual instrumentation, two NBS- designed instruments are described. Commercial equipment is based on similar principles.

For exact information on a particular instrument, the manufacturers' manuals should be

consulted.

Two signals are involved in our measurement. One is the calibrated 3.58 MHz signal

from the network via a color TV receiver. The

other is a 3.58 MHz signal that we have generated from our own oscillator. These two signals will be phase compared and the output

from the comparator used as an indicator to calibrate our oscillator. When you mix two signals and get their difference frequency, it

is called a beat note. For a violin or piano,

the beat note is often a few hertz. But for our oscillator it is much lower in frequency.

In fact, it usually takes about 10 seconds for

We know that Af/f for the U.S. TV networks is nominally -3000 x 10- 11

, so we can rearrange

the equation to see how long a beat note will

take.

At _ 27936 x 10

- (^) 3000 x 10

= 9.31 SECONDS AS THE PERIOD OF THE BEAT NOTE

This means that if the relative frequency of

the network color signal were exactly -3000 x 10- 11

, as is the case with ABC, the network

3.58 signal would change one cycle with re- spect to an NBS-control led 3.58 MHz oscillator

in 9.31 seconds. We now have our time

measurement scheme.

For example, if the network relative frequency were reported as -3010 x 10- 11

, the

period of the beat note would be shortened to:

135
T = 27936 x 10 -11

3010 x 10

-11 = 9.28 seconds.

Let's look at this example more closely.

A network frequency change of 10 parts in 10 11 resulted in a beat note period change of

9.31 - 9.28 = 0.03 second. So, if our timing measurement is in error by 0.03 second, the

frequency measurement will be in error by 10 parts in 10 11

. This tells us how well our time measurement scheme will work. An error

of 0.03 second is not very likely with the

electronic devices available today. Even stopwatches will be useful to us. We will now examine some methods of implementing the beat

note period measurement as used in the

equipment designed by NBS.

7.3.1 COLOR BAR COMPARATOR

One simple method devised by NBS to com- pare a local frequency source with the network

oscillators is to use a color bar comparator.

This is a small, low-cost circuit that can be

added to any color TV receiver. It is shown

in block form and schematically.

As shown on the

cuit connects only to terminals. However,

obtained if the recei

input connector that

signal directly to (Chroma) circuit in cation does not affect

recei ver.

block diagram, the cirthe TV receiver antenna

better results can be ver is modified with an feeds the color bar

the color processing

the set. This modifinormal operation of the

3.58 MHz

VCX0

5 MHz

N

D0UBLER

A schematic of the NBS color bar com- parator is included so that the circuit can be constructed and used. Notice the input jack

Jl on the left. It accepts the local frequency source at a frequency of 10 MHz or at

an integer submultiple; that is, 10 MHz divided by "N," where N = 1, 2, 3 up to

100. This lets you compare frequency sources

of many different frequencies if you wish—one

at a time, of course.

Two options are shown on the schematic:

the antenna TV interface circuit and also a video TV interface circuit for one model of

receiver. No matter how the signal gets into

the receiver, it will be processed just as if

it were normal picture information. The beat

of the local crystal oscillator with the net- work signal forms a vertical "rainbow" bar. The color of the bar changes across the width

of the bar as the oscillator being calibrated

changes with respect to the network oscillator. The measurement consists of using a stopwatch to measure how long it takes this

color change to occur. The stopwatch reading

is equal to the period of the beat note.

To use the color bar system for oscillator calibration, tune the receiver to a net- work color program, and set the oscillator to be calibrated so that the rainbow appears to move across the bar from right to left through

a complete color cycle (from red to green to blue and back to red) in about 10 seconds.

The color bar itself moves slowly across the screen from right to left. If the frequency

of your oscillator is far off, the colors in the rainbow pattern will change very rapidly

and the entire bar will move rapidly across

ANTENNA

BURSTS OF 3.58 V

AT HORIZONTAL RATE

455 3>

PHASE MODULATOR

FIGURE 7.3- BLOCK DIAGRAM OF COLOR BAR COMPARATOR.

136
FIGURE 7.4. SCHEMATIC, COLOR BAR COMPARATOR

137
the screen in the direction of the color

changes. The bar can be positioned to the

middle of the screen by the pushbutton labeled

"Horizontal Sync Phasing."

With the rainbow repeating colors in about 10 seconds, carefully adjust the crystal

oscillator until the period is:

result. Other versions of the FMC are commer- cially available that use a separate digital

display rather than the TV screen.

The FMC automatically performs the

operation of measuring the period of the 3.58

MHz beat note "T," computing At/T, scaling the

result for readout in parts in 10 11

, and displaying the one- and ten-period averages on

the TV screen. Readout is a series of 4-digit

numbers, representing the relative frequency

between the oscillator being calibrated and

the atomically-control led color subcarrier. By

referring to network data published monthly in

the NBS Time and Frequency Bulletin, a user of

the FMC can set his oscillator to agree with

the NBS standard to better than 1 part in 10 10 in five minutes and 3 parts in 10 11 in fifteen

minutes. Radio methods of calibrating oscillators traceable to NBS require several days

of "averaging" to achieve this resolution. Measurement uncertainties as a function of

averaging time that may reasonably be expected

are:

27936 x 10 11

seconds.

NBS DATA FOR NETWORK BEING VIEWED

If possible, check more than one network

to increase the confidence in calibration.

Also, by measuring over ten beat note periods,

the effect of reaction time with the stopwatch

will be reduced. Recall that for 1 period, an error of 0.03 second corresponds to a frequency error of 10 parts in 10 11

. For ten periods, a measurement error of 0.03 second

will result in a frequency error of 1 part in 10 11

.

AVERAGING TIME

10 seconds

100 seconds

15 minutes

30 minutes

MEASUREMENT UNCERTAINTY

3x10

6x10

2x10

1.5 x 10

10

-11

-11

-11

When making network frequency com- parisons, the FMC will also accept as input any oscillator whose frequency is 10/N MHz,

where "N" is any integer from 1 to 100. The

highest available frequency should be used

when there is a choice.

As mentioned before, the rainbow will be

of higher quality if the signal is connected

into the receiver as a video signal. The

improvement is due to the fact that antenna

injection also modulates the received audio

carrier. This results in a visible beat note that constantly changes with the audio content

of the program. The other example circuit

shown connects the signal directly into the

Chroma bandpass amplifier. The rainbow is then nearly perfect.

7.3.2 THE NBS SYSTEM 358 FREQUENCY

MEASUREMENT COMPUTER

For those users who require accuracies

approaching 1 part in 10 11

, another calibration method is available. The system 358

Frequency Measurement Computer (FMC) computes

and displays the oscillator's relative frequency directly on the TV screen. The user simply turns on the unit and waits for 15 minutes. He then comes back, reads off 10 values, and averages them to obtain the

The FMC consists of a 5- inch color TV re- ceiver with an additional electronics package

mounted to its base. Readout for the FMC is on the TV screen, with two columns of ten 4-digit numbers, and an additional analog

"phase cursor.

A. Block Diagram Overview

The FMC is divided into four main functional parts: relative frequency scaler; beat

note phase comparator and cursor generator;

data store and display; and relative frequency

generator. In this section, the term "offset"

is used interchangeably with "relative fre- quency.

"

1. Relative Frequency Scaler . The relative frequency or offset scaler processes the

beat note and 3.58 MHz signals to generate an output count proportional to relative fre- quency x 10- 11

. This output count is applied

directly to the single-period 4-digit counter.

138
<smm~—•

:

national mmtm or hamoawm

. SYSTfM 3M FCfOtSNCY MfASUHMIMr COMWTM.

FIGURE 7.5. THE NBS PROTOTYPE, SYSTEM 358 FREQUENCY MEASUREMENT COMPUTER.

OFFSET SCALER STORE a DISPLAY

4 DIGIT 6x40 COUNTER SR

4 DIGIT 6x40 COUNTER SR

DISPLAY

LOGIC

VIDEO

MIXER

DISPLAY

VIDEO TO TV

FIGURE 7.6. BLOCK DIAGRAM, SYSTEM 358 FREQUENCY MEASUREMENT COMPUTER.

139
A new measurement is stored in the 6 x 40

single-period shift register (SR) for readout.

The output count of the scaler is also applied

to the 4-digit ten-period counter through an inhibit gate and a 7 10 prescaler. The

ten-period counter therefore accumulates 1/10

of its total count on each single-period

measurement.

Theory of the Scaler . The scaler

accepts the beat note and the color subcarrier

frequency, f ls from the TV. The variable

divider generates output pulses at the rate R = Af x 10 4

; that is, 10,000 times the beat

note frequency. These rate pulses are gated

on for a time equal to 10 7 /f 1

. The count out- put to the data store counters is therefore: — x 10

11 (count = R x TG = Af x 10 4

x —\

f f

l

This rate output is accumulated in the 4-digit

counter for a period of time determined by

time gate, TG, which enables the "and" gate

preceding the counter. The total output count

for each measurement cycle can be expressed

as:

OUTPUT COUNT = R x TG

lf in4 10' Af x 10 x — ^x 10

11

.

Therefore, if Af/f x = 3000 x 10- 11

, the output

count accumulated will be — x 10

11 = [3000 x lO

-11

] x 10

11

The objective is to obtain a reading on a 4-digit counter at the output such that each

accumulated count is equal to 1 part in 10 11 relative frequency between f x and f2

. We can verify that this has been accomplished by the

following analysis. First, relative frequency

is defined as:

RELATIVE FREQUENCY = OFFSET =

f

l " f 2 Af

The beat note comparator generates f x - f2 =

Af. The beat note frequency, Af, is applied

as a start-stop gate to the period counter,

"P." The total count accumulated in "P" in one cycle of Af can be expressed as:

1 pn _

f 1x 10

Af

3000 x 10° = 3000 counts.

The scaling factor for the output is the

product of the 10 4 divider and 10 7 time gate.

These two factors may be partitioned in other

ways--for example, 10 5 and 10 6 or 10 3 and 10 8

; however, the partitioning used here is optimum

for the nominal 3000 parts in 10 11 offset to

be measured.

With the partioning used and an offset of

3000 parts in 10 11

, the period count will be

3,333 and the time gate, TG, will be on for

0.3 of the beat note period. The maximum

offset that can be measured is 10,000 parts in 10 11

, in which case the time gate is on con- tinuously, and simultaneously, the 4-digit

counter overflows. The minimum offset that

can be measured is determined by overflow of

the 16-bit period counter, P. This occurs at

an offset of approximately 150 parts in 10 11

.

For each cycle of Af, the period count is latched and applied to the rate generator,

where the output rate is

R =

Neither the 3.58 MHz color subcarrier nor

the 10/N MHz input to be compared with it has been mentioned in the preceding discussion.

Normally, f x is the 3.58 MHz input and f2 is the harmonically related 10/N MHz input. However, the unit will work equally well with any frequencies that have harmonically related

offsets in the range of 150 to 10,000 parts in

10 11.

The rate of output pulses from divider (-rP) is:

R =

f, x 10

Af

-4 At- = Af x 10

4

10

2. Beat Note Phase Comparator and Phase

Cursor Generator . The phase comparator sec- tion compares the phase of each 126th cycle of

3.58 MHz from the TV receiver with each 176th

cycle of the "A" input. This circuit performs

the same harmonic synthesis function as the

network oscillators which generate 3.579545454

... MHz by taking 63/88 x 5 MHz. The analog

140
voltage resulting from this phase comparison

is stored in a sample-and-hold integrator.

The analog "beat note" is further processed

through two paths, the phase cursor path for a visual indication of the beat note and a Schmitt trigger to condition the signal for

the TTL offset scaler input.

Illllli4

2999 3000 3000 3000

3000 3000

3005 3000

3000 3000

3000 3000

3000 3000

2975 3000

3000 3000

3000 3000

pleted and the data are loaded into the top 4 digits in the left column. The second 4 digits in the left column will now be intensified. On each following 10-second interval,

data are loaded into succeeding positions in the left column until ten single-period

averages have been accumulated. At this time,

the 10 single-period measurements are averaged

and then loaded into the top 4 digits of the

right-hand column. This process continues

until all 10-period averages have been loaded.

Two operator aids are included in this NBS equipment. If a large phase jump occurs

at any time in the sequence, the corresponding

single-period offset will be read out, but

that measurement wi 1 1 not be incorporated into

the 10-period average. On the next measure- ment cycle, that single-period average will be

rewritten and the count incremented. The TV speaker "beeps" as a reminder to the operator

each time a measurement is completed.

3. Data Store and Display . The gated

frequency from the offset scler is accumulated

in the two 4- digit counters in the data

store-and-display section. After each beat

note measurement cycle, the single-period

counter is gated on for 2.79 seconds. The

accumulated single-period count is then dumped

to the single-period store for readout. The

10-period 4-digit counter is preceded by a -HO

so on each single-period average, it accumulates 1/10 of its total count. At the end of

ten 1-period averages, the 10-period counter

contents are dumped to the 10-period store for

readout.

The readout data are presented as two columns of ten 4-digit numbers. The left

column represents single-period offset read- outs and the right column represents ten- period offset readouts. To start a measurement sequence, the user pushes the reset

button. All readouts are reset to zero and

the top 4 digits in each column are intensified, indicating that data will be loaded in these positions.

At the end of approximately 13 seconds,

the first single-period measurement is comB. Use of FMC with Unstable

Crystal Oscillators

The FMC is primarily intended to calibrate with stability of better than 1 part in

10 7 per day. In fact, if the oscillator being

calibrated will not hold still to better than

1 part in 10 7 (± 10,000 parts in 10 11

), the

digital readout cannot be used. About the

best one can do with a really poor-quality

oscillator is make the cursor "stand still"

momentari ly.

If the cursor moves less than one cycle

each three seconds, then the oscillator is within 1 part in 10 7 of the color subcarrier.

By switching between networks, one can obtain

a "concensus;" however, there is no way to really verify if the subcarrier is originating

from a network, as can be done by checking a good oscillator.

The FCC requires all stations to maintain

3. 5795454545. . . MHz ± 10 Hz on their subcarriers, so by comparing with several sta- tions, one is assured of being within 3 parts

in 10 6

. If the oscillator being calibrated is stable to 0.001% (1 part in 10 5 ), it is close

enough.

7.4 GETTING GOOD TV CALIBRATIONS

Although the frequency calibration

service using TV reduces the labor of calibrating an oscillator, it does not relieve the user of the responsibility of verifying that

"good" data are actually being processed. The user must be sure that the station being used

is actually transmitting network-originated

programs. (See Section 7.5).

141
At the present time, data for three net- works are available from NBS. The easiest

thing to do is to take successive readings, if

possible, on all three networks for 1.5 minutes each and verify that they differ relative to each other by approximately the cor- rect amount. Nominally, ABC operates at ex- actly -3000 parts in 10 11

.

Assume relative frequency readings were

taken with the following correct results:

FABC =-3131 xlO

F., D

~ = -3150 x 10 NBC

F rDC = -3130 x 10

The readings are consistent from one network

to another. If one of the three had differed

radically from its correct relative value, we could assume that the station involved was on

local programming. For example, if the

reading for NBC were -2460, then the NBC sta- tion would not be used at that time.

Measurements must be made only when

network-originated programs are on the air.

Network stations in the Eastern, Central, and

Pacific time zones typically carry 10 to 14 hours of network-originated programs per day.

The Mountain time zone carries about four

hours per day.

It should be noted that networkoriginated programs are not necessarily "live"

programs (such as football or basketball

games). It simply means that the programs are originated from the network centers in either

Los Angeles or New York City and are not tape

delayed by the local stations. Programs

originating at the local stations (such as the

channel 7 news as opposed to CBS Evening News) cannot be used.

The best choices are the daytime soap op- eras and quiz shows. These usually are daily

programs and originate from the New York and/

or Los Angeles network centers. Weekly evening programs are usual ly network-originated

except in the Mountain Time Zone. Once the

programming schedule in any given city is identified, a log can be made to indicate when measurements can be made. There is, however,

one further word of caution (see below).

7.5 THE DIGITAL FRAME SYNCHRONIZER

Some local stations are now using digital

frame synchronizers on their incoming network

lines. This is true of the ABC stations in the San Francisco and Chicago areas, which

provide network feeds to other local stations

in the area, as well as many other local sta- tions throughout the country.

The frame synchronizer stores one com- plete TV frame (1/30 second) in digital memory

and "reads" the TV picture under control of

the local station sync generator. Since most

sync generators are driven by a crystal oscillator, the color subcarrier is no longer referenced to the network atomic standard, and

the signals cannot be used for frequency calibration.

The ABC stations in Los Angeles and New

York also use frame synchronizers on the

incoming network lines, but these stations are

co- located with the network studios and use the network cesium standards for their 3.58

MHz reference. Therefore, WABC and KABC still

provide stable frequency references and can be

used for frequency transfer measurements.

In 1978, a questionnaire was sent to all network-affiliated TV stations to determine

which ones are using or plan to use frame syn- chronizers. The results of this survey are available upon request from the Time and Fre- quency Services Group, National Bureau of

Standards, Boulder, CO 80303.

At present, only a few of the largest TV

stations have two or more frame synchronizers.

The smaller stations that have a single frame

synchronizer will use it for their "electronic

news," not to hang across the incoming network

line. Of course, this situation may change in five years if the price of synchronizers

drops.

If you are using the TV networks for

frequency transfer, the best precaution is simply to use more than one network so that

data can be verified.

7.6 USNO TV FREQUENCY CALIBRATION SERVICE

The USNO also offers a TV frequency calibration service to users who can receive sta- tions WTTG (Washington, D.C.), WAPB (Annapolis, MD), or KTTV (Los Angeles, CA). The USNO

has stabilized the frequency of the cesium

standards at these stations, so the service

is somewhat different than the NBS service. For information, contact the Time Services

Division, U. S. Naval Observatory, Washington,

DC 20390.

142
WORKSHEET FOR USE WITH

COLOR BAR COMPARATOR

DATE: START TIME:

NETWORK NBC CBS ABC

ENTER PUBLISHED

DATA

COMPUTE PERIOD*

STOPWATCH

TIMES

AVERAGE:

COMPUTE

RELATIVE FREQUENCY**

27936 X 10

" n

= 27936 * '°" n= XXXX SECONDS BEAT PUBLISHED DATA XXXXXX NOTE PERIOD.

27936 X 10

" n

= 27936 *

10 ~"

= XXXXXX RELATIVE MEASURED TIME XXXXXX FREQUENCY OF YOUR

OSCI LLATOR.

Compare PUBLISHED DATA with COMPUTED RELATIVE

FREQUENCY to determine difference between your oscillator and the NBS frequency standard.

WORKSHEET FOR USE WITH

DIGITAL OFFSET COMPUTER

DATE: START TIME:

NETWORK NBC CBS ABC

ENTER PUBLISHED

DATA

10-PERIOD AVERAGES

100-PERIOD AVERAGE

YOUR OSC.

VS NBS

Compare the 100-period average with PUBLISHED DATA

for each network. If it is higher than PUBLISHED

DATA, then the oscillator being calibrated is too high in frequency. If it is lower, then the oscillator is too low In frequency.

FIGURE 7.7. SAMPLE WORKSHEETS FOR USING TELEVISION SUBCARR I ER FOR FREQUENCY TRANSFER

143

CHAPTER 8. FREQUENCY AND TIME CALIBRATIONS USING TV LINE-10

The previous chapter discusses the use of the TV network color subcarrier for frequency calibrations.

The television synchronization pulses can also be used

as a transfer standard for both time and frequency.

A television picture is made up of 525 lines which

are scanned or traced to produce the image you see on

the screen. First, the odd lines (called the odd

field) are traced, then the even lines (the even field)

are traced in between the odd. This scanning happens

so fast (60 times a second) that your eyes cannot de- tect it and you have the illusion of seeing a continuous picture.

Line- 10 (odd) is one of the 525 lines that make

up each picture; it can be used for both frequency and

time calibrations. It was chosen because it is easy

to pick out from the rest of the lines with relatively

simple electronic circuits. To calibrate an oscillator, a clock pulse from the oscillator is compared to

the trailing edge of the Line-10 (odd) synchronizing

pulse. Measurements by users of this service, made

with specially-designed equipment, will correspond to measurements made at NBS at the same time.

Now that we know what Line-10 is, let's say what

it isn't. It isn't the color subcarrier! In the previous chapter, we discussed using the color subcarrier

as a frequency calibration source. Line-10 has nothing

to do with the color subcarrier system. Line-10 works

with either color or black and white television sets. Some equipment that is designed for the color subcarrier system will have a Line-10 output pulse because

it is handy to do so, but the techniques involved are entirely different for the two TV-based systems. They

do have one thing in common, though. All measurements

using either system must be made on network (major U.S. television networks) programming if NBS traceability

is to be achieved. In addition, Line-10 measurements

must be made at exactly the same time of day that NBS

makes its measurements. (Neither of these requirements

apply if you are using Line-10 on a local basis.) This

will be discussed more fully later on.

Jit

; —JLL / - w > \u Z__

Z2S~

t~tt:.;: —zi '

FIRST THE ODD FIELD IS TRACED.

1^

J 2_

THEN THE EVEN FIELD IS TRACED.

FIELDS COMBINE TO FORM COMPLETE PICTURE.

8.1 HOW THE SERVICE WORKS

Assuming that you have a TV receiver and

a Line-10 pulse is available, what do you do

with it? You can use it to set your clock or

to measure and/or set the frequency of your

oscillator. It provides a means of checking

oscillators to within parts in 10 11

, but the

oscillator/clock should be of fairly high

quality (1 part in 10 8 or better).

8.1.1 USING LINE-10 ON A LOCAL BASIS

To understand how the service works,

let's discuss a real-life example: The Line10 method is used routinely to compare the

NBS frequency standard in Boulder, Colorado,

with the clocks at radio stations WWV and WWVB in Ft. Collins, Colorado. Both locations

are within common view of the Denver television stations. Let's assume that the clocks

in Boulder and Ft. Collins are synchronized. Now suppose we measure the arrival time of the

TV Line-10 synchronization pulse at Boulder

(location A) with respect to the NBS standard.

A short time later, this same pulse will

arrive at Ft. Collins (location B) and we can measure its arrival time with respect to the

Ft. Collins clock. Since the clocks are synchronized, the difference in the two clock

readings is only the differential propagation

delay, written t R

- t. as shown in figure 8.1.

145
B

FIGURE 8.1 DIFFERENTIAL TIME

DELAY, T

B

- TA

.

We now know the propagation delay between

Boulder and Ft. Collins. If we make another measurement and find that the difference in the two clock readings is not equal to the

propagation delay, then this means that one of

the clocks is running faster or slower than

the other. The discrepancy represents the

time difference between the clock at

Ft. Collins and the clock at Boulder.

The same technique can be used by anyone

who wants to transfer time and/or frequency

from one location to another. This works only

when both locations can see the same TV signal. An example of using this technique would

be a factory with scattered branches that wants to calibrate an oscillator at one of the

branches that is not within cabling distance

of the main factory. By using two Line- 10 units, the factory and the branch can both

look at a local TV signal and effect the

transfer.

Even if you don't know the propagation

delay, the Line-10 technique can be used to determine the frequency stability of your

oscillator compared to another. Using a divider, simply operate your oscillator as a clock. By watching the day-to-day differences, you can tell if your clock is gaining

or losing with respect to the clock used for

comparison.

Note that such a local signal can be any

kind of TV signal, not necessarily network TV. Also, the accuracy of transfer using this

method can be very great. The NBS link to Ft. Collins typically can achieve ten nanoseconds

time transfer. Local use of Line-10 is a cheap way to distribute frequency and time at high accuracy levels.

8.1.2 USING LINE-10 FOR NBS TRACEABILITY

In figure 8.1, we assumed that both locations were within common view of the same television station. However, in actual practice, a given TV program can be observed simultaneously over a large geographical region

because of the TV microwave distribution net- work. For example, this technique has been

used to compare clocks in the Boulder and

Washington, D. C. areas by making simultaneous measurements on programs originating from New York City (see figure 8.2).

NBS routinely makes readings on the three

major networks on both the East and West

Coasts. These data are published monthly in

the NBS Time and Frequency Bulletin, which is available upon request. A sample page of the

Bulletin is shown in figure 8.3.

If you want to compare your clock with

the NBS standard, you must make your measurements at the same time as NBS and, in addition, the measurements must be on programs

NEW YORK ORIGINATING STUDIO

-y\p ATOMIC FREQUENCY

Sri CONTROL

MICROWAVE PATH FROM

NYC TO DENVER, COLO. 4000-6400 km*

VHF PATH FROM DENVER TRANSMITTER TO NBS 30 km •DEPENDING ON NETWORK

WASHINGTON, D.C. AFFILIATE

FIGURE 8.2 TYPICAL ROUTING OF TV SIGNALS FROM NEW YORK CITY ORIGINATING STUDIOS TO DISTANT RECEIVERS.

146
UTC.

EAST COAST

AUG.

UTC(NBS) - LINE 10 (ODD) (in microsecond;)

NBC ABC

19:25 00 19:31 00 19: 26 00 1 9 : 32 : 00 19:27 00 19:33:00

15694 75 08672 31 31093 91 24071 .52 15667 06 08644.55

32005 47 24983 03 14018 02 06995.50 31872 57 24850.06

14949 71 07927 26 30308 97 23286.38 14884 43 07861 .91 31260 82 24238 38 13233 12 06210.59 31090 14 24067.63

1 4205 24 07182 67 29523 82 22501 .28 14015 66 06993 • 1

29771 69 22749 24 1 1663 10 04640.58 29525 03 22502.51

12715 81 05693 36 27954 14 20931 .62 12450 44 05427.93

29026 94 22004 47 10878 29 03855.73 28742 49 21719.93

12058 43 05035 96 27168 97 20146.43 1 1667 93 04645.40 28282 51 21260 09 10093 04 03070.54 27960 03 20937.53

00259 24 26603 42 25598 64 18616.28 15259 36 O8236.83

16567 44 09542 88 08522 81 01500.38 31551 49 24529.06

32873 49 25851 00 24813 56 17791 .00 14477 22 07454.66

158 14 94 08792 46 07737 72 00715.15 23746.32

32123 48 25101 04 "^^^4029 13 17006.61 06699.40

1

2

34

56

78

9

10

1

12

13

14

1516

1718

1920

FIGURE 8.3. NBS LINE-10 DATA FROM NBS TIME & FREQUENCY BULLETIN.

originating from a common source—New York

City (for the Eastern, Central, and Mountain

time zones) or Los Angeles (for the Pacific

Time Zone). Currently, measurements are made

by NBS on the East Coast networks at the following times: NBC— 2025 and 2031 UTC; CBS—

2026 and 2032 UTC; ABC— 2027 and 2033 UTC (During Daylight Saving Time, the measurements

are made beginning at 1925 UTC.) West Coast measurements (furnished by the Hewlett-Packard

Company in Santa Clara, California) are made

at: NBC— 2125 and 2131 UTC; CBS— 2126 and

2132 UTC; ABC— 2127 and 2133 UTC (all year

round).

The measurements at NBS are made on the

trailing edge of the horizontal Line- 10 (odd)

sync pulse which occurs approximately every

l/30th of a second. This means you must

already know your time to 1/30 of a second or better to be assured that you are measuring

the same pulse that is being measured by NBS.

All of this sounds fairly straightforward, but it isn't all that tidy. First of

all, the calibration isn't available to the user until he receives the data he needs in the mail. This means that he must keep careful Line-10 records and wait for the results.

Meanwhile the clock must be kept running so that when the data arrive and the comparison

is made, the clock error can be determined and

something done about it. This means that the

clock involved must be a pretty good one. It should be as good as 1 part in 10 8 or better.

Otherwise, the whole operation will be laborintensive and the user will not be able to predict the time very well. Nor will he know

the frequency very accurately if the system

has large variations. Line-10 provides a means of checking oscillators to within parts

in 10 11

, but the oscillators must be of high

enough quality to deserve the investment of

time and effort. If you need lower accuracy,

there are cheaper ways that require less

effort.

8.2 EQUIPMENT NEEDED

To use the Line-10 technique, you need a black and white or color TV receiver, a line-10 synchronized pulse generator, a digital counter-printer, and a precision clock.

Line-10 equipment is commercially available.

The TV receiver is adapted to bring out the

composite video signal that connects to the

147
LOCAL ATOMIC

CLOCK LINE-10 PREAMP

PULSE GENERATOR

SIGNAL FROM LOCAL TV TRANSMITTER

FIGURE 8.4 EQUIPMENT NEEDED FOR TV LINE-10 METHOD OF CLOCK SYNCHRONIZATION.

Line-10 pulse generator. This pulse is com- pared on a time-interval counter with a local one-pul se-per-second clock. The display on the counter (start with local clock, stop with

Line-10) will be a series of numbers (see

figure 8. 4).

8.3 TV LINE-10 DATA, WHAT DO THE NUMBERS MEAN ?

How do we get the numbers and what do

they mean? First of all, let's review the

basis for the frequencies used by commercial

television systems in the U. S. The FCC specifies the frequency used for the chrominance subcarrier to be 3.5795454 ... MHz ± 10

Hz. The networks get this number by taking a

5 MHz oscillator and multiplying its output by

63 and dividing by 88. Then, the FCC states that the horizontal rate for the picture shall be 2/455 of the color subcarrier. This is 15.7342657 kHz. Next, you get the vertical

rate by taking 2/525th of the nominal 15.7

kHz, giving a vertical rate of 59.94 Hz. But

Line-10, the one we want, only occurs every

other field so its rate is half of the vertical rate, or 29.97 Hz, and has a period of

about 33 milliseconds —33.3666666... to be

exact.

Now, in our Line-10 instrumentation setup, we start our time interval counter with

our clock at a 1 pps rate and stop it with the next Line-10 pulse. A stream of Line-10 data

is shown in figure 8.5.

A reading occurs each second, each reading being slightly different from the one pre- ceding it. Let's look at that difference.

Notice the second and third columns which are

reading milliseconds. The number increases

progressively by 1 millisecond per second.

This is because Line-10 occurs at a frequency

rate that is not an integer and appears to drift relative to our 1 pps. When the count approaches 33 milliseconds total, the time

interval counter is stopped by the next Line-10 pulse that comes along, but it must

always occur before 33 milliseconds elapse.

So instead of reading the number above

the double line plus the 1 millisecond per second which would equal 33.3101 + 1.00000 = 34.3101, it must stop at the very next Line-10

pulse which appeared at the counter stop input

at 34.3101 - 33.3666 = 0.9435. This is what

NBC LINE-10 (EAST COAST) FEBRUARY 11 UTC ms ys

20 35:41 025 309.93

20 35:42 026 309.96

20 35:43 027 309.99

20 35:44 028 310.02

20 35:45 029 310.05

20 35.46 030 310.09

031 310.11

032 310.14

033 310.17

000 943.53

001 943.56

002 943.59

003 943.62

004 943.65

FIGURE 8.5. PRINTOUT OF NBC LINE-10 DATA.

148
AVERAGE RELATIVE FREQUENCY (in parts in 1 0 1

1

MEASUREMENT PERIOD EAST COAST WEST COAST

NBC CBS ABC NBC CBS ABC 2-6 FEBRUARY -301 9.4

9-13 FEBRUARY -3019-5

16 - 20 FEBRUARY -3019.6

-2963.0

-2963.

-2963.2

-3012.3

-3012.4

-3012.7

-3019. 3019.

3019.

-3015.2

-3015.4

-3015-7

FIGURE 8.6. TELEVISION NETWORK FREQUENCIES RELATIVE TO THE NBS FREQUENCY STANDARD

AS PUBLISHED IN THE NBS TIME & FREQUENCY BULLETIN.

was printed out. The time interval counter

cannot ever print a number bigger than the

period of the highest frequency on the "stop"

input. Notice, too, that the last column in the data is also changing. The counter- printer displays 0.01 microsecond in the last

digit and is changing at a rate of 30 nano- seconds (0.03 microsecond) per second.

How does this happen? In addition to operating at a frequency that is a non- integer,

each TV station will have an error because its

oscillator frequency differs from the NBS frequency standard. It differs enough to show up on a high resolution record like this one. It shows up on the published NBS data in figure

8.6. On the date these data were recorded,

NBC's relative frequency was -3019.4 parts in

10 11

; that is, just over 3 parts in 10 8

. Just

like any clock that is running at a different

rate, it will accumulate a time error. In this case, the time error due to the basic

frequency difference between NBS and NBC is 30.194 nanoseconds per second. So our data

will show this amount of change, and it does.

Since NBC's frequency is low relative to NBS,

and its Line-10 pulse is stopping the clock

(our time interval counter, in this case), the

Line-10 pulses arrive later and later by the

30.194 nanosecond amount each second. This is evident in the last column of the printout

(figure 8.5) as an increasing number since the

time interval counter counts just a little

longer each second. If local programming is used, such as between Boulder and Ft. Collins,

this number can change by as much as 50 or 60 nanoseconds per second. It can also decrease

rather than increase.

Although you can print out several numbers for each network, only the first number

is used--the one that occurs right on the exact second that NBS is making measurements.

Taking several readings insures that the

equipment is working properly. Also, if you

happen to miss the first second, you can

reconstruct the data. This is not a recom- mended practice, but it can be done.

8.4 WHAT DO YOU DO WITH THE DATA?

Assume that data are taken daily, careful

records are kept, and the clock-oscillator

combination being used for TV Line-10 data is not disturbed. After all, if you are recording data for 30 days and waiting for a check

with NBS, you must not alter the setup. The

data points can be tabulated or, better yet,

plotted on graph paper. The accumulated time error of the oscillator is a measure of its

frequency offset. Therefore, the data serve

to calibrate the relative frequency of the

oscillator with respect to NBS. Notice that

we are making and recording time measurements

to get frequency. As you will read later, we

can also get time, but we must make some further measurements to get it.

Figure 8.7 shows the raw data for an oscillator being calibrated with the TV Line-10

system. The counter used for this measurement

had a resolution of 0.1 microsecond. For the

data taken, it appears that the oscillator

changed about 10 microseconds a day relative

to NBS. This works out to be about 1 part in

10 10 frequency error. Having three major U.S. networks gives a good self-check on the system

accuracy. Although the data will be slightly

different for each of the three networks, the

calibration error will be small.

8.5 RESOLUTION OF THE SYSTEM

How good is the system? It has been

known to produce consistent results of around

1 part in 10 11 over a period of several years.

If you are willing to pay attention to details, you can achieve similar results. Of course, you must have a fairly good TV signal.

149
NBS LINE- 10 DATA- JUNE USER'S LOCAL CLOCK

LINE- 10 DATA

Dl FFERENCE""

NBS - LOCAL CLOCK

DAI LY ACCUMULATION

(MICROSECONDS)

TOTAL

ACCUMULATION

DAY (NBC) (NBC) (MICROSECONDS)

7 02216.54 05339.9 -3123.4 0 0

8 19526.34 22660.

-3133.8 -10.4 -10.4

9 02374.93 05518.6 -3143.7 - 9.9 -20.3

10 18779.21 21934.0 -3154.8 -11.1 -31 .4

1

01722.58 04888.5 -3165.9 -11.1 -42.5

'Published in NBS Time & Frequency Bui 1 et i n . -This heading establishes a method for the user to learn if his clock is gaining or losing time. Since the NBS clock is always "on time," the user's data are subtracted

from the NBS reading. It is as if a time interval counter produced this column where

the NBS clock opens the gate and the user's clock closes it.

COMPUTING THE OSCILLATOR FREQUENCY

(Refer to Chapter 3 for explanation of the math)

Us ACCUMULATED IN 5 DAYS: -42.5 NO. OF MICROSECONDS (AVERAGE )_ 10^6 =

At

But one day has 86,400 x 1 0

b microseconds. So:

At -10.6 . ..-10

- —=- = 7- = -1 .23 x 1

86,400 x 10

b

., At Af . „. . 1

Al so: = —- = 1 .23 x 10

This is the relative frequency of the user's oscillator with respect to NBS. The

actual frequency of the user's oscillator can also be found. If we assume this is

a 2.5 MHz oscillator, then:

Af ^actual ^nominal

f f . . nom 1 na

in f

' , - 2.5 x 10

6 Substituting 1 .23 x 10~' U =

ac TMai2.5 x 10 b

Solving: f actual

= 2 - 500 , 000 . 0003075 Hz.

So the user's oscillator is higher than it's nominal value by a very small amount

FIGURE 8.7. COMPUTATION OF OSCILLATOR FREQUENCY USING TV LINE-10 DATA.

150
It also helps your results if you can receive

all three networks and cross-check the results

for consistency. On the other end of the spectrum, would it be worthwhile to use the Line- 10 system for lower-quality oscillators? The

answer is yes, but up to a limit. Aside from

the fact that other frequency calibration ser- vices are available on radio, the basis for a decision of whether to use Line-10 or not

rests with the amount of time the user is willing to invest. For, as the oscillator

quality goes down, the amount of attention the

system requires increases. It becomes necessary to look at the data more closely and

recognize and calibrate the larger frequency

shift that will occur.

If the user's oscillator has a frequency

error of 1 part in 108

, it will gain or lose

about 1 millisecond per day. Since the TV

Line-10 pulse is ambiguous (it occurs about

every 33 milliseconds), it would not be too

hard to keep track of the data. However, if

the local oscillator drifted and was off as much as 1 part in 106

, the data would get

harder to interpret. This would allow a change of 100 milliseconds per day and a different Line-10 pulse would be viewed by the user than that seen and reported by NBS. This

is the case for any clock that has a large

time error. Then, the user is stuck with

either noting the differences and plotting

them or correcting the data for whole periods

of the Line-10 sync signals. This is 33.3666

... milliseconds. So, although just about any

clock oscillator can be used, it is not as simple and straightforward if large clock

drifts are involved.

8.6 GETTING TIME OF DAY FROM LINE-10

Since the Line-10 data are directly

traceable to NBS, it contains time-of-day

information. As noted in the above paragraph,

it is ambiguous to 33 milliseconds. This

means that if you want to set a clock accu- rately (to within microseconds), you must

first set it to within 33 milliseconds using

some other scheme. A good way would be to use radio station WWV and an oscilloscope. Once

you are within the ambiguity range of the

Line-10 data, you can use Line-10 to set the

clock more precisely. You would need to know

I the path and equipment delays. The path delay

! would just about have to be measured by carrying a clock to the NBS in Boulder. Once the

| delays are known, and if the user records data

regularly from several networks, chances are

he can keep time to within several microseconds using Line-10.

The need to calibrate the path and re-

; solve the ambiguity may seem to penalize this

system and suggest to the user that he use

some other method, but there are not very many

ways to get accurate timing. Chapter 9 describes Loran-C techniques for accurate time

setting. It, too, needs to be calibrated and

is ambiguous in the same way. So the steps are the same and Line-10 costs less.

8.7 THE DIGITAL FRAME SYNCHRONIZER

As discussed in Chapter 7 (page 142), some local TV stations are using digital frame

synchronizers which affect the use of these

stations for frequency calibrations. The

frame synchronizer also eliminates the use of

these stations for Line-10 time transfer when

it is in the differential path. The ABC sta- tions in Los Angeles and New York also use frame synchronizers on the incoming network

lines so Line-10 reference is lost. For more information on the stations that are using

frame synchronizers, contact the Time and

Frequency Services Group, National Bureau of

Standards, Boulder, Colorado 80303.

8.8 MEASUREMENTS COMPARED TO THE USN0

The USN0 also publishes Line-10 measure- ments on the East Coast TV networks originating in New York City. Their measurements are made at the same time as the NBS measurements.

Users desiring to compare their clocks with

the USN0 may obtain the USN0 data from their

Time Services Bulletin, Series 4. For more

information, write to the Time Services

Division, U. S. Naval Observatory, Washington,

D. C. 20390.

8.9 LINE-10 EQUIPMENT AVAILABILITY

There are several manufacturers of TV Line-10 equipment. The NBS has also designed

equipment to recover the Line-10 pulse from a commercial TV receiver. The circuit diagram

is shown in figure 8.8. The instrument

accepts 0.5 to 1.5 volts of video signal of

either polarity from a TV receiver and makes

available a Line-10 pulse referenced to either

the leading or trailing edge of sync. (Note

that the Line-10 measurements published in the

NBS Time and Frequency Bulletin are referenced

to the trailing edge of the Line-10 sync pulse.) The problem is that you will have an error equal to the width of the sync pulse if you use the leading edge and NBS uses the

trailing edge. This design uses standard TTL integrated circuits and could be assembled by

a skilled technician.

For more information about manufacturers

of Line-10 equipment, write to the Time and

Frequency Services Group, National Bureau of

Standards, Boulder, CO 80303.

151
FIGURE 3.8. SCHEMATIC OF TV LINE-10 EQUIPMENT DEVELOPED BY NBS.

152
CHAPTER 9. LORAN-C TIME AND FREQUENCY METHODS

Historically, many different radio signals have been used for distributing time and

frequency. Some of the most successful

adapted for this purpose have been those of

the Loran-C navigation system. In the 1950' s, after its initial development for navigation,

work done at NBS showed that Loran-C signals

could be quite stable, and since then, they

have been controlled and used for frequency

and time calibrations.

The success of Loran-C frequency and time

signals is due to a number of things. Each

station is controlled by cesium standards.

Careful control and monitoring of the signals

by the U. S. Coast Guard and the U. S. Naval

Observatory allows a user to achieve rapid, accurate results. Groundwave signals provide

state-of-the-art frequency and time calibrations. Even the skywave signals, though

slightly less accurate, can become the basis

of excellent high-quality calibrations.

Loran-C coverage is increasing with the planned

addition of new chains.

9. 1 BASIC PRINCIPLES OF THE LORAN-C

NAVIGATION SYSTEM

Loran-C is a radio navigation system that

broadcasts on a frequency of 100 kHz using a bandwidth from 90 to 110 kHz. At this low

frequency, the radio waves follow the earth's

curvature; thus, they are relatively unbothered by the ionosphere and are very stable.

The Loran-C navigation system consists

of many synchronized "chains" or networks of

stations. These chains provide groundwave

coverage of most of the United States, Canada,

Europe, the North Atlantic, the islands of

the Central and West Pacific, the Philippines,

and Japan. Information about the various

chains is given in Table 9.1.

One station in each chain is designated

as a "master" station; the remaining stations

are "slaves." The master station transmits

groups of pulses that are received by the

slave stations. The slave stations receive

the master pulse groups and, at later times,

transmit similar groups of synchronized

pulses (figure 9.1).

On a ship or plane, the constant time

differences between the reception of the

master pulses and the corresponding slave

pulses establish a line-of-position that is used for navigation. Signals from three

separate Loran transmissions are needed to determine a line-of-position. For frequency

and time applications, only a single Loran station is needed.

The coverage and accuracy of Loran-C

signals depend upon the characteristics and

format of the transmitted signal. When the

system was designed, tradeoffs were made to make the system accurate and reliable. The

characteristics and format of the transmitted

Loran-C signals can be broken down into the

following major elements:

PULSE GROUP FORMAT

MASTER

SECONDARY W

SECONDARY X

SECONDARY Y

SECONDARY Z

-START OF GROUP REPETITION INTERVAL (GRI)

SPACING 1 ,000 ps

I I 1 1 I II v I 2,000 ys

1 G R I

-

SPACING 1,000 ys (ALL SECONDARY STATIONS)

I I I I I

I I I

I I I I I I I I

I M I I I I I

PHASE CODE

1ST GRI ALTERNATE GRI MASTER ++ +-4~

SECONDARY -I- ++4-++- FIGURE 9.1. LORAN-C PULSE GROUP AND PHASE-CODE FORMAT.

153
TABLE 9.1 LORAN-C CHAINS

STATION LOCATION

CTATTON

IDENTIFI- CATION

PEAK

RADI- ATFD n 1 LU POWER

(kW)

BASELINE

fODTNG

DELAY

Cm s

EMISSION

DELAY

(MS)

NAUTICAL

MILES

KILO- METERS

PATH LENGTH

(MS)

GREAT LAKES

REPETITION PERIOD: 99,300 |js (SS7)

DANA, IN 39° 51' 8" N 87° 29' 12" W 9930M

(9960Z)

*

(OPERA!

400

IONAL FEBR JARY 1980)

MALONE , FL 30° 59' 39" N 85 10' 9" W 9930W

(7980M)

* 800

14355.1

SENECA, NY 42° 42' 51" N 76° 49' 34" U 9930X

(9960M)

* 800

31162.1

BAUDETTE, MN

48° 36' 50" N 94° 33' 18" W 9930Y

U. S. WEST COAST REPETITION PERIOD: 99,400 |js (SS6)

FALLON, NV

39° 33' 6" N 118° 49' 56" W

9940M 400

GEORGE, WA

47° 3' 48" N 119 44' 40" W

9940W

1 cc\r\ loUU 2,797.0 1

1 , uuu 1 Q

These stations are members of two or more chains and are therefore dual or triple-rated. They

transmit on two (or three) rates with two (or three) pulse repetition periods.

I'

154
STATION LOCATION

STATION

IDENTIFI- CATION

PEAK

RADI- ATED

POWER

(kW)

BASELINE

CODING

DELAY

(ps)

EMISSION

DELAY

(MS)

NAUTICAL

MILES

KILO- METERS

PATH LENGTH

(MS)

MIDDLETOWN, CA

38° 46' 57" W

122° 29' 45" W

9940X 400 1 ,094.5 27,000 28094.5

SEARCHLIGHT, NV 35° 19' 18" N 114° 48' 17" W

9940Y 500 1 ,967.2 40,000 41967.3

NORTHEAST U. S. REPETITION PERIOD: 99,600 ps (SS4)

SENECA, NY 42° 42' 51" N 76° 49' 34" W

9960M

(9930X)

*

800

CARIBOU, ME 46° 48' 27" N 67° 55' 38" W

9960W

(5930M)

350 2,797.2 11 ,000 13797.2

NANTUCKET, MA 41° 15' 12" N 69° 58' 39" W9960X

(5930X) 275 1 ,969.9 25,000 26969.9

CAROLINA BEACH, NC 34° 3' 46" N 77° 54' 47" W9960Y

(7980Z) 700 3,221.7 39,000 42221.7

DANA, IN**

39° 51' 8" N 87° 29' 13" W

9960Z

(9930M) 400 3,162.

54,000 57162.1

SOUTHEAST U. S. REPETITION PERIOD: 79,800 ps (SL2)

MALONE, FL 30° 59' 39" N 85° 10' 9" W

7980M

(9930W)

*

800

These stations are members of two chains and are therefore dual-rated. They transmit on two rates with two pulse repetition periods.

155
STATION LOCATION

STATION

CATION

PEAK

RADI- ATED

PUWtK

(kW)

BASELINE

CODING

UtLMY

(MS)

EMISSION

UtLAY

(MS)

NAUTICAL

MILES

KILO- METERS

PATH LENGTH

(US)

GRANGEVILLE, FL 30° 43' 33" N 90° 49' 44" W

7980W 800 • 1 ,809.5 11 ,000 12809.5

RAYMONDVILLE, TX 26° 31 ' 55" N 97° 50' 0" W

7980X 400 4,443.4 23,000 27443.4

JUPITER, FL 27° 1 ' 58" N 80° 6' 54" W7980Y 275

2,201.9 43,000 45201.9

CAROLINA BEACH, NC 34° 3' 46" N 77° 54' 47" W

7980Z

700 2,542.7 59,000 61542.7

WEST COAST OF CANADA

REPETITION PERIOD: 59,900 lis (SHI)

WILLIAMS LAKE, BC 51° 57' 59" N 122° 22' 2" W

5990M 400

SHOAL COVE, AK

55° 26' 21" N 131° 15' 20" W

5990X

(7960Y)

it

• 400 2,343.6 11 ,000 13343.6

GEORGE, WA

47° 3' 48" N 119° 44 1 40" W

5990Y 1600 1 ,927.4 27,000 28927.4

CENTRAL PACIFIC

REPETITION PERIOD: 49,900 us (SI)

JOHNSTON ISLAND

16° 44' 44" N 169° 30' 31" W

4990M 275

UPOLO POINT, HI 20° 14' 49" N 155° 53' 10" W

4990X 275 804.2 1 ,489.3 4,972.2 11 ,000 15972.4

*These stations are members of two chains and are therefore dual-rated. They transmit on two rates with two pulse repetition periods.
STATION LOCATION

STATION

IDENTIFI- CATION

PEAK

RADI- ATED

POWER

(kW)

BASELINE

CODING

DELAY

(MS)

EMISSION

DELAY

(MS)

NAUTICAL

MILES

KILO- METERS

PATH LENGTH

Cm s )

KURE, MIDWAY ISLAND

ooo oo ' yio" ki *M N 178° 17' 30" W

4990Y 300 849.6 1 ,573.4 5,253.

29,000 34253.1

NORTH PACIFIC

REPETITION PERIOD: 99,900 us (SSI)

ST. PAUL,

PRIBILOFF IS. , AK

oi y 1 U N 170° 14' 60" W

9990M 275

ATTU, AK

kOO A Q 1 A C II KI Z3<L 43 40 n 173° 10' 52" W

9990X 275 3,875.3 11 ,000 14875.3

POINT CLARENCE, AK

CtrO "l/|l /in" KI 00 14 4U N 166° 53' 14" W

9990Y 1000 3,069.29,000 32069.1

NARROW CAPE,

KODIAK IS. , AK

C"70 oc i on" KI b/ 2b 20 N 152° 22' 11" W

9990Z

(7960X)

*

400 3,590.5 43,000 46590.5

NORTHWEST PACIFIC

REPETITION PERIOD: 99,700 (js (SS3)

IWO JIMA, BONIN IS.

o/iO /in I /ill hi 24 48 4 N 141° 19' 29" E

9970M 1000

MARCUS ISLAND

O It O 1 T 1 /-ill h I 24 17 8 N 153° 58' 52" E

9970W 1800 692.9 1 ,283.2 4,283.9 11 ,000 15283.9

HOKKAIDO, JAPAN

42° 44' 37" N 143° 43' 09" E

9970X 400 2,002.3 6,685. 1 30,000 36685.1

*These stations are members of two chains and are therefore dual-rated. They transmit on two rates with two pulse repetition periods.

157
STATION LOCATION

STATION

IDENTIFI- CATION

PEAK

RADI- ATED

POWER

(kW)

BASELINE

CODING DELAY (^)EMISSION

DELAY

(MS)

NAUTICAL

MILES

KILO- METERS

PATH LENGTH

(MS)

GESASHI, OKINAWA

26° 36' 25" N 128° 8' 56" E

9970Y 400 721.8 1 ,336.8 4,463.2 55,000 59463.2

YAP, CAROLINE IS. 9° 32' 46" N 138° 9' 55" E

9970Z 1000 929.4 1 ,721.3 5,746.8 75,000 80746 8

GULF OF ALASKA

REPETITION PERIOD: 79,600 ms (SL4)

TOK, AK

63° 19' 43" N 142° 48' 32" W

7960M

NARROW CAPE,

KODIAK IS. , AK

57° 26' 20" N 152° 22' 11" W

7960X

(9990Z)

*

400 2,804.5 11 ,000 13804.5

i mi

SHOAL COVE, AK

55° 26' 21" N 131° 15' 20" W

7960Y

(5990X)

* 3,651.

26,000

\

C _7U J J. , J.

i

NORTH ATLANTIC

REPETITION PERIOD: 79,300 ms (SL7)

ANGISSOQ, GREENLAND

59° 59' 17" N 45° 10' 27" W

7930M 1000

!

i

\

SANDUR, ICELAND

64° 54' 27" N 23° 55' 22" W

7930W

(7970Y)

* 1500 657.9 1 ,218.5 4,068.1 11 ,000

1

15068.1

1

1.

EJDE, FAROE ISLAND

62° 17' fifl" N 7° 4' 27" W

7930X

(7970)

* 400 1100.3 2,037.8 6,803.8 21 ,000 27803.8 i>

CAPE RACE,

NEWFOUNDLAND

46° 46' 32" N 53° 10' 28" W

7930Z

(9930X) 1500 843.0 1 ,561.2 5,212.2 43,000

[ 48212.2

1

These stations are members of two chains and are therefore dual-rated. They transmit on two rates with two pulse repetition periods.

158
STATION LOCATION

STATION

IDENTIFI- CATION

PEAK

RADI- ATED

POWER

(kW)

BASELINE

CODING

DELAY

(MS)

EMISSION

DELAY

(us)

NAUTICAL

MILES

KILO- METERS

PATH LENGTH

(MS)

NORWEGIAN SEA

REPETITION PERIOD: 79,700 us (SL3)

EJDE , FAROE ISLAND

62° 17' 60" N 7° 4' 27" W

7970M

(7930X)

* 400

BOE, NORWAY

68° 38' 7" N 14° 27' 47" E

7970X 250 654.7 1 ,212.5 4,048.2 11 ,000 i cnyi o o loWo.c

SYLT, F. R. GERMANY

54° 48' 30" N 8° 17' 36" E

7970W 275 657.6 1 ,217.8 4,065.7 26,000 JUUOJ ^OOfiR . 7/

SANDUR, ICELAND

64° 54' 27" N 23° 55' 22" W

7970Y

(7930W) 1500 476.2 882.0 2,944.5 46,000 48944.

JAN MAYEN, NORWAY

70° 54' 53" N 8° 43' 59" E

7970Z 250 520.2 963.4 3,216.3 60,000 62216.3

MEDITERRANEAN SEA REPETITION PERIOD: 79,900 us (SL1)

SELLIA MARINA, ITALY

38° 52' 21" N 16° 43' 6" E

7990M 250

LAMPEDUSA, ITALY

35° 31' 21" N 12° 31' 30" E

7990X 400 284.0 526.0 1 ,756.0 11 ,000

KARGA BARUN, TURKEY

40° 58' 21" N 27° 52' 2" E

7990Y 250 529.4 980.4 3,273.3 29,000 32273 .

ESTARTIT, SPAIN

\ 3° 12' 16" E

7990Z 250 646.9 1 ,198.0 3,999.7 47,000 50999.7

* These stations are members of two chains and are therefore dual rated. They transmit on two rates with two pulse repetition periods.

159
1. Carrier Frequency 4. Phase Code

2. Pulse Shape 5. Blink Codes

3. Pulse Group 6. Spectrum

The characteristics and format of the Loran-C

signals and the important tradeoffs which

were made in their selection are discussed

below.

9.2 BASIC LORAN-C FORMAT

The basic format of the Loran-C trans- missions, for a single rate, which would be

received at a point in the coverage area of a Loran-C chain, is shown in the line labeled

"phase code" in figure 9.2. Signals from

other chains (adjacent and distant) would be superimposed on these signals (because all Loran-C signals share a carrier frequency of

100 kHz) but would have different Group Repetition Intervals (GRI). Master transmissions

are labeled "M" in the figure and consist of

8 pulses (represented by the short vertical

lines) separated by 1000 microseconds and a ninth pulse 2000 microseconds after the

eighth. The secondary transmissions are labeled "S" in the figure and consist of 8

pulses separated by 1000 microseconds. These

pulses have the general envelope shape shown

in the lower right corner of figure 9.2, and

are positively and negatively phase coded as shown in the middle of the figure. Phase

coding, station occupancy bounds, GRI,

standard track point, etc., will be discussed

in greater detail

NOTE: The following information is in- cluded here for those readers who

have a need to understand Loran-C operation in detail. An average

frequency and time user may not wish to pursue this subject in such depth and may skip to section 9.3 on page 166.

FIGURE 9.2. LORAN-C SIGNAL FORMAT (CHAIN OF SIX STATIONS AT GRI CODE 9930).

160
9.2.1 CARRIER FREQUENCY i(t) = 0,t < T

All Loran-C transmissions are timeshared

(time multiplexed) on a carrier frequency of

100 kHz. The choice of carrier frequency and

the propagation medium determine the range to which the signals will be propagated (ground- waves and skywaves) and how stable they will

be. High frequencies (greater than 200 kHz)

have the best groundwave propagation characteristics. The best frequency for the Loran-C

navigation system might have been in the band

150 to 200 kHz, but a frequency in this band

was not available because it would have interfered with existing European broadcast sta- tions. Loran-C signals at 100 kHz can be

transmitted without inordinate difficulty,

have good groundwave range and good stability. Loran-C transmissions may occupy a band

from 90-110 kHz (see pulse shape).

9.2.2 TRANSMITTED PULSE SHAPE

The shape of the standard Loran-C pulse

envelope as a function of time is given by:

f / +

>, _ t 2 -2t/At

f(t) - t e p,

where Atp is the time to peak of the pulse

and is 65 microseconds for a standard Loran-C

pulse.

This pulse rises to approximately 50% of

its peak amplitude in 25 microseconds. The

rise time of the Loran-C pulse should be as fast as possible to allow maximum pulse amplitude at the receiver before the pulse has been

contaminated by the arrival of skywaves. A squarewave would rise more quickly, but the

20 kHz bandwidth allowed for Loran-C pulses

would be exceeded. The standard Loran-C

pulse offers a good compromise between fast

rise time and required bandwidth. Other

pulse shapes having faster rise times that meet bandwidth requirements have been proposed, but not adopted. Transmitting equip- ment limitations place additional constraints

on the type of pulse selected.

9.2.3 TRANSMITTED PULSE

The Loran-C transmitted pulse shape is defined in terms of the base current in the

transmitting antenna. The antenna base current, i(t), is given by the following:

i(t) = A ( ^-S )

exp [-2 ( i-^-E )] sin (u>n t + i»,

where:

T < t < \

A is a constant related to the peak

current (in amperes)

t is time (in microseconds)

t is the time origin for the envelope; also called ECD (in

microseconds)

t is the time of the pulse envelope

p peak (65 + x)

At is the rise time of the pulse

P envelope (65 microseconds)

uy. is the angular carrier frequency

(0.2 n radians/microsecond)

(j> is the phase code (0 or n radians, + 180°)

For time (t) greater than t , i(t) is con- trolled to satisfy radiated spectrum requirements. To prevent contamination of the leading edge of a Loran-C pulse by the tail of

the previous pulse, ideally, the amplitude

of the tail should be well attenuated before

the next pulse starts. If the skywave of the

tail of the pulse is considered, then a Loran-C pulse should be attenuated as fast as possible after attaining peak amplitude.

Unfortunately, the bandwidth constraint

must be considered. A compromise between

these two requirements is to allow a pulse

length of 500 microseconds. By requiring the

amplitude of the pulse at 500 microseconds to

be down 60 c"3 from the peak of the pulse, the

bandwidth limitation can be met and the pulsetail skywave contamination problem can be avoided. Precise timing control of the leading edge of the Loran-C pulse is required to permit users to obtain accurate navigational

i nf ormation.

161
TRANSMITTER

TRIGGER

PULSE

TIME REFERENCE

13 15

10 20 30 40 50 60 70 80 90

ZERO CROSSINGS IN MICROSECONDS

FIGURE 9.3. STANDARD LORAN-C PULSE (ECD = 0, POSITIVELY PHASE-CODED).

1.0

0.8

0.6

0.4

UJQ

B 0.2

ENVELOPE TAG

~ POINT y

- (33yS) /

-

10 \ r°l T

40 50 60 tyS

~

ECD = +3yS=T 1 / 1

" D PULSE TIME 1 REFERENCE (PTR) \

FIGURE 9.4. LORAN-C PULSE WITH ECD = +3.0 MICROSECONDS.

162
9.2.4 GRAPHICAL REPRESENTATION Zero Crossing :

A graphical representation of the stan- dard Loran-C pulse is shown in figure 9.3. The main aspects of the pulse are defined as follows:

The instant at which the direct coupled

voltage (or current) reaches its zero

DC level.

Pulse Time Reference (PTR): A. Envelope-to-Cycle Difference

The positive-going zero crossing nearest

to the 30 microsecond point on the pulse

envelope for a positively phase coded

pulse (negative-going zero crossing for

a negatively phase coded pulse). In receivers, this point is often referred

to as the "Track Point."

Reference Delay (D ^):

The fixed delay between

triggers and the PTR. transmitter

Pulse Start:

The time 30 microseconds minus the algebraic value of the ECD earlier than the

PTR.

Time-to-Peak

The time interval from pulse start to the

peak value of the envelope of the pulse.

AT is 65 microseconds.

P

Pulse Tail:

The portion of the pulse after the peak

has occurred.

Envelope-to-Cycle Difference (ECD) is the displacement between the start of the

Loran-C pulse envelope and a zero crossing of

the Loran-C carrier. ECD arises because the

phase (carrier) velocity and the group (envelope) velocity of the Loran-C signal differ.

As the signal propagates over sea water, the

ECD decreases because the phase velocity

exceeds the group velocity. ECD shifts over

land paths are not always well behaved. The

ECD of the transmitted signal is controlled by

pre-distorting the transmitting antenna cur- rent pulse so that a signal with the desired

ECD is obtained in the near far-field. ECD is illustrated in figure 9.4 (ECD = +3 microseconds). In the transformation between the

transmitting antenna base current and the

electric field in the far field, the carrier

advances approximately 2.5 microseconds

(because of the imaginary term in the theta

component of the electric field vector) which

results in a shift in ECD of +2.5 microseconds. In general, the ECD of the antenna base

current waveform is adjusted to zero such that

the ECD in the near far-field is +2.5. In general, this ECD decreases as the signal

propagates such that it is approximately zero

in the prime service area. ECD is an impor- tant parameter because if it is too great a

TABLE 9.2. ECD TABLE: HALF CYCLE PEAK AMPLITUDE vs ECD

HALF CYCLE

NO.

NOMINAL TIME

OF HALF CYCLE

us

ECD (us)

-4 -3 -2 -1 0 +1 +2 +3 +4

1 +2.5 + 066 + 050 + 036 + 025 + 016 + 009 + 004 + 001 000

2 +7.5 166 144 122 102 083 066 050 036 025

3 + 12.5 + 290 + 264 + 239 + 214 + 190 + 166 + 144 + 122 + 102

4 +17.5 419 393 367 341 315 290 264 239 214 1-5 +22.5 + 545 + 520 + 496 + 470 + 445 + 419 + 393 + 367 + 341

6 +27.5 659 637 615 592 569 545 520 496 470

7 +32.5 + 758 + 740 + 721 + 701 + 680 +, 659 + 637 + 615 + 592

8 +37.5 840 825 810 793 776 758 740 721 701

NORMALIZED TO PEAK AMPLITUDE (t

p = 65 us); i.e. , AMPLITUDE AT t p = 1

163
receiver will not be able to identify the

proper "Track Poi nt.

B. ECD Determination

A standard measurement process has been

defined to determine the ECD of the Loran-C

pulse. Amplitude samples of the first eight

half cycle peaks or quadrature samples at 2.5

microsecond intervals about the PTR through

the first forty microseconds of the pulse are

taken. A best-fit standard Loran-C pulse is determined for the measured pulse using a minimum mean square error (MMSE) estimator. A cost function which is a function of both peak

pulse amplitude (A) and ECD (t) is minimized.

This cost function is minimized by solving two partial differential equations. The peak

amplitude and ECD of the measured data are found from this best-fit pulse. An rms pulse

shape error computation is associated with

this determination of ECD. Confidence limits

on ECD are being established for various mag- nitudes of rms pulse error. At Loran-C trans- mitting stations, the ECD is determined by

measuring half cycle peak amplitudes through

the first forty microseconds of the pulse.

C. Pulse Group

Loran-C receivers use carrier phase mea- surements of the pulsed 100 kHz signals. High

accuracy is achieved by allowing separation of

the groundwave and skywave components of the

Loran-C signal due to pulse modulation. Be- cause all chains operate on the same frequency, some method of chain selection is necessary. This is achieved by transmitting signals from different chains on different group repetition intervals (GRI) (see figure 9.2).

In selecting the number of pulses to transmit

per GRI and their spacing, tradeoffs had to be

made to maximize the number of pulses per second which could be received (to increase

effective receiver signal-to-noise ratio (SNR)

for a given peak power), to minimize interference from first-hop skywaves and multi-hop

skywaves, and to provide sufficient time for

receiver recovery (processing).

D. Existing Format

The stations in a particular Loran-C chain transmit groups of pulses at a specified

group repetition interval (GRI). Possible

TABLE 9.3- GROUP REPETITION INTERVALS

(GRI In Tens of Microseconds)

9999 8999 7999 6999 5999 ^999

9998 8998 7998 6998 5998 4998

9997 8997 7997

•

•

6997 5997 4997

•

•

9991 8991

•

7991 6991

•

5991

•

4991 9990 (SSI) 8990 7990 (SL1) 6990 5990 (SHI) 4990 (SI)

9989 8989

•

7989 6989 5989

•

4989

9971 8971 7971 6971 5971 4971 9970 (SS3) 8970 7970 (SL3) 6970 5970 (SH3) 4970 (S3)

9969 8969 7969 6969 5969 4969

9931 8931 7931 6931 5931 4931 9930 (SS7) 8930 7930 (SL7) 6930 5930 (SH7) 4930 (S7)

9929 8929 7929

•

6929

•

5929

•

4929

•

9000

•

8000

•

7000 6000 5000 4000

164
values for GRI range between 40,000 microseconds (25 groups per second) and 100,000 microseconds (10 groups per second) in steps of 100

microseconds. The master station transmits a total of nine pulses as shown in figure 9.2. The first eight pulses are separated by 1000

microseconds and the ninth pulse is separated

from the eighth by 2000 microseconds. The

ninth pulse is used for identification of the master signal. Secondary stations transmit

eight pulses which are separated by 1000

microseconds.

E. GRI

The GRI in Loran-C chains are selected to improve receiver operation while reducing

interference from adjacent chains (cross rate interference). The GRI must be long enough to prevent overlap of the master and any secondary signals in the coverage area. Time must

be allowed for between the last secondary's

eighth pulse and the master's first pulse in

the next group.

Pulses Per Group

With multiple pulsing in a GRI, it is possible to increase the effective average

pulse rate and effective average power without

increasing the peak power or losing the advantages of pulse transmission and timesharing.

Each doubling of the number of pulses radiated

results in twice the previous average radiated

power (3 dB) and increases the far-field

strength by the square root of two. Thus,

going from one to eight pulses results in 9 dB more average power. In this manner, the range

of the system is maximum for a given peak

power. Groups of pulses in the series 2, 8,

16, etc., provide the best phase codes for

skywave rejection.

Pulse Spacing

Loran-C pulse spacing is chosen to be

1000 microseconds to provide enough time for

first or second hop skywaves to decay before

the next pulse arrives.

Phase Coding

Phase coding serves several purposes:

(1) It is a means to identify master and

secondary stations automatically.

(2) It allows for the automatic search

process.

(3) It rejects multi-hop skywaves.

Through the use of phase coded signals and

cross-correlation techniques in receivers,

automatic identification of the master's and

secondaries' signals is possible. Phase

coding extends over a period of two GRI (called the Phase Code Interval (PCI) as shown

in figure 9.2. Master and secondary phase

codes are shown in figure 9.5. Basically,

when skywaves, whose phase codes differ from

the desired groundwaves, are received during

receipt of the groundwave, the skywave signal

is cancelled.

STATION

GRI MASTER SECONDARY

A + + -- + - + - + + + + + + __ +

B +--+ + + + + - +_ + _+ + __

FIGURE 9.5. LORAN-C PHASE CODES.

F. Blink Codes

Blinking is used to warn navigation users that there is an error in the transmissions of

a particular station. Blink is accomplished

UNUSABLE

TD(S)

0N-0FF PATTERN * 12 SECONDS *•

NONE

X

Y

Zw

XY

xz

xwYZYW

ZW

XYZ XYW

XZW YZW

XYZW m = APPROXIMATELY 0.25 SECONDS ON mm = APPROXIMATELY 0.75 SECONDS ON

FIGURE 9.6. LORAN-C BLINK CODE.

165
at the master station by turning the ninth

pulse on and off in a specified way as shown

in figure 9.6. The secondary station of the

unusable pair blinks by turning its first two pulses on and off. The first two pulses of

the secondary station are turned on (blinked)

for approximately 1/4 second every 4 seconds.

All secondaries use the same code. Most modern navigation receivers automatically

detect secondary station blink only as this is sufficient to trigger the alarm indicators.

G. Spectrum

The transmitted spectrum is defined to

be the spectral density of the time function

f(t), where f(t) is

t

2 -2t/65

1 kHz

125 Hz

1/GRI Hz

1/(2 x GRI) Hz

1000 microsecond pulse

spacing

Blocks of 8 pulses with

1000 microsecond spacing

Group repetition interval

Phase code interval

NOTE: When observing the spectrum on analyzers whose minimum IF bandwidths are greater than the

spacing between the spectral

lines to be observed, the fine

spectra will be a result of the

spectrum analyzer's IF response

rather than the Loran-C signal.

I. Harmonics

The spectral density function is given by

I- 2

|F(u))

s(uo = [IfMi]

'max.

To assure that Loran-C transmissions do not interfere with other services in adjacent

parts of the radio spectrum, the spectral

level of any harmonic relative to the value

at 100 kHz doesn't exceed the following

levels:

where:

and

F(u») =f

f(t) e"

jujt dt

iu = angular frequency (rad/sec).

The Loran-C spectrum follows provisions of the

Radio Regulations of the International Telecommunications Union (ITU). The total energy

outside the 90 - 110 kHz band is less than 1% of the total radiated energy. The energy

below 90 kHz and above 110 kHz are no greater

than 0.5% of the total radiated energy. As a practical matter for Loran-C type signals,

this is achieved if the spectral density of

the radiated signal at 90 kHz and 110 kHz is down at least 20 dB relative to its value at

100 kHz.

H. Fine Spectra

There are several hundred spectral lines

in the near band spectrum of the Loran-C

signals. These spectral lines result from

the various periodic elements of the Loran-C

transmissions. Among the most significant

are 1 ines every:

HARMONIC

2nd

3rd

4th

LEVEL

-70 dB

-80 dB

-85 dB 5th (or greater) -90 dB

9.3 WHAT IS THE EXTENT OF LORAN-C COVERAGE ?

There are two methods of signal propagation: groundwave and skywave. Using the

groundwave, Loran-C timing is possible to about 1500 miles over land and 2000 miles over

the ocean. Skywave reception beyond 5000

miles is possible.

9.3.1 GROUNDWAVE SIGNAL RANGE

Radio energy from each Loran-C transmitter radiates in all directions. A portion of

the energy travels out from each transmitting

station parallel to the surface of the earth.

This is the groundwave.

Useful Loran-C groundwave coverage ex- tends from approximately 1500 to 2000 miles.

During periods of good reception, this range may be greater and during periods of high

noise and interference, it may be less.

166
However, based on typical noise and interference situations, 1500 miles is a reasonable

estimate of the rel iable groundwave range of

Loran-C signals from a station having 300

kilowatts peak pulse power.

The ability of the Loran-C timing re- ceiver to indicate accurate time readings is dependent upon the relative strength of the

received signals and the level of the local

noise interference. As the signal-to-noise

ratio decreases, the performance of the re- ceiver decreases. As a result, accuracy

deteriorates at receiver locations far away

from the stations. For timing purposes,

long-term averaging may be employed to reduce

errors.

9.3.2 SKYWAVE SIGNAL RANGE

Some portion of the Loran radio signal

radiates upward from the transmitting antenna

and is reflected from the electrified layer

of the atmosphere known as the ionosphere.

This signal is called the skywave. If the

groundwave signal has traveled a long distance, it will be reduced in amplitude and

weakened. The receiver will pick up both the

groundwave and the skywave signals. The

receiver cannot easily lock to the groundwave

because it is weak and noisy, so it will lock

to the skywave signal.

But the skywave signal "moves" around

because it is reflected off the ionosphere.

This movement is caused by the corresponding

motion of the ionosphere due to the action of

the sun. The most pronounced effects occur at sunrise and sunset, but there are also smaller

changes throughout the day and night. Each of

these path changes looks like a change in the

signal phase at the receiving point.

Because the skywave signals are stronger

than groundwave signals at great distances,

skywave lock-on is possible at distances

beyond where the groundwave signals can be

received. A strong skywave signal allows the

receiver to average and reproduce its skywave

phase plot day after day. What we are saying

is that, even though the skywave signal moves

around, it does so in a predictable way. With

good record- keeping, the skywaves can yield

very good time and frequency data.

The next section will explain how Loran works and how it can be used for timing. Keep

in mind that Loran can supply high accuracy

frequency and time calibrations, but under

weak or noisy signal conditions, cycle selection is difficult and requires a skilled operator. If this extreme timing accuracy is not required, other techniques should be examined.

9.4 WHAT DO WE GET FROM LORAN-C?

Loran-C uses several modulation techniques to improve its use for navigation.

These translate directly into an ability to disseminate both frequency and time with

greater accuracy. Foremost among its characteristics is the fact that the signals are transmitted as pulses--and pulses at a carrier

frequency as low as 100 kHz are unusual.

FIGURE 9.7. THE 100 kHz LORAN-C PULSE.

Why does Loran use pulses? The accuracy

of the system is based on the fact that the

early part of the pulse—the one that leaves

the transmitter first—will travel along the

ground and arrive before the less stable sky- wave pulse that bounces off the ionosphere--

so, the groundwaves and skywaves can be sorted

out at the receiver. If your location or reception conditions do not permit groundwave

reception and use, the timing accuracy

achieved by using skywaves may be many times

worse. But, even with skywaves, frequency

calibration accuracy is still very good.

9.4.1 SIGNAL CHARACTERISTICS

In addition to transmitting pulses for

separation of ground and skywave signals, the

pulses are transmitted in groups for identification of the particular station being re- ceived. The slave stations in a particular

Loran-C chain transmit eight pulses to a group. For identification, the master station

transmits a ninth pulse in each group.

Loran station identification is also

aided by separately phase-coding the master

and slave pulses. Each group of pulses is coded by a phase reversal process which en- ables the receiver to eliminate certain types

of interference. Also, since all Loran-C

broadcast at 100 kHz, it is possible to re- ceive many different Loran chains at the same

time. Therefore, the pulses in each chain are transmitted at slightly different rates so the

receiver can distinguish between them.

167
V

LORAN-C TIMING

RECEIVER

STOP

TRIGGERS

SYNCHRONIZED

TO LORAN-C

TIME

INTERVAL COUNTER

1PPS

FREQUENCY

STANDARD

LOCAL CLOCK

1 PPS

A START

TRIGGERS SYNCHRONIZED

TO NULL EPHEMER I S TABLE

LORAN RATE GENERATOR AND

SYNC CIRCUITRY

FIGURE 9.8. INSTRUMENTATION FOR UTILIZING PULSES WITHIN THE LORAN-C PULSE GROUPS.

All these subtle differences in trans- mission pulse rates give rise to the need for

a fairly complex receiving system. Although

relatively simple Loran-C navigation equipment

exists, the use of Loran-C for timing requires

greater interaction by the operator. As might

be expected, the cost of receiving equipment

is higher than less complex VLF or high frequency radio equipment. Manual signal lock-on,

which is required with some Loran-C receivers, means that the operator skill required is fairly high. Still, once the equipment is installed and the operator gains a little

experience, the results can be exceptionally

accurate. As a frequency and time calibration

system, Loran-C is almost unequaled.

Figure 9.10 shows how signals from a Loran-C chain are timed. Plus and minus signs over individual pulses indicate the phase

code, where (+) represents a relative phase

of 0° and (-) represents 180°. All master

stations are phase-coded one way, and all slaves are phase-coded another.

9.4.2 TIME SETTING

To make time calibrations, the user of Loran-C must take into account the fact that

the pulse repetition rates used are not at one pulse per second, nor are they a convenient

LORAN-C NAVIGATION CHAIN

PHASE

CODE

MASTER

IDENTIFIER

MASTER

SLAVE Xf

SLAVE Y

bx

MASTER • RECEIVER

TIMING OF LORAN-C SIGNALS

• MASTER IDENTIFIER

TT1TTT 1 TTTTTm

SLAVE X SLAVE Y_ MASTER \ SLAV E X

( + SLAVER MASTER

1

X EMISSION

DELAY « Y EMISSION DELAY

GRPTTTTTTT TITTTTTT 1 TflTT

z

—GROUP INTERVAL A GROUP INTERVAL 4 ^— GRP-

(SS<}.,GPR=100,000ys) ONE FRAME Xi< 1 OOOys . -Qf^t- U/\/\(l(|l/WfrFIGURE 9.9. TIMING OF LORAN-C SIGNALS.

168
IB)

FIGURE 9.10. STEPS IN LOCKING ONTO A LORAN-C

SIGNAL: THE PULSES SHOULD BE CLEARLY VISIBLE ABOVE THE NOISE,

AS IN (A) . (B) MOVE THE PULSES

TO THE LEFT AND INCREASE 'SCOPE SWEEP SPEED TO 1 ms/cm. (C) SHOWS THE FIRST OF THE EIGHT PULSES AT A FASTER 'SCOPE SWEEP SPEED. NOTE THAT THE SINE WAVES ARE DOUBLED, INDICATING LACK OF CODING. (D) RESOLVE THIS BY ADJUSTING RECEIVER.

(E) SHOWS THE FINAL STAGE OF PULSE POSITIONING WITH THE BRIGHT SPOTS

ON THE WAVEFORM AS THE RECEIVER

TRACKING POINT.

169
multiple thereof. Instead, a time of coincidence, TOC (when a Loran pulse coincides with

a one-second pulse) may occur as infrequently

as every sixteen minutes. After the receiver

is locked to the incoming signal, the one pulse-per-second timing output must be syn- chronized at the time of coincidence. This is done by pushing a button on the receiver just

before the correct second arrives. Assuming

power is not lost, and the receiver does not

lose lock, the timing pulses will stay "on time." It is important to keep a battery

supply connected to the receiver to prevent

loss of synchronization in case of power

outages.

Loran-C is most often used when the

required timing accuracies are equal to or better than ten microseconds. Remember that

any user of a timing system that provides

accuracies of this order must take into account path and equipment delays. In addition, he must pay very close attention to how

the receiving equipment is maintained and

have the necessary battery backups. Simply

stated, as with any scheme for time and frequency calibrations, if you are looking for

high accuracies, you must be willing to pay

the price in equipment complexity and

operator attention.

The most difficult part of Loran-C timing

is recognizing the sample point where the re- ceiver is locked onto the signal. Since

Loran-C operates at 100 kHz, it is possible to lock onto carrier zero crossings separated by

the 10 microsecond period. An oscilloscope

display of Loran-C pulses is helpful and under

noisy conditions, it is of great assistance to the operator to have a graphical plot made of

the received pulse showing the exact position

of the sample point. Some receiving equipment

sold for Loran timing usually incorporates

this plotting capability.

Setting Local Time .

In a timing system, it is often convenient or necessary to operate "on time" rather

than simply with a known time offset. The

equipment needed is basically a clock that has

a display for the time-of-day readout.

The following steps show how to synchronize local time by Loran-C:

1. Obtain a coarse clock synchronization to UTC by reference to WWV. Allow for propagation and receiver

delays if known.

2. Acquire and track a Loran-C trans- mission from either a slave or master transmitter.

3. On an upcoming time-of-coincidence

(TOC), operate the clock-sync con- trols so that clock circuits will

synchronize the internal receiver

time signal when the TOC occurs.

4. Using Loran-C emission and propagation delay figures (available,

along with TOC tables, from USNO,

Time Services Division, Washington,

DC 20390) and the receiver delay

from the operating manual, the

clock time can be corrected to be "on time.

This completes synchronization. Subsequent "one-second" pulses from the receiver

wi 1 1 be "on time.

9.4.3 FREQUENCY CALIBRATIONS USING LORAN-C

Frequency calibration using Loran-C is less complicated than time recovery, but the

operator must still acquire the station and

lock the receiver to a correct operating

point. Under noisy reception conditions, the

oscilloscope display of the received pulses

may be too noisy to allow the tracking point

to be positioned accurately. Unless the

operator can identify eight pulses, the

receiver may fail to lock because of the

inherent phase coding.

Assuming that lock-on has been acquired,

the receiver will track the local oscillator

against the received signal and produce a phase plot. The chart then simply represents

the beat note between a local standard and

the received Loran signals.

How to Measure Frequency .

The frequency of a local frequency stan- dard is measured in two steps. First, a phase-tracking record is prepared using the

signal radiated by a synchronized Loran-C transmitter. The record should cover a period

that is appropriate to the desired measurement

accuracy. For example, in most locations, a groundwave phase record extending over twentyfour hours is adequate for relative frequency

determinations having a probable error of one or two parts in 10 11 to 10 12

. Shorter periods

may be used—or the skywave rather than the

groundwave may be tracked--when extreme accu- racy is not needed.

The second step is to reduce the data

provided by the record to yield frequency

difference. The basic expression used for

this purpose, as discussed in Chapter 3, is:

Af = _ t

Z

" t

l

f T

170
'800 1200 1600 2000 0000 0400 0800 1200 1600

2.5 MHz STANDARD VS CAPE FEAR LORAN-C, l/4"/Hr 10 ys FULL SCALE

FIGURE 9.11. SAMPLE RECORD OF RECEIVED LORAN-C SIGNAL, NOON SEPTEMBER 26 THROUGH NOON SEPTEMBER 27, 17-65 MICROSECONDS.

where Af/f is the relative frequency of the

local frequency source, and t x and t 2 are

initial and final phase or time differences,

respectively, obtained from the record over

the averaging time interval, T.

A sample phase record is shown in figure

9.12. Here we see that the relative phase

between the local standard and the Cape Fear

Loran-C signal stood at t x = 4.25 microseconds

at noon. At the following noon (86,400 seconds later), the relative time had moved

across the chart once and then to 5.9 microseconds. Thus, the relative frequency difference for the indicated twenty- four hour

period was:

Af _ At _ , -(3.45 + 10 + 4.1) v ~ ~ v in > f T 8.64 x 10

=2.03 x 10" 10

9.5 HOW GOOD IS LORAN-C?

The frequency and time services provided

by Loran-C are capable of providing calibrations at very high accuracy levels. Of course the accuracy achieved will depend on propagation conditions, the path from the

transmitter to the user, and the length of

time over which the measurements are averaged.

At this writing, Loran-C is one of the

principal methods of intercomparing the

atomic standards at the NBS, the USN0 and the

National Research Council in Canada. This

scheme is also extended to the European Loran network and affords a means for the BIH (Paris) to compare all of the world's main

time scales. The results reported for this

system of intercomparison are impressive--

approaching 1 part in 10 13 over several

months.

Typical values for groundwave reception over land could easily approach a few parts

in 10 12 for a one-day average. Corrections

to Loran data are provided by the USN0, and

the NBS also monitors and reports daily Loran phase readings. Users who want very accurate

results from Loran-C must correct the

observed data by using the published data. This information is available by TWX on a daily basis from the USN0 or by mail weekly

from the USN0 and monthly from NBS.

171
9.5.1 GROUNDWAVE ACCURACY

For timing, the accuracy of the ground- wave at a particular location depends upon several factors that contribute to the errors

in the measurements. The approximate errors

are:

1. Error in transmitted signal, 0.2

microseconds.

2. Errors caused by propagation path,

up to 0.2 microseconds over water;

up to 1 microsecond over land.

3. Receiver errors, 0.02 microsecond

typical

4. Errors caused by atmospheric noise

and interference, up to 0.5 microsecond for interference. Longer

averaging times can be used with

noisy signals.

9.5.2 SKYWAVE ACCURACY

At frequencies near 100 kHz, skywave

stability depends on the reflecting medium--

the ionosphere. Frequently, the ionosphere

causes small signal amplitude fades or phase

variations for long periods during total daylight on the path. As a general rule, expec- ted one- or two-hop skywave stability is approximately several microseconds and is quite usable for timing.

9.6 ARE THE DATA VALID?

The Loran-C groundwave, unlike other VLF

or LF transmissions that are used for frequency calibration, has no characteristic phase

signature such as the diurnal phase shift of

the skywave at VLF or the WWVB-type of periodic phase offset and return. If, in the

absence of "built-in" phase disturbances of

this type, substantiation of the Loran phase

record is desired, it can be accomplished in two ways:

1. Some Loran receivers provide for a signal envelope record showing the

position of tracking points that

can be made on a second chart

recorder. See figure 9.13.

2. A concurrent record of signal amplitude can be made. However, this

does not provide evidence of tracking as clearly as the recycling

envelope trace suggested above.

Shown are two plots of the received Loran

pulse. Notice the sample point where the

- 0

TRACKING POINT

AT END OF SECOND

CYCLE

FIGURE 9.12. PLOT OF THE RECEIVED LORAN-C SIGNAL.

172
receiver is tracking the signal. Although

this method does not always guarantee cycle

count, it at least will allow the operator to reset the receiver at the same point after the

signal is lost and reacquired.

9.7 SUMMARY

Loran-C broadcasts provide an excellent

source of signals for both frequency calibrations and time recovery. Loran may not be the

best choice for all users, especially those

who have only routine low- to-medium accuracy

calibrations to perform. Balanced against

this, however, is the fact that there are to

be a number of chains in operation and the

signal strength will be high. It can also be

expected that more Loran manufacturers will

enter the marketplace. This should provide

lower cost instruments that are easier to operate.

Like all other services described in this

book, Loran-C requires that the operator

expend more effort for very accurate results.

This is especially true for accurate time

recovery-the user must not confuse the

resolving power of Loran-C (in the nanosecond

region) with the accuracy (1 to 2 microseconds) that the average user can achieve. It

is also worth repeating the suggestion to any potential user of Loran-C (or any other ser- vice) to use the manufacturer's published

material. If any doubts exist, try to obtain

a receiver on loan and gain some first-hand

experience before investing in any technique

for time and frequency calibration.

173

CHAPTER 10. SATELLITE METHODS OF TIME AND FREQUENCY DISSEMINATION

For many years people have realized that

an earth satellite would be the nearly ideal

way to broadcast radio signals for time and

frequency calibrations. At this writing, two systems are operational and another is being

planned.

NBS provides a continuous time code via

the GOES (Geostationary Operational Environmental Satellite) satellites. The U. S. Navy

operates the TRANSIT series of navigation

satellites with major participation by the

Applied Physics Laboratory of the Johns Hopkins University and the U. S. Naval Observatory. The operation of these two systems will

be explained in this chapter.

In addition to GOES and TRANSIT, the

proposed Global Positioning Satellite System

(GPS) is in the experimental stage. When it

becomes operational, it will offer users

several advantages. First, its frequency

band, signal strength, and the use of state- of-the-art atomic oscillators on board each of

the planned 24 satellites will allow highaccuracy calibrations. Second, its predicted

popularity should make available a line of

equipment that will be versatile and easy to

use. However, since no firm operational date

can be specified, GPS will not be covered in this chapter.

10.1 THE GOES SATELLITE TIME CODE

10.1.1 BACKGROUND

A satellite signal is no different than

any other radio signal so far as the user is concerned. Table 10.1 gives the main specifications for GOES. Note that it broadcasts in the UHF band (which means small antennas are

used), the system supports two satellites at

this time, and the satellites are nearly sta- tionary. That is, if you point the antenna to one part of the sky, you don't have to move it to keep a satellite in view.

The user should not be discouraged by the

difference in frequency or the operating techniques needed for frequency and time calibrations using GOES. In fact, tests have shown

that GOES is one of the easiest services to use and that it can be used in areas where

other signals are nearly impossible to re- ceive. The equipment provided by manufacturers is usually quite sophisticated and nearly

automatic in its operation.

GOES is an operational descendant of NASA's Applications Technology Satellites.

TABLE 10.1 SPECIFICATIONS FOR THE GOES

SATELLITES

FREQUENCY: 468.8250 MHz (Western GOES)

400.00/0 MHz (.eastern GUbSJ

POLARIZATION: RIGHT-HAND CIRCULAR

MODULATION: CPSK (± 60°) DATA RATE: 100 BPS

SATELLITE

LOCATION:*

135° W (Western GOES)

75° W (Eastern GOES)

SIGNAL

STRFNGTH

AT EARTH'S

SURFACE

(OUTPUT FROM

ISOTROPIC

ANTENNA):

-139 dBm

CODING: MANCHESTER

BANDWIDTH: 400 Hz

*0ther satellite locations may be used occa- sionally when operations must be switched to in-orbit spare satellites.

The GOES satellites are in orbit 36,000 kilo- meters above the equator. They travel at about 11,000 kilometers per hour and remain

continuously above nearly the same spot on earth. They are thus termed geostationary.

Since they have the same regions of earth in

constant view, they can provide 24-hour,

continuous service.

The advantages of geostationary satellites for time broadcasting are numerous. They are almost always in view and provide a source of continuous synchronization. Non- geostationary satellites, on the other hand,

offer exposure to the user only for short

periods of time at intervals ranging from

about one hour to many hours or even days.

Thus, non-geostationary satellite systems

usually have a number of satellites in orbit.

Because of the bandwidth and its center

frequency, the GOES system operates on the

technique of using time measurements to calibrate frequency. As other chapters in this

book explain, a time variation can be trans- lated into a frequency difference. When the user buys a GOES receiver, he will have an instrument that expects the user to connect

his 1 pps clock for comparison. This implies

175
that the user must have such a clock and maintain it with batteries, etc., in order to use the GOES signals.

There is nothing to prevent manufacturers

from using frequency measurements directly and

from building equipment to do this. However,

GOES has a time-code output (perhaps its major

application is for its time code) and since

the manufacturers felt the largest number of users wanted the time code, they developed

their equipment for time recovery. Frequency

calibrations obtained using GOES time data

fall into the accuracy range from 1 part per

million to 1 part in 10 9 (for a 1-day measurement period) with respect to NBS, depending on the sophistication of the equipment used.

The path from the NBS master clock to the user via a geostationary satellite is, in effect, a 1 ine-of-sight path allowing the use of high carrier frequencies that are largely

unaffected by the ionosphere and troposphere.

This means negligible fading and path length

variations which are so characteristic of

terrestrial HF signals. The 1 i ne-of-sight

path also means that free space transit time

computations will work for most timing applications.

The NBS master clock is located on the

ground rather than in the satellite itself.

The satellite is only a "bent pipe" or "trans- ponder" used to relay signals. This allows

for easy control and maintenance of the sys- tem, thus guaranteeing better performance and

rel iabi 1 ity.

Because of these advantages, the GOES satellite can provide a time message, repeated

continuously, to clocks in its view. It can control the frequency rate of the slaved

ground clock to eliminate the need for highquality oscillators and can also provide position data to correct for propagation delays.

10.1.2 COVERAGE

There are three GOES satellites in orbit, two in operational status with a third serving

as an in-orbit spare. The two operational

satellites are located at 135° and 75° West

Longitude and the spare is at 105° West Longitude. The western GOES operates on 468.825

MHz; the eastern on 468.8375 MHz. The earth

coverages of the two operational satellites

FIGURE 10.1. COVERAGE OF THE GOES SATELLITES.

176
TC WORD

1/2 S

INTERROGATION

MESSAGE

TC WORD

1/2 S

INTERROGAT ION MESSAGETC WORD

1/2 S

[ NT E RR OGAT T ON MESSAGETC WORD

1/2 S

INTERROGATION

MESSAGE

—

30 S

TIME CODE FRAME CONSISTING OF: •SYNCHRONIZATION WORD •DAYS, HOURS, MINUTES, SECONDS •UNIVERSAL TIME CORRECTION •SATELLITE POSITION

FIGURE 10.2 INTERROGATION CHANNEL FORMAT

40 BITS 32 20 16 16 I 08 SYNC WORD

TIME OF YEAR LONG LAT EXPERIMENTAL USE

BEGINS ON

00 AND 30 SECONDS UTC

UT1 TIME CORRECTIONS

SATELLITE POSITION

TIME FRAME

-30 SECONDSFIGURE 10.3 TIME CODE FORMAT.

are shown in figure 10.1. The heavy oval

lines overlap the points on earth where each

satellite is 7° above the horizon. The

lighter lines represent a 3° elevation angle.

10.1.3 SIGNAL CHARACTERISTICS

The signal characteristics are summarized

below. The two satellite signals are both

right-hand circularly polarized and separated

in frequency by 12.5 kHz. The data rate is 100 b/s requiring 400 Hz of bandwidth. The

data are Manchester coded and phase modulate

the carrier ± 60°, thus providing a carrier

for the application of conventional phase-lock

demodulation techniques.

The main purpose of the GOES satellites

is to collect environmental data from remote ground sensors. The time code was added to provide time and a date to the collected data.

10.1.4 TIME CODE FORMAT

The time code is part of the interrogation channel which is used to communicate with

the remote environmental sensors. Interrogation messages are continuously being relayed

through the GOES satellites. The format of

the messages is shown in figure 10.2.

Each interrogation message is one-half

second in length or 50 bits. The data rate is 100 b/s. The time-code frame begins on the

one-half minute and takes 30 seconds to com- plete (see figure 10.3). Sixty interrogation

messages are required to send the 60 BCD time-code words constituting a time-code

frame.

The time-code frame contains a synchronization word, a time-of-year word (UTC), the

UT1 correction, and the satellite's position

in terms of its latitude, longitude, and

height above the earth's surface minus a bias

of 119,3000 microseconds. The position information is presently updated every 4 minutes.

As shown in figure 10.2, an interrogation

message contains more than timing information.

A complete message consists of four bits

representing a BCD time code word followed by

a maximum length sequence (MLS) 15 bits in length for message synchronization, and ends

with 31 bits as an address for a particular

remote weather data sensor.

177
20 40

100 120 160 E 180 W 160 140 120 100 80 60 LONGITUDE

0 20 40

FIGURE 10.4. WESTERN SATELLITE POINTING ANGLES.

100 120

LONGITUDE

160 E 180 W 160 140 120 100 80 40 20 20 40

20 40

FIGURE 10.5. EASTERN SATELLITE POINTING ANGLES.

178
LONGITUDE

LONGITUDE FIGURE 10.6. WESTERN SATELLITE MEAN DELAYS.

For total delay, add 133.5 ms to downlink delays shown.

LONGITUDE

100 120 140 160 E 180 W 160 UO 120 100 80 60 SO .'(1 0 .'() 10 LONGITUDE FIGURE 10.7. EASTERN SATELLITE MEAN DELAYS.

For total delay, add 124.5 ms to downlink delays shown.

179
10.1.5 ANTENNA POINTING

Pointing an antenna to either satellite

is relatively simple. Because of the large

beamwidths of low-gain antennas (< 10 dB),

pointing into the general direction of the

satellite is usually sufficient. However,

the antenna must be located so it has an unobstructed path to the satellite. Figures

10.4 and 10.5 provide detailed information for antenna pointing with elevation and azimuth

angles provided. For example, the pointing

directions to the eastern satellite from San Francisco can be obtained from figure 10.5 as approximately 119° azimuth and 24° elevation.

Satellite positions may change with time. Users should check the monthly NBS Time and

Frequency Bulletin for current status.

10.1.6 PERFORMANCE

The GOES time code can be used at three

levels of performance: uncorrected, corrected

for mean path delay only, and fully corrected.

A. Uncorrected

The path delay from point of origin

(Wallops Island, Virginia) to the earth via

the satellite is approximately 260,000 microseconds. Since the signals are advanced in time by this value before transmission from

Wallops Island, they arrive at the earth's

surface nearly on time (within 16 milliseconds).

B. Corrected for Mean Path Delay

Accounting for the mean path delay to any point on the earth's surface, but ignoring the

cyclic (24-hour) delay variation, generally

guarantees the signal arrival time to ± 0.5

millisecond. For example, the mean delay to San Francisco through the eastern satellite is 130.5 milliseconds downlink (from figure 10.7)

plus 124.5 milliseconds uplink delay, for a total of 255 milliseconds. Since the time is advanced by 260 milliseconds before leaving

Wallops Island, the time arrives at San Francisco 5 milliseconds early.

C. Fully Corrected

The cyclic path delay variation on GOES

is a result of the satellite orbit or path

around the earth not being perfectly circular

and not in the plane of the equator. The

orbit is actually an ellipse and has a small

incl ination--usual ly less than 1°. To compensate for these and other effects, the satellite position is included with the time mes- sage for correction of path delay by the user. (See [18] and [19] for details on how this is accomplished.) This correction provides path

delays accurate to ± 10 microseconds under

normal operating conditions. The ultimate

accuracy of the recovered time depends upon knowledge of user equipment delays and noise

levels as well as path delay.

Typical delay variations for the eastern satellite are shown in figure 10.8. The

peak-to-peak values change as the satellite's

inclination and eccentricity are varied by

orbit maneuvers and natural perturbations.

10.1.7 EQUIPMENT NEEDED

A. Antennas

Many antennas have been used by NBS witn

varying results. The best performance has been obtained using a right-hand circularly

polarized helix antenna (fig. 10.9) with about

10 dB gain. Excellent results have been obtained with the microstrip antenna shown in figure 10.10. Dipoles and loops have also

worked at lower levels of performance. All of these antennas had gains in the range of

3 to 10 dB.

6pm 4pm 2pm 12n 10am 8am 6am 4am 2am 12mn 10pm 8pm 6pm

FIGURE 10.8 TYPICAL DELAY VARIATIONS FOR THE EASTERN SATELLITE.

180
FIGURE 10.9. RIGHT-HAND CIRCULARLY POLARIZED

HELIX ANTENNA USED TO RECEIVE

GOES SIGNALS.

B. Receivers

The NBS-built receiver shown in figure

10.11 is completely automatic, requiring no tuning or auxiliary equipment to operate. Com- mercial versions of this equipment are also

available. For information on manufacturers,

contact the Time and Frequency Services Group,

524.06, NBS, Boulder, CO 80303.

10.1.8 SATELLITE-CONTROLLED CLOCK

The clock shown in figure 10.11 was designed to interface with a receiver and

achieve bit and frame synchronization, strip

the time code from the interrogation message,

FIGURE 10.10. MICR0STRIP ANTENNA USED TO RECEIVE GOES TIME CODE.

FIGURE 10.11. GOES TIME CODE RECEIVER.

181
obtain time code frame sync, and read the time

code message into memory. The clock then up- dates the memory time by counting cycles of

the interrogation channel data clock, a 100 Hz square wave, and verifying its memory time

through continual comparisons with consecutive

time code messages from the satellite [20].

This clock was designed to address the

majority of user's needs at minimum cost. This

clock was not designed to "squeeze" the last

bit of performance from the GOES timing system. The clock does not directly use the

satellite position data to correct its output

but can display the data for manual computation of the delay.

A. Delay Corrections

The usefulness of potentially highly accurate timing signals relayed through geo- stationary satellites is complicated by the

computation of the propagation delay from the

transmitter through the satellite back down to the receiver. Computation of this delay is usually accomplished through the manipulation

of orbital elements. From the orbital elements, six constants which describe the

satellite's position and velocity at a given

instant of time, and a complete description of

perturbing forces, it is possible to compute

the satellite's position at other times.

Once satellite position is known, the

free space propagation delay to any receiver

follows directly from simple geometry. The

computation of satellite position from orbital

elements, however, is complicated and best

accomplished using a digital computer.

This type of computation would be difficult for most of the expected users of the

GOES time code. However, NBS has developed

programmable calculator programs, and a "smart" clock which uses the satellite's

position to compute the delay to high accuracy.

B. Programs for Computing

Free-Space Path Delay

Several programs to compute the total

free space path delay from Wallops Island,

Virginia, through the satellite to any point

on the earth's surface are available from the

Time and Frequency Services Group, 524.06,

NBS, Boulder, CO 80303. These programs are written for use with programmable calculators.

10.1.9 SMART CLOCK

A "smart" clock with receiver is shown in figure 10.12. This is essentially an addition

FIGURE 10.12. SMART CLOCK.

182
of a second microprocessor to the "satellitecontrolled clock" for the calculation of the

free-space propagation delay from the CDA to the clock via the satellite. This delay value

is then used with a delay generator to com- pensate for the free-space path delay.

The "smart" clock uses the same type of

four-bit microprocessor as the "satellitecontrolled clock." The microprocessor is interfaced to a large scale integration scientific calculator array (math chip) to provide the floating arithmetic and mathematical

functions required in the delay calculation.

A 1 pps output and satellite position as longitude, latitude, and distance from a reference orbit is obtained continuously from

the satellite-controlled clock operating on

the satellite's transmitted interrogation

channel signal.

User position is entered into the system

via thumbwheel switches, and transmitter position (Wallops Island, Virginia) is contained

in the microprocessor software. The computed

delay drives a programmable delay generator

to correct the 1 pps from the satellite con- trolled clock. The resultant output from the

programmable delay generator is a compensated

1 pps, adjusted to be in agreement with the master clock at Wallops Island, which is in turn referenced to UTC(NBS). The hardware is

a multiprocessor system consisting of two microprocessors plus a slaved scientific calculator chip and a delay generator.

468 MHz \,

^ GOES S SATELLITES

ANTENNA

The delay calculation is dependent on knowledge of the broadcasting satellite's

position. This position is predicted in advance by a large scale scientific computer

operating on orbital elements obtained from

NOAA and sent to Wallops Island via telephone

line to be broadcast along with the time code

from each satellite. The delay correction

system will work with any satellite in a synchronous orbit as long as the satellite's

position is known. A calculation is made and

the result is latched into the delay generator

once per minute. A complete up and down delay

calculation requires the execution of about

200 key strokes representing data and mathematical operations and functions under the

control of the microprocessor and its associated transistor-transistor-logic (TTL) components.

A block diagram of the smart clock, in- cluding receiver and antenna, is shown in figure 10.13. The smart clock has been com- pletely documented (schematics, board layouts,

and software)[19].

10.1.10 CLOCK CALIBRATION

To use the GOES signals to calibrate or set another clock, one needs to determine the

relationship of the satellite controlled

clock's 1 pps relative to UTC(NBS) and to the

clock being calibrated. The relationship

RECEIVER

USER'S LONGITUDE LATITUDE

1 0 5 ,, 2 6

THUMBWHEEL

SATELLITE-CONTROLLED

DIGITAL CLOCK

/^.PROCESSOR (4004)

SATELLITE

LONGITUDE,

LATITUDE, RANGE

4 0 ,, 0 0

SWITCHES

4004 -BASED DELAY-COMPUTING

/^PROCESSOR

512 kHz

1PPS UNCORRECTED

POR DELAY

PROGRAMMABLE

DELAY GENERATOR

(TTL)

LSI/MOS

SCIENTIFIC CALULATOR CHIP

DISPLAY LEDS

2 5 2 4 2 9

/xSECONDS DELAY

1PPS CORRECTED

FOR DELAY

FIGURE 10.13. SMART CLOCK BLOCK DIAGRAM.

183
between the satellite-controlled clock and the

clock being calibrated is usually determined

by use of a time interval counter. For example, the clock to be calibrated may start the

counter, then the satellite controlled clock

stops it. The relationship between UTC(NBS)

and the user's clock is then known by combining this measured result with the computed

difference between the satellite clock's 1 pps and UTC(NBS).

An example of the clock calibration

procedure is given below. The clock to be calibrated is located in Boulder, Colorado,

where the GOES satellite signals are being

received.

Signals received in Boulder (105.26° W Longitude and 40.00° N Latitude) showed the

satellite location to be:

Since the Wallops Island clock is set to be 260,000 us early with respect to UTC(NBS):

WALLOPS ISLAND CLOCK - UTC(NBS) = + 260,000 us

Then, by adding these two equations:

SATELLITE CLOCK - UTC(NBS) = - 6,351 us (1)

The measured time interval between the user's clock and the satellite signal is stated as:

134.92° W Longitude

0.38° S Latitude

+46 us Distance

USER'S CLOCK - SATELLITE CLOCK = +6,548 us (2)

Using a calculator, the following delays are calculated:

WALLOPS ISLAND TO SATELLITE

SATELLITE TO BOULDER

TOTAL PATH DELAY

To this we must add:

MEASURED EQUIPMENT DELAY

to get the total delay from

Wallops Island to the user's satellite clock 1 pps.

TOTAL DELAY:

133 606 ps 127 553 ps

261 159 ps

+ 5 192 ps

266 351 ps

This result can also be written as:

SATELLITE CLOCK - WALLOPS ISLAND CLOCK

= - 266,351 ps,

where the result is negative because the

satellite clock is late with respect to the

Wallops Island clock (see Chapter 3 for sign

conventions)

Then, by adding equations (1) and (2):

USER'S CLOCK - UTC(NBS) =+ 197 ps. (3)

The smart clock automatically computes

the relationship between UTC(NBS) and the

output of the satellite-controlled clock and

adjusts the difference to be zero; i.e.,

UTC(NBS) = satellite-controlled clock. The

calibration using the smart clock is therefore

read directly from a time interval counter.

10.1.11 RESULTS

More than two year's of data from several

satellites have been recorded at NBS Boulder

using the measurement configuration shown in figure 10.14. The results, on strip chart

recorders, have been of the form illustrated

in figure 10.15. The phase shift in the

uncorrected output is clearly evident. The

corrected output is generally a straight line

since the satellite position data are updated

every four minutes. Remember that the time

code message and position data are repeated

every half-minute but new position data are input once every 4 minutes.

The equation relating the time recovered

from the satellite to the master clock at Wallops Island is given in eq. (4):

184
ANTENNA

RECEIVER

lpps FROM

NBS MASTER CLOCK

"

DATA DATA CLOCK

DIGITAL CLOCK

SATELLITE

POSITION

RECEIVING

LOCATION

(LONGITUDE

& LATITUDE )

1 pps

TIME INTERVAL COUNTER

START STOP HZ

(UNCORRECTED)

DELAY CORRECTION

lpps (CORRECTED FOR FREE SPACE

PATH DELAY)

TIME INTERVAL COUNTER

START STOP 3 t

UTC(NBS)-SAT/NBS- (CORRECTED)

FIGURE 10.14. MEASUREMENT OF GOES SIGNALS.

UTC(NBS) - SAT/USER

1 = (UTC - CDA)

/FREE SPACE \

[ PROPAGATION)

\ DELAY /

(IONOSPHERE &^ TROPOSPHERE

DELAY ,

(CDA EQUIP DELAY)

/ satellite \

(transponder)

\ DELAY /

/ RECEIVER \

I & CLOCK

\ DELAY /

Term 1 is known to better than 1 microsecond

using the data logger which compares the CDA

clocks to Loran-C and TV Line-10. This term

will be controlled in the next generator

system to be installed at Wallops Island to

± 3 microseconds or better. Terms 2, 4, 5 and

6 are expected to remain constants at the few

microseconds level and can for the most part

be calibrated out of a user's system. Term 3 can be computed as discussed previously, with

its accuracy dependent upon the quality of the orbital data given to NBS by NOAA. All measurements to date indicate this term will

be accurate to a few tens of microseconds.

Figure 10.16 shows the results of one month's data taken at NBS/Boulder using orbit

predictions derived from three sets of orbital

elements extrapolating as much as 22 days from

their date. The results indicate a system

capability of 10 microseconds. Over a period

\ * s V

CORRCCTED FOR FREE SPACE

. DELAY AT A MIN INTERVALS

FIGURE 10.15. SATELLITE SIGNALS CORRECTED AND UNCORRECTED.

185
UTC(NBS)-G0ES1/NBS

CORRECTED FOR CLOCK DRIFT

& EQUIPMENT DELAY CHANGES

AT CDA WALLOPS ISLAND, VA O— AVERAGE OF 48 MEASUREMENTS

TAKEN AT HALF HOUR INTERVALS

OF 43019 OF 43033 OF 43040

_J I I I I I 1 I 43031 43035 43039 43043 43047 43051 43055 43059 MJD

FIGURE 10.16. TYPICAL RESULTS.

of one year, charts have deviated from the

"straight line" by as much as 25 microseconds.

Using two receivers, there was no correlation

in the drift of the receiver and we conclude

that the problem resides with the receiving

equipment only. A better definition of per- formance and accuracy cannot be made until

data are gathered from widely separated locations in the Western Hemisphere.

Because of the spacing of frequency

assignments to the land-mobile users, there is far less interference to the eastern satellite

signals than to the western satellite. Therefore, the eastern satellite should be used by

those user's situated in large urban areas.

B. Outages

10.1.12 PRECAUTIONS

A. Interference

The "land-mobile" services and the GOES interrogation channel use the same frequency

allocations (468.8250 and 468.8375 MHz) , which

means the time code may suffer interference

from land-mobile transmissions. This is par- ticularly true in urban areas where there is a high density of land-mobile activity. The

satellite frequency allocations are secondary

to the land-mobile services. Therefore, any such interference must be accepted by the time

signal users. Complaints to the FCC will not result in any adjustments in favor of such

users. The spectrum use by satellite and

land-mobile is shown in figure 10.17.

Although the GOES satellites transmit

continuously, there may be interruptions

during the periods of solar eclipses. The

GOES satellites undergo spring and autumn eclipses during a 46-day interval at the

vernal and autumnal equinoxes. The eclipses

vary from approximately 10 minutes at the

beginning and end of eclipse periods to a maximum of approximately 72 minutes at the

equinox (fig. 10.18). The eclipses begin 23 days prior to equinox and end 23 days after

equinox; i.e., March 1 to April 15 and September 1 to October 15. The outages occur during local midnight for the satellites. In the future, it is expected that GOES will oper- ate through eclipse periods using batteries.

There will also be shutdowns for periodic maintenance at the Wallops Island ground

LAND

MOBILE- WESTERN

GOES 1

EASTERN

GOES vzzzzzzwzzzzm

LAND MOBILE

YZZZZZZZZZZZZZZA

468.8250 468.8375

(MHz)

468.8500

FIGURE 10.17. FREQUENCY USE.

186
MARCH APRIL'

7 17 27 6 16

50

40

30

20

10

/

/

>vV

/PARI

ECLI

IAL PSE // t /

\ > \\

A / \\

\\

A \\

/

/ /

FULL ECLIP 5E

I \ \\

h

1

\\

\\

1

1

\\

\\

1 \\

\\11

II

f-

- SEPTEMBER -« OCTOBER

FIGURE 10.18. SOLAR ECLIPSE TIME

station. These scheduled outages will be

reduced in the future as redundancy is added

to the Wallops Island facility.

C. Continuity

NBS cannot give an absolute guarantee to the long-term continuance of the GOES time

code. The GOES satellites belong to N0AA and

are not under the direct control of NBS. However, NBS and N0AA have agreed to include

the time code in the GOES satellite trans- missions to the maximum extent possible. The

GOES system presently has a sufficient number

of satellites to operate into the late 1980' s, and it is expected that the time code will be

included throughout this period.

10.2 THE TRANSIT NAVIGATION SYSTEM

10.2.1 GENERAL

These Navy navigation satellites have

been operational since 1964. Known as the

TRANSIT satellites, their design, power level,

and coverage provide an excellent source of

time and frequency signals for calibration

purposes. The signal format used on the sys- tem lends itself well to time recovery from

which frequency calibrations can be derived

using the techniques previously described in this book.

A major difference between TRANSIT and

GOES is that TRANSIT, being an orbiting system, experiences a much larger Doppler shift

of its radio signal. This is no real problem

for the user since the overall system design

takes this into account and provides enough

information to the users so that accurate time

recovery can be achieved.

The biggest difference between GOES and

TRANSIT is that the TRANSIT system can only be

used intermittently--when a satellite is in view of the receiver.

This system is a good candidate for users who have a worldwide requirement. TRANSIT is,

in fact, the only time and frequency system

that will work anywhere in the world. GOES is limited at present to the western hemisphere

and its oceans.

The satellites broadcast at about 400

MHz. The message sent contains a number of

characteristics which allow position fixing

and also time recovery. As with the GOES

system, TRANSIT allows a user to recover time

signals which can be used as a clock or to steer an oscillator's frequency. Unlike GOES,

however, the TRANSIT time signal format does not include day-of-the-year information.

Accuracy of the overall system is comparable

to GOES. Time to better that 10 microseconds

and frequency accuracies to 1 part in 10 10 (for measurement periods of 1 day) can be

achieved.

Although TRANSIT is primarily a navigation satellite, the responsible Navy group

has recently allowed changes in its operation

to provide even more support for its frequency

and time function. Coupled with this is the

plan to launch additional satellites in the

TRANSIT series. Users planning to obtain

equipment to use TRANSIT, especially at remote locations, should contact either the U. S. Naval Observatory or the Navy Astronautics

Group for the latest information on satellite

passes, etc. In addition, manufacturers of TRANSIT equipment are a good source of information.

At this writing, the TRANSIT system is supported by five orbiting satellites. These

are in a circular polar orbit at an altitude

of about 1100 kilometers. Thus, a user on the

earth will see a satellite every 90 minutes or

so. In contrast to a GOES-type of transponder,

187
TRANSIT SIGNAL GENERATION

OSCILLATOR

5 MHi —80 „pm FREQUENCY MULT

X80

399 968 MHi

(*400 MHil

149 988 MHr

(=1S0 MHilw

FREOUENCV MULT

X30

PHASE MODULATION

CLOCK ^

FREQUENCY DIVIDE ADJUST RECEIVER ^96 uSEC MEMORY ~

SATELLITE CLOCK CONTROL SYSTEM

OSCILLATOR

FREQUENCY DIVIDE

1 - 104.159HZ p - 9.6 uS

INJECT

i » 1627HZ

p - 614.449 »jS

-2 1 ±4 -32

ADJUST

9 6 uSEC .

1. SATELLITE OSCILLATOR DESIGN CENTER FREQUENCY = 4,999,600

2. DIVIDER CHAIN RATIO (READ) = 1 6 x 3 x 2 x 32 x 32 = 98,304

3. 98,304/4,999,600 = 19,662373 ms/BIT

4. 19,662373 x 6103 = 119.9994626 SECONDS

5. 120.0000000 - 119.9994624 = 537.6 MICROSECONDS

6. DIVIDER CHAIN RATIO (DELETE) = 48/4999600 = 9.6 MICROSECONDS

7. 537.6 ys/9.6 ys = 56 CLOCK CORRECTION BITS/2 MIN. INTERVALS

FIGURE 10.19. TRANSIT SATELLITE CLOCK CONTROL SYSTEM.

TRANSIT has an accurate clock on-board each

of its satellites. These are monitored from

earth-control stations and corrections are sent to the satellites periodically to keep

them on time (see fig. 10.19).

In addition to position information, the

satellites send a time mark each two minutes.

By monitoring the signals from four groundmonitor stations, the Navy is able to steer

the on-board clocks by special commands. For

the user, this means that at any time he can

be assured that a TRANSIT signal has been

carefully controlled for accuracy.

There is much redundancy in the overall

system control. New control data are sent

to the satellites every 12 hours or so. Al- most all the ground equipment used for both

navigation and timing contains a fairly

188
ADD/DROP CONTROL 1 MHZ

IN

400 MHZ

IN

MICROCOMPUTER *

NNSS RECEIVER

DATA

IN

ENABLE

6 DIGIT DECADE COUNTER

DATA

ys COUNT

LATCH

STROBE (FTM*)

FTM DETECTOR

1 PPS OUT

FIGURE 10.20. TRANSIT TIMING RECEIVER BLOCK DIAGRAM.

sophisticated computer-controlled receiver

that allows nearly hands-off operation.

As with the GOES system, the satellite

message is designed to allow the receiver to synchronize itself, locate the required bits

in the data stream and produce a time mark.

Accuracy of time recovery using TRANSIT

depends on several factors. Again, the potential user is encouraged to talk with manufacturers and to contact the USNO, Time Services

Division, Washington, DC 20390 before proceeding. At this writing, time recovery accuracy

falls within from 10 to 50 microseconds. With

suitable care of an operating clock on the

ground, this service can provide very valuable

calibration information.

10.2.2 EQUIPMENT NEEDED

Equipment costs are higher for TRANSIT

than for GOES, but still not unreasonable

considering the few alternatives.

Antennas are small, vertical, and omnidirectional, making installation simple.

The instrumentation needed by' a user

will depend on his particular application.

In general, he will need a frequency source

and clock. Purchase of a receiver for TRANSIT

will then allow comparisons to the specifications of the equipment.

It is expected that manufacturers will

differ in their implementation of TRANSIT

receiving equipment, but a typical requirement

is that the user obtain the time of day from

another source to within 15 minutes in order

to resolve the time ambiguity of TRANSIT. For some applications, it may also be necessary to dial in the user's longitude and latitude.

See figure 10.20 for a typical receiver

block diagram.

189

CHAPTER 11. AN INTRODUCTION TO FREQUENCY SOURCES

An instrument or device that produces a steady and uniform signal output in whatever

form is said to have a frequency of operation.

The new quartz crystal wristwatches , for example, vibrate their crystal frequency source at 32,768 times in one second. Most clocks con- tain an identifiable source of frequency, be

it balance wheel, crystal, or pendulum.

Frequency sources are also used in another way. Besides counting their vibrations or oscillations for timekeeping, they

are used to set musical instruments and to calibrate many types of electronic devices. A radio dial is marked off in frequency units as are the familiar television channels. The TV

channel selector, when set to channel 4, tunes

the receiver to obtain a signal at a frequency

of about 70,000,000 hertz or 70 MHz.

This chapter deals with the properties of

frequency sources and their possible applications to real situations. The idea is that

once you understand what the frequency generator will do, you can more intelligently

choose the right type of device. Some of the

physics of the devices are discussed and

finally a table is presented that compares

several devices.

The first part of this chapter deals with

the use of frequency sources to drive clocks,

which is an important application of many

high-quality oscillators. There is a strong

relationship between time-ordered events and

frequency division. Communications systems

especially rely on these two means of separating channels of communications. For example,

there are a number of stations on the AM radio

dial. These are spaced in frequency at about

every 10 kHz. Another way to share the spec- trum would be to have all the stations at the

same frequency, but at different times . The

same spectrum is thus time-shared. For obvious reasons of cost and convention, we choose

rather to run all the stations at the same

time, but on different frequencies.

Common usage allows manufacturers and

writers to refer to frequency sources as standards. You will see and hear the term frequency standard mentioned when what is really meant is a frequency source. The word "stan- dard" should properly be reserved for only

those frequency generators that serve as references for others. In the U. S. the primary frequency standard resides in Boulder,

Colorado, at the National Bureau of Standards

Laboratories. But there can be many secondary

standards that are calibrated with the single

primary standard. Here is where the popular

usage of the term "standard" gets misused.

People use it to describe high-quality oscillators because their stability is so good that

it approaches that of the nation's standard.

It is probably not possible to prevent this

misuse, so the reader should be warned to note carefully what is meant by the words used to describe the various frequency sources.

11.1 FREQUENCY SOURCES AND CLOCKS

Notice that most clocks—in particular,

the very accurate and precise ones--are based

on high-quality frequency sources. The reason

for this is the intimate relationship between

frequency (symbol v, "nu") and time (symbol

t). See the definition of frequency in the

glossary for more information. If we look at

a series of events which are occurring in a somewhat regular fashion; e.g., the rise of

the sun every morning, we can state how many

of these events occur in a given time period.

This number would be the frequency of this

series of events. In our example, we could

say that the frequency of sunrises is v = 7 events per week, or v = 365 events per year. "Events per week" or "events per year" would

FREQUENCY

SOURCE

5MHz FREQUENCY

DIVIDER

60Hz

DIVIDER & PULSE

GENERATOR

1 PULSE

PER SECOND

(1Hz)

ELECTRIC

CLOCK

CLOCK

(COUNTER/

ACCUMULATOR)

CLOCK-TICK

OUTPUT

FIGURE 11.1. EXAMPLE OF A CLOCK SYSTEM.

191
EVENTS

PERIOD —

FREQUENCY = NUMBER OF EVENTS PER UNIT OF TIME

ACCUMULATED CLOCK TIME = TOTAL NUMbER OF EVENTS ALLUMULAILU CLUCK IIML NUMBER OF EVENTS PER UNIT OF TIME

POSSIBLE DEFINITION:

UNIT OF TIME = A SPECIFIC NUMBER OF PERIODS OF

A WELL-DEFINED EVENT GENERATOR

FIGURE 11.2. DEFINITION OF TIME AND FREQUENCY.

be called the unit which we used for our frequency number. This frequency number is different for different units. In our example,

we assume that we know somehow what a week or

a year is; i.e., we relied on some understood

definition of our unit of time.

Now, what is the time interval between

the events? The answers for our examples are

simple: One sunrise succeeds the other after

t = 1/7 week, or t = 1/365 year, where we used

"week" and "year" as two possible choices for our unit of time.

We have learned two things: (a) for

periodic events, the time between the events t is related to the frequency v of their occurrence in the following simple way:

1 v =

t

and (b) that periodic events can be counted to define time; i.e., the generator of the periodic events-the frequency source--can be used

as a clock. The frequency becomes a clock by

the addition of a counting mechanism for the

events.

In the above example, the frequency used

is that of the rotating earth. The time

between recurring events is one day. The

rotating earth has served mankind for thousands of years and remained until very recently the source for the definition of time in- terval. The counting mechanism which made it

a clock was the recording of days and years.

The need to get along for many days without celestial observations, and to measure

time intervals which are very much shorter

than a day, brought about the invention of

clocks. Although there are other types of

clocks like the sand clock or the water clock

and the sundial, we will discuss only clocks

based on electronic frequency generators.

The first clocks based on the pendulum

were invented about 400 years ago. This type

of clock is still widely used today. The pen- dulum may be a suspended weight (gravitational

pendulum) like in "grandfather" clocks, or the

balance wheel (torsion pendulum) of modern

wri stwatches. The object of our discussion

here is today's most advanced oscillators and

clocks; however, a close look at traditional

clocks will show all the essential features

which we will recognize again in our later

discussion of quartz crystal and atomic

clocks.

The pendulum in our clock is the frequency determining element. In order to have a frequency source, the pendulum has to be set

and kept in motion. Thus, a source of energy

is necessary together with the means to transfer this energy to the frequency-determining

element. In a wristwatch, this source of

energy is typically the winding spring or battery. The energy is transferred by a means which is controlled by the pendulum itself

(feedback) in order to cause energy transfer

in the proper amount at the proper time in synchronism with the movement of the pendulum

or balance wheel. We now have a frequency

source. The tick motion of a balance wheel

could be picked up acoustically, for example,

and used as a frequency source. This is actually done by jewelers when they adjust the

rate of a clock: the tick-frequency is com- pared to some (better) frequency source. In order to have a clock, a read-out mechanism is necessary which counts and accumulates the

ticks (more accurately, the time interval

between the ticks) and displays the result.

In our example of a wristwatch, this is accom- plished by a suitable set of gears and the

moving hands on the clock face or digitally.

We have just discussed how the addition

of a mechanism to a frequency source which

counts and accumulates and possibly displays

the result creates a clock. This task can also be performed by an electronic frequency

192
FEEDBACK

ENERGY TRANSFER

ENERGY

SOURCE

FREQUENCY DETERMINING

ELEMENT

FREQUENCY

SOURCE OUTPUT

CLOCK (COUNTING ACCUMULATING

DISPLAY )

FIGURE 11.3. FREQUENCY SOURCE AND CLOCK.

11.2 THE PERFORMANCE OF FREQUENCY SOURCES

The performance of a frequency source is usually described in terms of its accuracy,

reproducibility, and stability. These are defined as:

ACCURACY : How well does it relate to the definition? In the case of frequency, this means

how well it relates to the definition of a second (in terms of cesium) mentioned

previously.

divider which, for example, derives a frequency of 60 Hz directly from a 5 MHz crystal

oscillator (fig. 11.1). The 60 Hz voltage can

be used to drive an electrical clock motor

similar to those driven by the 60 Hz power

line frequency that we use at home and at

work. Or, an additional electric pulse gen- erator may be used that generates one very

sharp electrical pulse per second. The time

interval of 1 second between the pulses (corresponding to a frequency of 1 Hz) is derived

from the output of our frequency source. The

pulses can be directly used in time comparisons with those of other similar clocks, or they can drive a counter/accumulator.

The electric power line is a very good

frequency source for many applications. A clock driven from the power line will keep

excellent time. This is because the power

system is carefully controlled to maintain its

frequency within definite limits. By increasing or decreasing the individual generator

frequencies, the flow of power in the interconnected system can be regulated. Each power

company is notified in advance to set their

frequency to a particular value so that the

millions of clocks on the system—those in homes and offices across the country—will

gain or lose time as required to keep the

clocks correct. The time corrections are usually done at night.

The basic unit of time is the second

(symbol s). It is defined with reference to a frequency-determining element. Since 1967, by

international agreement, this element or

"natural pendulum" has been the cesium atom. The second is defined in the official wording

as "the duration of 9,192,631,770 periods of

the radiation corresponding to the transition

between the two hyperfine levels of the ground

state of the cesium-133 atom." Accordingly,

the frequency of the cesium pendulum is 9,192,631,770 events per second (the cesium

atom is a very rapidly oscillating pendulum).

The unit of frequency is then defined as hertz

(symbol Hz) which means the repetitive occurrence of one event per second (the use of

"hertz" is preferred to the older term "cycle

per second" , cps).

REPRODUCIBILITY : If you built a number of such frequency

sources and adjusted them, how well would they

agree in frequency? This term obviously

applies to sources that are manufactured and

then tested to see how they differ.

STABILITY : Once the device is set to a given frequency, how well does it generate that value

during some period of time?

It is obvious from these definitions that

frequency accuracy will be of great interest

in scientific measurements and in the evaluation and intercomparison of the most advanced

devices. Good reproducibility is an asset in applications where it is important to expect

some agreement among several devices. How- ever, the stability of a frequency source is usually most important to the user.

The frequency stability (symbol o, "sigma") of a frequency source will depend on many things that might cause frequency

changes. Frequency stability can be measured

by taking a reasonably large number of successive readings on an electronic counter which

counts the frequency of the device to be

evaluated. Each counter reading (in hertz) is obtained by counting the output frequency for some specified time (the sampling time "tau",

symbol t). This sampling time can usually be

chosen by simply adjusting a knob on the counter; for example, a sampling time of 0.1

s or 1 s or 10 s may be chosen. The result

can be expected to change as t changes. So the stability needs to be defined for a certain "tau".

Further, it might be expected that variations in the readings of measured frequency

would tend to average out if observed long

enough. This is not always so. The stability

of the frequency source (sigma or a) will

therefore usually depend on the sampling time

of the measurement and tends to get smaller

with longer sampling times, t. But, again,

there are many exceptions to this.

193
It may happen that the frequency fluctuations at some later time are partially caused

by, or depend to some degree upon, the previous fluctuations. In this case, the computed

value of a will depend on the particular way

in which the many counter readings are aver- aged and evaluated. Another influence on mea- sured stability depends on whether the counter

starts counting again immediately after com- pletion of the preceeding count or if some time elapses ("dead-time") before counting

starts again.

Finally, the electronic circuits used in

measuring frequency stability will have a finite response time; i.e., they cannot follow

frequency fluctuations faster than some given

rate. For example, our eyes cannot register

light fluctuations which occur faster than

about every 1/10 of a second; using v = 1/t, we say that the eye has a frequency response

of 10 Hz, or that its bandwidth is only 10 Hz; i.e., the eye cannot follow frequencies higher

than 10 Hz.

frequency stability of a

gv = 10 Hz were mea- sured at a carrier frequency of v = 5 MHz (MHz = megahertz = million Hz) then the fractional

or relative frequency stability would be

a = o% . =2x10°.

y ov/v

We denote the kind of a we are using by a subscript, 6v ("delta nu"), referring to frequency fluctuation (measured in Hz) or 6v/v =

y, referring to relative frequency fluctuations (dimensionless). A stability of one part in a million is thus

a

y

= 1 x

10" 6

,

and one part in a trillion is written as

a =1x10

y

In order to measure frequency stabilities

for sampling times smaller than some value x, our measurement equipment has to provide for

an electronic frequency bandwidth which is larger than 1/t.

To summarize: A recommended way of pro- perly measuring and describing frequency sta- bility is the following: (a) make sure that

the frequency bandwidth of the total measuring

setup is larger than 1/t, where t is the smallest, desired sampling time; (b) use a counter with a cieadtime as small as possible (the

dead-time should be less than the reciprocal

bandwidth; if not, use the computation pro- cedures that exist to allow for larger deadtimes); (c) take a sufficiently large number

of readings at a given sampling time which is held constant and compute:

ADDITION OF THE SQUARES OF THE DIFFERENCES BETWEEN SUCCESSIVE READINGS

2 x TOTAL NUMBER OF DIFFERENCES USED

There is a sound basis for using the

relative frequency stability o^ instead of

using frequency stability directly, a is

a numeric and is independent of the actual

operating frequency of the oscillator being

discussed. Thus, it is possible to compare

the stability of a 10 MHz oscillator with one that has an output at 10 kHz.

In fact, many high-quality oscillators

have outputs at 1,5, and 10 MHz. But an application for these sources might require

the generation of a different frequency, e.g., the television color subcarrier signal at 3.58

MHz. If the original source, before the syn- thesis of the TV signal, had a relative frequency stability of one part per million (1 x 10- 6 ), then the 3.58 MHz signal would have the same relative frequency stability a .

This assumes, of course, that the

synthesis or generation of new frequencies

does not change the stability of the frequency

source. This is generally true for most

appl i cations.

(In the scientific literature this a is often

called the square root of the Allan variance.)

The counter readings can be taken in Hz; a

will then have the dimensions of Hz; (d)

repeat step (c) for other sampling times t and

tabulate or plot a as it depends on x.

Commonly, a will be expressed as a relative value. Existing literature often uses the term "fractional" instead of relative, so the reader should treat the two terms equally;

they mean the same thing. The value obtained

for the frequency stability is divided by the

carrier frequency. For example, if a

11.3 USING RELATIVE FREQUENCY STABILITY DATA

Suppose you have a clock. Let's say its

frequency source is a crystal oscillator, and

you want to know the time error. If you know

enough about the oscillator, you can compute

the time error. You can even predict the error ahead of time—fairly accurately, too.

First, let's define a few things that we

will need to write an equation. There are only about three things that can happen to our

194
clock—assuming we don't drop it or pull the

plug!

1. The clock could be reading the wrong time

because we didn't set it at all or be- cause we set it wrong. Call this the

initial time error At0

. Until we correct

it, we have to have it in our equation as

a constant.

3. There is one more possible error. Was the oscillator "on frequency" when- it started to drift? If it was not, then we must account for this offset. To do this, we define the relative frequency

offset as:

2. The frequency of the oscillator driving

the clock may be fluctuating. If that is the case, then we will have a term in our equation that relates the fluctuation and

the length of time it has been drifting.

Let's call this length of time t. It would be the time from zero time (when

the clock was right or we measured the error At0 ) till now. From what we said

about the stability, we see that we need

a term such as:

a

y

(T) X t.

Actually, we know the value of t that we

want. It's simply t, the time since we

last set the clock. So our term becomes

approximately:

Oy(T ^ t) X t.

This then takes care of the instabilities

of the oscillator frequency driving our

clock.

This term means that our oscillator may have had a frequency error when it started to drift. In other words, we allow for an arbitrary starting point.

To review then, we have an oscillator

connected to a clock and it may or may

not have had a time error At in addition

to having or not having some drift x t from either the correct or incorrect

frequency Av/v x t.

Addition of these time errors gives us the clock error at time t:

At = At + a (t = t) x t + — x t. o y

v

' v

The equation shows how the relative fre- quency offset and stability accumulate

time error as t grows larger. Let's plug

in some numbers and see what happens to our clock. Assume the oscillator was off

frequency by 1 part in 10 11

, fluctuates

by 1 part in 10 11 per day, and had a 1 microsecond error yesterday at this time.

Reading the graph we see that our clock error would be about three microseconds.

FIGURE 11.4. RELATIONSHIPS BETWEEN CLOCK ACCURACY , FREQUENCY

STABILITY, AND FREQUENCY OFFSET.

195
We get almost 1 microsecond error from

fluctuations, 1 microsecond from frequency offset and one from our initial set- ting error.

11.4 RESONATORS

The performance of an oscillator depends

a lot on its basic frequency-determining element. This element determines the frequency

by its resonance behavior. Thus, it is called

a resonator. Some examples of resonators are

a rod clamped only at one end that can vibrate; a block of solid material that can con- tract and expand and thus vibrate; an electrical capacitor-coil combination (tank circuit)

in which the energy can oscillate back and

forth; and a coil in which an electric current

can create a magnetic field which can oscillate between its two possible polarities (a

magnetic dipole). These devices have one thing in common--they will vibrate or oscillate if they are excited. The method of excitation may be a mechanical pulse, an electrical pulse, or a sudden surge of an electric or

VIBRATING ROD

V I BRAT I N BLOCK

OSCILLATING

CAPACITOR-COIL

(TANK) CIRCUIT OSCILLATING

DIPOLE ANTENNA

* *.

U prr>o-rY^ S OSCILLATING MAGNETIC

(S)l _oc>_J(N) DIPOLE ANTENNA

FIGURE 11.5. EXAMPLES OF RESONATORS.

OSCILLATION

OF MAX. INTENSITY

EXCITING

PULSE ENDS

DECAYING

OSC ILLATION

OF A RESONATOR

RESONANCE

BEHAVIOR OF

A RESONATOR

FREQUENCY

FIGURE 11.6. DECAY TIME, LINEWIDTH, AND Q-VALUE OF A RESONATOR.

196
magnetic field. These devices exhibit resonance. They are resonators with a welldefined center frequency which is a characteristic of the length of the rod, the thickness

of the block, the size of the capacitor and

coil, or the length of the antennas. Once

excited, the oscillations will gradually die

out. The decay time is determined by the

losses in the resonator. Some of these losses are internal friction, or electrical resistance. In any case, the oscillation energy is finally transformed into heat. If there were

no losses, the oscillations would never stop

and we would have the ideal resonator. The

greater the losses, the faster the oscillations will decay. We can use the decay time

of the oscillations (symbol T) to describe the

quality of our resonator. The larger T is,

the better the resonator. "Better" for us means that the device favors one frequency

over all others.

An alternate way of measuring resonance behavior is to use a separate oscillator,

couple it to the resonator, and slowly sweep

the frequency of the oscillator. We will find

a signal frequency at which the resonator will

oscillate, with maximum intensity. Above or below this resonance frequency, the response

of the resonator will lessen until it ceases

to respond. This frequency interval around

resonance, for which the resonator response is relatively strong, is called the resonance

linewidth (symbol W). There is a simple relationship between W and T, the decay time:

W = - Hz.

T

This equation is an approximation and

applies only to certain types of resonance. Another factor would have to be inserted if

the resonance curve were different from that

shown in figure 11.6. However, this factor is nearly equal to one: T is actually defined as the decay of the amplitude to 37% of its

original value: 37% is 1/e, e is the base of

natural logarithms and equals 2.71828.

As we did in the case of a (page 194),

it is convenient to consider the linewidth in

a fractional way. The fractional linewidth

would be W/vo where the symbol v 0 is used for

the resonant frequency. A more popular term

is the "quality factor" (Q) of the device. Q

is defined as the reciprocal fractional linewidth

For accurate frequency sources and good

clocks, we want to have elements with very

high Q's. A Q value of one million (106

) is very common. For example, a 1 MHz crystal

oscillator with a Q of a million would have a linewidth of 1 hertz. The oscillator would

then favor operation at a frequency within 1 hertz of a megahertz. If this oscillator were

measured, its frequency should be within its one hertz range, i.e., within a part per

million. In fact, it can be made much better

than that. Parts per billion are typical.

In summary then, we want high-quality

resonators if our end product is to have good

accuracy and stability characteristics. The

discussion that follows in this section deals

with several types of high-quality frequency

sources.

Many kinds of frequency-determining

elements have been and are being used in frequency sources. They can be grouped into

three classes:

mechanical resonators electronic resonators

atomic resonators

As far as mechanical resonators are con- cerned, we will only discuss one kind, the

quartz crystal. Other mechanical resonators

like the pendulum and the tuning fork are of

less importance in today's high performance

frequency sources, although they have been

historically very important and are still

widely used in watches. For similar reasons, we will also omit the discussion of electronic

resonators such as LC tank circuits and micro- wave cavities. Atomic resonators form the

heart of our most accurate frequency sources

and clocks, and will therefore be extensively

discussed.

11.5 PRIMARY AND SECONDARY STANDARDS

At this point, we should briefly discuss

the terms "primary frequency standard" and

"secondary frequency standard". These terms refer to the application of the devices; any frequency source, regardless of its accuracy

or stability, can be a primary frequency standard, if it is used as the sole calibration

reference for other frequency sources. A secondary frequency standard is a device which

is occasionally calibrated against a primary

frequency standard but operationally serves as the working reference for other frequency

sources.

The machines used for primary standards,

like those at NBS, Boulder, NRC, Canada, and

in Germany and England, are in a class by

themselves. Because of the way they have been

built and operated, they can be evaluated.

This means that experimental data is taken and

197
FIGURE 11.7. THE U.S. PRIMARY FREQUENCY STANDARD, NBS-6.

PRIMARY DIRECT

STANDARD

CALI BRATION

I

SECONDARY

STANDARDS

OTHER

STANDARDS

FIGURE 11.8. HIERARCHY OF FREQUENCY STANDARDS.

198
used to calculate the errors in their output

frequency due to all known causes. It is because of this careful evaluation that the

accuracy can be stated without having to check

these machines against others. This concept

will be discussed in detail in connection with

the cesium beam frequency sources.

11.6 QUARTZ CRYSTAL OSCILLATORS

Quartz crystal oscillators are widely

used. They range from the tiny units found in wristwatches to elaborate instruments in laboratories. Sometimes they are called crystal oscillators and sometimes quartz oscillators. Whatever the name, they dominate the

field of good frequency sources.

The quartz crystal in the oscillator is a mechanical resonator. The resonator's oscillations have to be excited and sensed. This

is done by taking advantage of the piezoelectric effect in the quartz crystal. The piezoelectric effect (figure 11.9) results when

mechanical compression of the crystal generates a voltage across the crystal. Conversely, the application of an external voltage

across the crystal causes the crystal to expand or contract depending on the polarity

of the voltage. A crystal is not a homogeneous

medium but has a certain preferred direction;

thus, the piezoelectric effect has a direcQUARTZ CRYSTAL

APPLIED PRESSURE APPLIED VOLTAGE GENERATES VOLTAGE CAUSES CONTRACTION

FIGURE 11.9. THE PIEZOELECTRIC EFFECT.

tional dependence with respect to the orientation of the crystal. In order to take advantage of the piezoelectric effect, one has to cut a crystal resonator from the original

crystal block in a well-defined way with

respect to the crystal lographic directions.

The raw material used today is natural or synthetic quartz. A crystal is cut out of the

raw crystal material in the desired orientation with the aid of optical techniques which

allow the determination of the crystal lo- graphic axes. The high-precision, final

orientation of the cut and the tuning to the

desired frequency is then done by grinding and

etching, controlled by x-ray methods.

The quartz crystal can be cut and electrically excited in a variety of ways. The most common types of vibrations (modes) are

the longitudinal and thickness modes, the

flexure (bending) mode, the torsional mode,

and the shear mode. In order to use the

piezoelectric effect, metal electrodes have to be attached to the crystal surfaces so that

the desired mode is excited.

The electrodes are typically created as extremely thin metallic coatings by vacuum evaporation of metals. Electric leads are attached to the electrodes by soldering. They

also serve as the mounting support, thus freely suspending the quartz crystal. In order to least upset the mechanical vibrations of the

crystal, the electrode-support leads are attached at points where no vibrational motion

occurs (nodes). The crystal is then packaged

and the case is either filled with a protective gas or else evacuated.

Using the crystal, an oscillator can be

built by adding an electronic amplifier feedback and a power supply. The oscillator out- put frequency is determined by the quartz crystal resonator. Its frequency in turn is determined by the physical dimensions of the

crystal together with the properties of the

crystalline quartz used. The actual operating

frequency thus depends on the orientation of

the cut, the particular mode, and the dimensions of the crystal. As an example, we find

that the resonance frequency, for a longitudinal mode of vibration, is approximately:

v = 2.7 x 10 3 x y , o L

'

where L is the length of the crystal. If L is in meters, the resonance frequency v 0 will be

in hertz. The equation allows us to estimate

the size of the crystals. For example, a 100

kHz crystal will have a length of just a few

centimeters. We see that the manufacture of quartz crystals with resonance frequencies

above 10 MHz is hardly possible. However, we

can excite resonators not only in their so- called fundamental mode (previously discussed), but also at multiples (overtones) of this fundamental resonance frequency. An example of this is the violin string which

also can be caused to vibrate at frequencies

which are multiples of its fundamental.

Quartz crystals which are designed for excitation at multiples of their fundamental reso- nance are called overtone crystals.

199
FLEXURE

LONGITUDINAL

TORSIONAL

THICKNESS SHEAR

QUARTZ

CRYSTAL

V

-ENCLOSURE

ELECTRODE

-(THICKNESS

EXAGGERATED)

ELECTRICAL

-& MOUNTING

LEAD

-SUPPORT

CONTACT

LEADS

FIGURE 11.10. PRINCIPAL VIBRATIONAL MODES

OF QUARTZ CRYSTALS. FIGURE 11,11. TYPICAL QUARTZ

CRYSTAL MOUNT.

FUNDAMENTAL

FEEDBACK

AMPLIFIER

POWER

SUPPLY

QUARTZ

CRYSTAL

RESONATOR

-OUTPUT

2nd OVERTONE

3rd OVERTONE

4th OVERTONE

FIGURE 11.12. QUARTZ CRYSTAL

OSCILLATOR.

FIGURE 11.13. FUNDAMENTAL AND OVERTONE

RESONANCE FREQUENCIES.

200
11.6.1 TEMPERATURE AND AGING OF CRYSTALS

Temperature and aging are two effects,

among others, which are important in the design of crystals and crystal oscillators.

There is a temperature dependence of the

quartz crystal that affects resonance frequency, and there is also a drift of the resonance frequency as time goes on.

The temperature dependence is caused by a slight change in the elastic properties of the

crystal. Certain cuts (crystal lographic

orientations of the crystal) minimize this

effect over a rather wide range of temperatures, most notably the so-called "AT" and

"GT" cuts. Temperature coefficients of less

than one part in 100 million per degree (Celsius or kelvin) are possible. In other words,

the fractional frequency change will be less

than 10- 8 with one degree of temperature

change. This small effect requires care in the design of a crystal oscillator if very

high frequency stabilities over longer times

(hours or days) are desired. If large environmental temperature fluctuations are to be

tolerated, the crystals themselves are enclosed in electronically regulated ovens which maintain a constant temperature. In certain crystal oscillators this is done to better than 1/1000 of a degree.

An alternate solution to the temperature

problem is the so-called temperature-compensated crystal oscillator or TCX0. An additional frequency-determining element in the

oscillator, which can be just a small capacitor, allows us to tune the oscillator over a limited range. If a temperature sensor is added to cause a change in this capacitor, the

change in resonance frequency of the crystal resonator can be cancelled. Capacitors whose

values change with an applied voltage (varactors) are used. The applied voltage is derived from a temperature-sensing circuit.

The TCX0 does not necessarily require

further temperature control by an oven. However, there is a drawback to this approach.

By adding another frequency-determining element, the crystal resonator loses a corres- ponding part of its control on the output frequency of the whole oscillator. The long-term

stability (days) of TCXO's is therefore below

that of crystals with a good oven control.

TCXO's are used in small, usually portable

units of relatively lower performance. They

are used where frequency stabilities from day

to day and frequency changes (over some tens

of degrees of temperature) of not better than

10- 9 are needed.

Drift, or aging, is a common behavior of

all crystal oscillators. It is a nearly

linear (uniform) change in resonance frequency

with time. Drift often is negative, meaning

that the resonance frequency decreases. A frequency decrease could be interpreted- as an increase in the crystal size. Many things

have been considered as the cause: contamination (depositing of foreign material) on the

surfaces; changes in the electrodes or the

metallic plating; reforming of loose (from

grinding and etching) surface material; or changes in the internal crystal structure.

All of these are possibly caused or enhanced

by the vibrating motion of the oscillating

crystal. Recent crystal holder design improvements, combined with clean vacuum enclosures, have led to a reduction of aging to about 10- 11 per day. For a 5 MHz crystal with

a thickness of a little less than a millimeter, this aging corresponds to an absolute

thickness change of only 10- 11 of a millimeter, or less than 0.1% of the diameter of an atom.

When a crystal oscillator is first turned

on, it will not usually oscillate at its original frequency. It will exhibit a "warm-up"

due to temperature stabilization of the crystal resonator and its oven. For some time

then, often many days, a large but diminishing

drift occurs until it reaches its operating

point. Its frequency then might also be substantially different (as compared to its

stability and aging performance) from its

frequency before power was turned off.

11.6.2 QUARTZ CRYSTAL OSCILLATOR PERFORMANCE

Crystal resonators have Q-values which

are typically in the range from Q = 10 5 to almost 10 7

. These are very high Q-values as compared to most other resonators except

atomic resonators.

These high Q-values are an essential pre- requisite for the excellent stability perfor- mance of crystal oscillators. The best available oscillators have stabilities of a few

parts in 10 13 for sampling times from one second to a day. There is some experimental

evidence that some crystal resonators may, in the future, perform even better! The limitations are primarily caused by noise from electronic components in the oscillator circuits.

This noise may possibly be reduced by selection of low-noise components—transistors,

capacitors, etc., and by special circuit

designs. Thus, there is a reasonable chance

that crystal oscillator stability can reach

values of better than 10- 13 for sampling times

of seconds to hours. For times shorter than one second, stability is often determined by

additive noise in the output amplifiers. This can be reduced by a crystal filter in the

output. The long-term stability beyond

several hours sampling time is determined by

201
FIGURE 11.14. FREQUENCY STABILITY OF THE BETTER QUARTZ CRYSTAL OSCILLATORS.

aging and by external influences such as line

voltage variations, temperature fluctuations,

etc. A specification of accuracy is not easily stated for crystal oscillators. Without

frequency calibration, they can be manufactured by controlling the dimensions of the

quartz used to within 10- 6 of a nominal fre- quency. If they are calibrated against a high

accuracy frequency source, they maintain this

calibration (accuracy) in accordance with

their long-term stability characteristics. A crystal with a low aging characteristic of

10- 11 per day will maintain its accuracy to a few parts in 10 8 for a year without recalibration.

It is apparent, therefore, that crystal

oscillators require calibration at least once

a year. They may need it more often depending

on the application. Frequency adjustments are made with a small added capacitor in much the same way as was discussed in connection with

the TCXO. The most stable crystal oscillators, with the lowest aging rate, cost approximately $2,000. They have a volume of a few

thousand cubic centimeters, and require input

power of about 10 watts. They have an elaborate crystal oven for temperature control

well-designed electronics, and usually several

output frequencies which are derived from the

oscillator with frequency dividers and multipliers. These high performance devices use

2.5 or 5 MHz crystal resonators having

Q-values of a few million.

Cheaper and smaller crystal oscillators

are available in a variety of designs with

corresponding reductions in frequency stability and/or environmental insensitivity. Costs

can go down to below $100, sizes to a few

cubic centimeters, and power requirements to

less than 0.1 watt. The reliability of

crystal oscillators is usually not limited by

the crystal itself, but also depends on the

accompanying electronic circuits.

11.7 ATOMIC RESONANCE DEVICES

Commercially available atomic frequency

oscillators are based on resonances in atoms

at microwave frequencies — in the range from 1 to 100 GHz (gigahertz or billion hertz). We may picture atomic resonance as a magnetic

dipole antenna. We typically deal with a great many of these dipole antennas (atomic

resonators), and we can, as a simplification,

separate them into two kinds: Receiving antennas receive energy from a magnetic field

that oscillates at their resonance frequency,

much like a TV antenna at home. Transmitting

antennas radiate energy at their resonance frequency just like the transmitting antenna

of a broadcast station. A physicist might say that the atoms that act like a receiving antenna are in the "lower state" and those

acting like a transmitting antenna are in the

"upper state.

There is a peculiarity with atoms: an atom changes from the upper to the lower state upon emission of a well-defined amount of energy. Correspondingly, an atom changes

from the lower to the upper state after receiving an equal amount of

energy at the atomic resonance

frequency.

If we have a great many atoms as in a gas, we will find that the number of upper

202
GAS, CONTAINING AN EQUAL

AMOUNT OF THE TWO KINDS OF ATOMS

•W.WI •w.w

OtQ>0>0»0>OtO>0>0>0«0> >

•o*o*o,

o

,

o*oofo#6*0#o *<:> *<5 *0*o«o«o*o*

O O O O O o

o o o o o o ATOMIC BEAM

ATOMIC BEAM SOURCE

KIND 1 - ATOMS

(LOWER STATE)

MAGNET

STATE SELECTOR)

KIND 2 - ATOMS

(UPPER STATE)

VACUUM CHAMBER

FIGURE 11,15. SPATIAL STATE SELECTION.

state atoms is nearly equal to those in a lower state. This has an important consequence: If this gas is placed in a magnetic

field that oscillates at the atomic resonance

frequency, all atoms will resonate. Nearly

half of the atoms receive (absorb) energy from

the field, and the other half emit (add)

energy of an equivalent amount to the field.

The net effect is almost zero. The gas as a whole acts as if it has almost no resonance

even though the individual atoms do resonate. From this we see that in order to achieve

atomic resonance, we have to somehow change

the relative amounts of the two kinds of

atoms. The upper or the lower state has to be

in the majority. The way in which this is done determines the design of our atomic

frequency oscillator.

11.7.1 STATE SELECTION

There are two important methods used to accomplish what is called state sel ection--the

change in the relative numbers of the two

kinds of atoms (upper state and lower state):

Spatial state selection relies on an actual sorting procedure where the two atomic states are sorted into different directions in space. One of the states can then be used;

the other is discarded. An actual system may

produce an atomic gas by heating some sub- stance in an oven. The atoms leave the oven through a hole and form an atomic beam in an adjacent vacuum chamber. The atomic beam is then passed through a rather strong magnet

which causes the separation of the beam into two beams, one containing the upper state atoms, the other the lower state atoms. Recall that our picture of an atom is that of

a magnetic dipole antenna. The magnet exerts

a force on these magnetic dipoles. The force

acts in opposite directions for the two different atomic states (we note this without

further explanation).

Optical state selection takes advantage

of the fact that atoms have more than just one kind of resonance. Other resonances typically

correspond to infrared or visible (light)

frequencies. We can excite one of these resonances by shining intense light of the

LIGHT

BEAM

GAS CFAL STATE SELECTED

ThROUGH ACTION

OF THE LIGHT

FIGURE 11.16. OPTICAL STATE SELECTION.

203
right frequency on the atoms. If the light is filtered carefully, very monochromatic--one

color (a very well-defined frequency)--l ight

is obtained. If the light frequency is chosen

properly, only one kind of atom will resonate.

It will be found that this light frequency is different from the corresponding light reso- nance of the other kind of atom. Light reso- nance can thus "remove" one kind of atom and

provide for the desired majority of the other

kind.

11.7.2 HOW TO DETECT RESONANCE

We are now almost in a position to assemble our atomic frequency oscillator. We still

need some means to detect atomic resonance. A short waveguide or microwave cavity is typically used. Such a cavity has microwave reso- nances which are determined by its physical

size. The electrical losses of this cavity

are determined by the electrical conductivity

of the cavity material. A cavity may be shaped like a cylinder or a box. It resonates

in a way quite similar to an organ pipe in the case of sound waves. The best known example

of a microwave cavity is the microwave oven where food is cooked by placing it into the resonance field of a cavity.

In order to observe atomic resonance, we

have to place the state-selected atoms inside

this cavity and subject them, for some specified time, to a microwave radio signal at their resonance frequency. The microwave

signal will change the relative number of atoms in the two states. If all atoms were

initially in the upper state, we will find

some in the lower state after the microwave

signal has acted upon them for some time. If the frequency of the microwave signal equals

the atomic resonance frequency, the transfer

of atoms from one state to the other reaches a maximum. The center of the atomic resonance

is thus found by monitoring the number of atoms in one of the two states while varying

the microwave frequency until the number of atoms in the lower state reaches a maximum, or the number of atoms in the upper state reaches

a minimum. An electric signal can be obtained

from the state selector which is fed back to the crystal oscillator generating the micro- wave signal. Thus, an automatic servo can be built that keeps the crystal oscillator locked

to the atomic resonance.

Detecting the effect of the microwave

signal on the atoms can be done in three ways. Two of these relate to the methods of state

selection.

In atom detection , the atoms which leave

the cavity as an atomic beam are passed

through the field of a magnet that spatially

sorts the two states. An atom detector is placed to intercept one of these states. The

output of the detector indicates the number of atoms in the upper (or lower) state.

With optical detection , the atoms are optically state selected. A photo-detector

for light can be placed in a position such

that the light which has interacted with the atoms is detected. Since the light removes atoms from one of the states, its intensity

NO SIGNAL

STATE SELECTED

ATOMIC BEAM MICROWAVE

CAVITY MAGNET M

NO SIGNAL

MICROWAVE SIGNAL

(OF ATOMIC RESONANCE

FREQUENCY )

STATE SELECTED ! ATOMIC BEAM ! MICROWAVE

CAVITY MAGNET

,0*

MAXIMUM SIGNAL

FIGURE 11.17. ATOM DETECTION.

204
NO SIGNAL

FILTERED

LIGHT

GAS CELL

MICROWAVE

CAVITY

PHOTO

DETECTOR

MAXIMUM SIGNAL

M-ICROWAVE SIGNAL

(OF ATOMIC RESONANCE

1 FREQUENCY)

FILTERED

LIGHT

MICROWAVE

CAVITY

PHOTO

DETECTOR

MINIMUM SIGNAL

GAS CELL

FIGURE 11.18. OPTICAL DETECTION.

will change if atoms that resonate at the

light frequency are added. Such an addition

takes place as a result of the microwave

signal. The microwave, in effect, transfers

atoms from a state with no light interaction

to another which takes part in the light

resonance. The light at the photo-detector,

therefore, is a measure of the number of atoms

in one of the states.

For microwave detection , the microwave

radio signal is transmitted through the cavity

to a microwave detector. The atoms will

either add energy to the signal if they are initially in the upper state or subtract

energy if they are initially in the lower

state. The microwave power level at the

detector is thus a measure of the number of atoms changing state.

11.7.3 ATOMIC OSCILLATORS

The frequency-determining element is the

atomic resonator which consists of a cavity

containing the state-selected atoms and some means of detecting the change in the number of atoms in the two states. A control signal,

related to the number of atoms that changed

their state due to the action of the microwave

signal, is fed back to the crystal oscillator.

The oscillator and the associated frequency

multiplier provide the energy-transferring

means. As discussed before, good crystal

oscillators are available at frequencies of

several megahertz. The atomic resonances are at gigahertz frequencies. The crystal oscillator frequency therefore has to be multiplied

by a factor of about 1000. Some power supply

has to provide the energy necessary to drive

the oscillator, multiplier, and possibly the atom state selector.

Very high Q values can be achieved with

atomic resonators. Recall that resonator Q is related to the time T, which describes the

average duration of oscillations of the resonator. In atomic resonators, we find two chief causes for the dying out of oscillations. First is the collision of the resonating atoms with each other and with surrounding

walls. Each collision usually terminates the

oscillation. The second is rather obvious.

The atoms may simply leave the region of microwave signal interaction. In an atomic

beam machine, the atoms enter the cavity, traverse the cavity in a certain time, and

then leave the cavity. As an example, let us assume that we have a cavity of one meter

length which is traversed by atoms with an average speed of 100 meters per second (a

typical value). We assume an atomic resonance frequency of 10 GHz. The interaction time T is then 1/100 second, the linewidth is 100 Hz,

and the Q value is 10 8

. This Q value is con- siderably better than that of a quartz oscillator.

205
MICROWAVE SIGNAL

(OF ATOMIC RESONANCE

FREQUENCY

T7

NO ATOMS

MICROWAVE

RECEIVER

MICROWAVE

CAV ITY

CONSTANT SIGNAL DUE

TO MICROWAVE SIGNAL

STATE SELECTED

ATOMIC BEAM «

MICROWAVE SIGNAL

(OF ATOMIC RESONANCE

FREQUENCY

MICROWAVE

CAVITY

CHANGE IN SIGNAL

DUE TO ATOMS

FIGURE 11.19. MICROWAVE DETECTION.

11.7.4 ATOMIC RESONATOR FREQUENCY

STABILITY AND ACCURACY

FEEDBACK

QUARTZ CRYSTAL

OSCILLATOR

AND FREQUENCY MULTIPLIER

POWER SUPPLY

ATOMIC

RESONATOR •OUTPUT

FIGURE 11.20. ATOMIC FREQUENCY OSCILLATOR.

Before we discuss what might cause changes in the output frequency of an atomic

oscillator, we should make a very important

distinction between quartz crystal and atomic

oscillators: The atomic resonance frequency

itself is given to us by the nature of our

device. Unlike the quartz crystal, it will not drift or age. Thus, atomic resonators with Q values of 10 8 or higher may be expected

to have accuracies of one part in 10 8 or

better. This is because we will not be able

to shift the frequency further away from resonance than the linewidth.

The following listing is not exhaustive

but gives only the minor perturbing effects on atomic resonators:

Random noise in the crystal oscillator,

the detector, the microwave cavity, and the

206
frequency synthesizer will cause corresponding

changes of the output frequency of the atomic

oscil lator.

The microwave cavity is itself a resonator. Thus, we have an additional frequencydetermining element in addition to the atoms. The cavity influences the output frequency by

pulling the combined resonance frequency to a value which usually lies between the reso- nances of the atom and cavity. This neces- sitates the tuning of the cavity itself so it

is equal to the atomic resonance. The re- quirements for tuning are relaxed if the Q

value of the atomic resonance is as high as possible and the cavity Q as low as possible.

The microwave signal will never be just

one single frequency but will have a certain

distribution of frequencies or spectrum. If the exciting microwave signal has an unsym- metrical distribution of frequencies, a frequency pulling occurs which is much like the

mechanism of cavity pulling. This effect can

be made negligible, however, by careful

electronic design.

Collisions between the atoms and between

the atoms and the walls of a vessel (gas cell)

in which the atoms may be contained will not only shorten the duration of the oscillation

but also cause a frequency shift. Obviously,

these effects can be minimized by having low atom densities and no walls, if possible.

An apparent change in frequency when the source moves relative to an observer is called

Doppler effect. Everyone experiences this by

listening to an approaching train whistle or automobile sounds. Here, the moving objects

are the oscillating atoms and the "observer"

is the microwave cavity. The Doppler effect

can be greatly reduced by choosing a particular direction of the atomic beam with respect

to the direction of the oscillating magnetic

microwave field in the cavity. In our acoustic example, this corresponds to the passing

of the whistle at some large distance instead

of having the whistle move directly towards

the observer. An alternate way of reducing

the Doppler effect is the containment of atoms

in a region which is small compared to the

wavelength of the microwave radiation; e.g.,

by placing a gas cell inside of the cavity.

Of the several effects which are discussed here, magnetic field change is the only

one that directly affects atomic resonance.

Recall that we used a small magnetic dipole antenna as the model for the atomic resonance.

If this antenna is placed in an external

static magnetic field (as that of a permanent

magnet), the tuning of the antenna changes;

that is, the atomic resonance frequency

changes. This necessitates magnetic shielding, which is a characteristic design feature

of all presently used atomic frequency oscillators. The shielding is usually quite elaborate and reduces the effect of magnetic

fields, such as the earth's magnetic field, to 1% or less of their external value. This

residual magnetic field can be measured quite

precisely by using the atomic resonance

itself. In fact, the overall measurement

precision is so good that magnetic field

effects do not seriously limit the accuracy of

atomic frequency oscillators. However,

changes in the external magnetic field, or the

movement of the device through a varying

field, as in a relocation, may perturb the

frequency of the atomic oscillator.

Generally speaking, one tries to minimize

all of the effects listed above and to keep

their influence as stable with time as possible. Such an approach suffices for most

applications. But the magnetic effect is an exception. The residual magnetic field inside

the shields must be evaluated and possibly

reset any time after changes in the external

field occur, say, after moving the device.

The magnetic shields may also have to be

demagnetized. For laboratory standards, how- ever, where frequency accuracy is the primary

purpose, all effects must be evaluated in a series of experiments, and reeval uations must

be done occasionally in order to detect

changes.

11.8 AVAILABLE ATOMIC FREQUENCY DEVICES

We will now discuss the design and per- formance of the three types of atomic frequency oscillators which are currently in operational use.

11.8.1 CESIUM BEAM FREQUENCY OSCILLATORS

For a cesium oscillator, atomic resonance

is at 9,192,631,770 Hz. The oscillator is based on the atomic beam method using spatial state selection and atom detection. An oven contains the cesium metal. If heated to about

100°C, enough cesium gas will be produced to form an atomic beam which leaves the oven through one or many channels into a vacuum

chamber. This chamber is evacuated to less

than 10- 9 of atmospheric pressure. The beam traverses first the state-selecting magnet,

then the microwave cavity. Typically, a cavity with separated interrogation regions is used. This design offers certain advantages

over a simple cavity of a length equivalent to the separation of the two regions. In the

cavity, an external microwave signal acts on the beam. The beam finally reaches the atom detector after passing another state-selecting

magnet. The atom detector is simply a

207
QUARTZ CRYSTAL OSCILLATOR & FREQUENCY SYNTHESIZER

FEEDBACK

FIGURE 11.21. CESIUM BEAM FREQUENCY

OSCILLATOR.

tungsten wire which is heated to about 900°C

by passing an electric current through it. This wire is biased with a few volts dc and

cesium atoms which hit it become electrically

charged or ionized and can be collected on an auxiliary electrode. The stream of electrically-charged atoms at this electrode repre- sents an electric current which is amplified,

detected, and used in the feedback network.

The speed of the atoms and the length of

the cavity determine the Q value of the atomic

resonator. Typical atom speeds are 100 meters per second. In non- laboratory devices, which

have to be reasonably small, the cavity is about 0.2 meter long. The corresponding

interaction time T is two-thousandths of a second. We can calculate the linewidth as a few hundred hertz and a Q value of 10 7 (see

section 11.4). In laboratory devices, one can

10

10

10

12

i 10

-1 5

1 0" 10 10 10 J 10 H 10 = SAMPLING TIME (SECONDS)—*

a) Typical performance.

b) Typical performance of recently

available high beam intensity tubes.

Individual units may perform slightly worse or better than shown. Modern lab- oratory standards perform still better.

FIGURE 11.22. FREQUENCY STABILITY OF COMMERCIAL CESIUM BEAM FREQUENCY OSCILLATORS.

use very long cavities. Cavities of up to 4 meters long are used, leading to Q values of

10 8. The fractional frequency stability of

laboratory and commercial devices can reach

parts in 10 14 at sampling times of less than one hour to days. The short-term frequency

stability is limited by fluctuations in the

atomic beam intensity, "shot noise," which is basic and unavoidable. These fluctuations

affect the frequency stability less as more intense atomic beams are used. This approach,

which is becoming available in both commercial

and laboratory devices, improves the stability. In contrast to commercial devices, the

laboratory oscillators are designed to allow a more complete and easier evaluation of all effects on the frequency. Cesium oscillators

are used extensively where high reproducibility and long-term stability are needed for

sampling times of more than a day. For most applications, cesium oscillators need not be calibrated. They are the work horses in most

of today's accurate frequency and time distribution services.

11.8.2 RUBIDIUM GAS CELL

FREQUENCY OSCILLATORS

Rubidium atomic resonance is at 6,834,682,608 Hz. The oscillator is based on the gas cell method using optical state selection and optical detection. The gas cell contains rubidium gas at about 10- 9 of atmos- pheric pressure. In order to reduce the

effect of collisions among the rubidium atoms, argon, an inert buffer gas, is introduced into

the cell at about 1/1000 of atmospheric pressure. This allows lifetimes of the rubidium

atom oscillations of about 1/100 second. The

oscillation lifetime T is still limited by atom collisions. We can calculate a corre- sponding linewidth of about 100 Hz and a Q

value of 10 7

. Atomic collisions as well as the simultaneous action of the light and the

microwave signals on the same atom cause frequency shifts of the order of 10- 9

. These

QUARTZ CRYSTAL

OSCILLATOR & FREQUENCY

SYNTHESIZER

FEEDBACK

LAMP

FILTER

CAVITY WITH

GAS CELL

FIGURE 11.23. RUBIDIUM GAS CELL FREQUENCY OSCILLATOR.

208
frequency shifts depend strongly on the com- position, temperature, and pressure of the

buffer gas and on the intensity of the light. As a result, rubidium gas cells vary in their resonance frequency by as much as 10- 9

, depending on the particular setting of the

frequency shifting parameters during manufacture. Since these influences cannot be

expected to remain unchanged with time, rubidium oscillators need initial calibration and

also recal ibration because they exhibit a frequency drift or aging like crystal oscillators. The stability performance of rubidium

oscillators is nevertheless quite spectacular.

At one-second sampling times, they display a stability of better than 10- 11 and perform

near the 10- 13 level for sampling times of up to a day. For longer averaging times, the

frequency stability is affected by the frequency drift which is typically 1 part in 10 11 per month. This is much less than the drift

of crystal oscillators. Commercial devices

with the above-mentioned performance are available as well as rubidium oscillators of

lower performance at significantly reduced

costs.

,-'2

J_

2 3 4 5

1 10 10 10 10 10 SAMPLING TIME ( SECONDS ) —

FIGURE 11.24. FREQUENCY STABILITY OF COMMERCIAL RUBIDIUM GAS CELL FREQUENCY OSCILLATORS.

11.8.3 ATOMIC HYDROGEN MASERS

Maser is an acronym meaning "microwave

amplification by stimulated emission of radiation." The atomic resonance frequency is at 1,420,405,752 Hz. The oscillator is based on the atomic beam method using spatial state selection and microwave detection. All natural hydrogen gas is composed of hydrogen

molecules. Each hydrogen molecule is formed

by chemical bonding of two hydrogen atoms. The beam source is a radio frequency gas discharge in molecular hydrogen. This discharge

produces atomic hydrogen with high efficiency.

The atomic hydrogen beam leaves the source through one or many channels into a vacuum

chamber. The beam then traverses a state- selecting magnet and enters a storage bulb in the microwave cavity. The storage bulb is made from quartz glass which has low electric

losses and thus does not significantly alter

the cavity Q. The storage bulb is evacuated

to less than 10- 11 of atmospheric pressure.

Its inner walls are lined with Teflon. This

coating allows many collisions of the hydrogen

atoms with the walls without significantly

disturbing the oscillations of the atoms. The

underlying physical mechanisms are not yet

fully understood. The storage bulb is typically 0.15 meter in diameter and dimensioned

in such a way as to hold hydrogen atoms for

about one second.

FEEDBACK

QUARTZ CRYSTAL

OSCILLATOR & FREQUENCY

SYNTHESIZER

FREQUENCY

COMPARATOR

MICROWAVE

OUTPUT

CAVITY WITH STORAGE BULB

FIGURE 11.25. HYDROGEN MASER OSCILLATOR.

Rubidium oscillators are used wherever

excellent medium-term--minutes to a daystability is needed. They are used where

reduced costs, size, and weight, as compared

to cesium oscillators, are important. Rubidium devices are used where a crystal oscillator, with its more frequent needs for recal i- bration and its greater environmental sensitivity, would not suffice.

After about one second, the atoms leave

the bulb and thus also leave the microwave

cavity. We can calculate a linewidth of about

1 Hz and a Q value of about 10 9

. This is the

highest Q value in all presently-used atomic

standards. Higher Q values are realized in experimental devices such as methane-stabilized helium-neon lasers and ion storage

devices as well as in microwave cavities which

209
FIGURE 11.26. FREQUENCY STABILITY OF A HYDROGEN MASER OSCILLATOR.

The curve is a composite of the best documented per- formances of several differently designed devices.

However, there is sufficient evidence to indicate

that a device could be built that performs as indicated. Programs are underway twoard this goal.

are cooled to the superconducting state. If the intensity of the hydrogen beam, which

consists only of upper state atoms (emitting

atoms), is sufficiently large and if the

cavity losses are sufficiently low, selfoscillation will start in the cavity. The maser itself will generate a microwave signal. We then have a maser-oscil lator with an output

frequency directly derived from the atomic

resonance. A crystal oscillator can be locked

to this frequency by frequency comparison

techniques. When compared to a cesium oscillator, the hydrogen maser is not quite as accurate. This is because of experimental

difficulties in the evaluation of the frequency shift due to the collisions of the

hydrogen atoms with the Teflon surface of

the storage bulb. In order to obtain selfoscillation, the cavity Q must be relatively

high. Thus, cavity pulling is relatively

strong, and it limits the long-term stability

over periods longer than several days. Values

of long-term stability are not better than

those of cesium oscillators. However, for

periods of a few seconds to a day, the hydrogen maser has the best stability of all existing oscillators. Its application is rather limited to uses where these stabilities

are critical and where a rather bulky device

is not a handicap. Unlike cesium and rubidium

oscillators, hydrogen masers have not yet been

evaluated under adverse environmental conditions. The number of hydrogen masers in use

is very small compared to the numbers of

cesium beam and rubidium devices.

11.9 TRENDS

All devices which were discussed in this

report have improved over the past years in one or more of the following aspects: reliability, size, weight, stability performance,

and environmental insensitivity. A considerable amount of effort is still being expended

to go further in this direction; however, it

would be inappropriate here to try to estimate

the improvement of performance specifications

which might (or might not) be achieved in the

future.

There are several other devices, designs,

and concepts which have been studied or are being investigated with some potential for

future use in frequency standards. However,

they will not be discussed here.

210
TABLt 11.1 COMPARISON OF FREQUENCY SOURCES

LISTED ARE TYPICAL CHARACTERISTICS OF FREQUENCY SOURCES (1975). THE CRYSTAL OSCILLATORS RANGE FROM SMALL PLUG-INS TO THE BEST LABORATORY UNITS. VALUES ARE GIVEN FOR THE VERY SMALL CESIUM AND RUBIDIUM OSCILLATORS AS WELL AS THE LARGER STANDARD UNITS. THE HYDROGEN MASER IS SHOWN FOR COMPARISON SINCE VERY FEW HAVE BEEN SOLD COMMERCIALLY.

CHARACTERISTICS

KIND OF OSCILLATOR

CRYSTAL CESIUM RUBIDIUM

GAS CELL HYDROGEN

MAS E R

BASIC RESONATOR FREQUENCY 10 kHz TO 100 MHz 9,192,631 ,770 Hz 6, 83*4 ,682 , 608 Hz 1 ,1420 ,405 ,752 Hz

OUTPUT FREQUENCIES PROVIDED 10 kHz TO 100 MHz 1 , 5, 10 MHz

TYP 1 CAL

1

, 5 , 10 MHz TYPICAL

1 , 5 , 10 MHz TYPICAL

RESONATOR Q

h 6 10 TO 10° 10 7 TO 10

8

I0

7 10 9

RELATIVE FREQUENCY STABILITY, SHORT-TERM, 1 SECOND

10" 6 TO 10" 12 5 x 10~" TO

- 1 3 5 x 10 3

2 x 10 TO

- 1 2

5 x 10

IZ

5 x lO" 13

RELATIVE FREQUENCY STABILITY,

LONG-TERM, 1 DAY

10" 6 TO I0~

12 lO"' 3 TO lO""1 -1 2

5 x 10 TO

3 x lO"' 3 10~' 3 TO 10""*

PRINCIPAL CAUSES OF LONG-TERM INSTABILITY

AGING OF CRYSTAL, AGING OF ELECTRONIC

COMPONENTS, ENVIRON- MENTAL EFFECTS

COMPONENT

AGING

LIGHT SOURCE AGING,

F 1 LTER 6 GAS CELL AGING, ENVIRON- MENTAL EFFECTS

CAVITY PULLING,

ENV 1 RONMENTAL

EFFECTS

TIME FOR CLOCK TO BE IN ERROR 1 MICROSECOND

1 s TO 10 DAYS

1 WEEK TO

1 MONTH 1 TO 10 DAYS

1 WEEK TO

1 MONTH

FRACTIONAL FREQUENCY REPRODUCIBILITYNOT APPLICABLE. MUST CALIBRATE.1 x 10"" TO

2 x lO"

12

1 x lO"'

0

5 x lO"' 3

FRACTIONAL FREQUENCY

DRI FT

1 x 10" 9 TO

- 1 1

1 x 10 PER DAY

< 5 x lo"' 3

PER YEAR

1 x 10""

PER MONTH

< 5 x lO" 1

PER YEAR

PRINCIPAL ENVIRONMENTAL

EFFECTS

MOTION, TEMPERATURE,

CRYSTAL DRIVE

LEVEL

MAGNETIC FIELD, ACCELERATIONS,

TEMPERATURE

CHANGE

MAGNETIC FIELD, TEMPEkATURE

CHANGE, ATMOSPHERIC PRESSURE

MAGNETIC FIELD, TEMPERATURE

CHANGE

START-UP AFTER

BEING OFF

SECONDS TO HOURS

(MAY HAVE SYSTEMATIC OFFSET)30 TO 60 MINUTES 10 TO 60 MINUTES FEW HOURS

RESONATOR RELIABILITY, TIME

BETWEEN REPLACEMENTS

VERY RELIABLE 3 YEARS 3 YEARS NO DATA

tvpitai 1 It 1 UnL c

5

i

1 7cit , nipir LUD 1 I CENTIMETERS

10 TO 10,000 10,000 TO 700,000 1 ,000 TO 20 ,000 300,000

TYPICAL WEIGHT IN KILOGRAMS (POUNDS)

0.1 TO 10

(2 OZ TO 22 LB)

16 TO *t00 (35 TO 70 LB)

1 TO 20

(3 TO 1(5 LB) 200

(<t60 LB)

POWER CONSUMED, WATTS 0.1 TO 15 30 TO 200 12 TO 35 1*0 TO 100

ESTIMATED PRICE $100 TO $5000 $15,000 TO $20,000 $3,000 TO $10,000

$100,000 TO $200,000

211

I 7; X 341 4 I
I

I

;

i

1
CHAPTER 12. SUMMARY OF AVAILABLE SERVICES

In the previous chapters, we discussed the various time and frequency dissemination ser- vices that can be used for calibrations. The following tables should serve as a quick reference

to these services. Table 12.1 summarizes the major characteristics of the different services.

Tables 12.2 and 12.3 show the different means of calibrating time and frequency sources and the

accuracies that can be obtained.

TABLE 12.1. CHARACTERISTICS OF THE MAJOR T&F DISSEMINATION SYSTEMS

DISSEMINATION TECHNIQUES

ACCURACY

FREQUENCY

ZATION

ACCURACY

FOR nATF Un 1 L TRANSFER

AMRTni ITTY

COVERAGE

FOR

ACCURACY

VLF

RADIO

GBR, NBA, OMEGA,

ETC.

1 x 10-11 ENVELOPE

1

1 - 1

1 flU mc1Mb

1 CYCLE NEARLY

u LUDn

Lp

RADIO

STANDARD FREQUENCY

BROADCAST (e.g. , WWVB)

1 x 10 "

PHASE 24h

ENVELOPE

1 - 10 ms YEAR

USA - LIMITED

(WWVB)

LORAN-C 5 x 10" 12 1 us (GND)

50 us (SKY)

T0C 15 MIN PHASE 10 us SPECIAL

AREAS

HF/MF

RADIO STANDARD FREQUENCYBROADCAST (e.g. , WW) 1 x 10" 7 1000 us

CODE - YEAR

VOICE - 1 DAY TICK - 1 s

HEMISPHERE

TELE- VISION

(VHF/SHF

RADIO

PASSIVE LINE-10

, -11 1x10 1 us 33 ms NETWORK

COVERAGE

COLOR SUBCARRIER 1 x ID'

11 NA NA

SATELLITE (UHF

RADIO)

GOES o in -103 x 10 30 us 1 YEAR WESTERN

HEMISPHERE

TRANSIT 3 x 10" 10 30 us 15 MINS

PORTABLE

CLOCKS

PHYSICAL

TRANSFER

1 x 10" 13 100 ns N/A LIMITED BY TRANSPORTATION

213
TABLE 12.2. HOW FREQUENCY IS CALIBRATED

COMMERCIALLY AVAILABLE

FREQUENCY SOURCE

ACCURACY NEEDED (RELATIVE)

METHOD OF CALIBRATION

TUNING FORKS

1 x 10-2 TO

1 x 10" 3 COMMERCIAL FREQUENCY COUNTERS; STANDARD T&F BROADCASTS

ELECTRIC POWER GENERATORS

= 2 x 10" 5

(1 x 10" 3 Hz IN 60 Hz)

COMMERCIAL FREQUENCY COUNTERS; STANDARD T&F BROADCASTS

QUARTZ OSCILLATORS

1 x

10" 3 TO

1 x

10" 10

COMMERCIAL FREQUENCY COUNTER UP TO 1 x 10- 7

; FREE-RUNNING ATOMIC

OSCILLATORS OR STANDARD T&F BROAD- CASTS FOR 1 x 10- 7 OR BETTER

ATOMIC OSCILLATORS 1 x 10-9 TO = 1 x 10-13

COMMERCIAL CESIUM STANDARDS, HYDROGEN

MASERS AND NATIONAL PRIMARY STANDARDS. RUBIDIUM OSCILLATORS CAN BE CALIBRATED

ON SITE WITH A COMMERCIAL CESIUM STAN- DARD OR A HYDROGEN MASER TO AN ACCURACY OF ABOUT 5 x 10- 12

. FOR CALIBRA- TION AT A DISTANCE, USE THESE SAME SOURCES AND LINE-OF-SIGHT MICROWAVE OR

AN LF OR VLF BROADCAST. FOR ACCURACIES IN THE RANGE OF 5 x 10- 12 TO

1 x 10- 13

, COMPARE TO A NATIONAL

PRIMARY STANDARD AND USE ON-SITE CALI- BRATION OR LF OR VLF BROADCASTS.

TABLE 12.3. HOW TIME IS CALIBRATED

i

SOURCE OF TIME ACCURACY NEEDED

METHOD OF CALIBRATION

REGULAR (ESCAPEMENT)

WATCHES AND CLOCKS

10 SEC = 1 x 10 _4

/DAY

(10 TIMES BETTER THAN USUAL WATCH)

TELEPHONE TIME FROM NBS OR CERTAIN SELECT CITIES; STANDARD RADIO BROADCASTS

ELECTRIC CLOCKS

1 SEC = 1 x 10" 5 /DAY

(MORE ACCURATE THAN MOST

POWER COMPANIES CAN HOLD)

TELEPHONE TIME FROM NBS OR CERTAIN SELECT CITIES; STANDARD RADIO BROADCASTS

QUARTZ WATCHES AND CLOCKS

0. 1 SEC = 1 x 10 _6/DAY

(STABILITY THIS GOOD BUT CAN'T BE SET THIS WELL)

TELEPHONE TIME FROM NBS OR CERTAIN SELECT CITIES; STANDARD RADIO BROADCASTS

PRECISION QUARTZ CLOCKS 1 (js = 1 x 10 _10/DAY

CESIUM CLOCK: STANDARD

RADIO BROADCASTS

ATOMIC CLOCKS

FROM = 3 x 10" 3 SEC TO

= 3 x 10- 6 SEC ON YEARLY

BASIS (i.e. , 1 x lO- 10 TO

1 x 10- 13

)

NBS OR USNO TIME SCALES

VIA PORTABLE CLOCK OR LF OR VLF BROADCASTS

214
WWVH BROADCAST FORMAT

UR IS IDENTIFIED BY

•H< TONE

1 MINUTE IS 1000 Hi TONE

BY

• THE 29th & 69th SECONO PULSE OF EACH MINUTE IS OMITTEO

STATION 10

STATION

\\N0

-. AUOIO V TONE

BEGINNING Of EACH HOUR IS 1011*11* 0 8 SECONU LONG. 1500 Hi TON* • BEGINNING Uf EACH MINUTE IS I0EN1

(I 8 SECONO LONG 1?00 H/ TONE • I »i ?9th » Mttl SECONO hi:'-' 0* E

FIGURE 12.1. WWV/WWVH HOURLY BROADCAST FORMAT.

215

BIBLIOGRAPHY

[1]

[2]

Barnes, J. A., "A Non-mathematical

Discussion of Some Basic Concepts

of Precise Time Measurement," On Frequency (published by Tracor,

Inc.), Vol. II, No. II (May 1971).

Blair, Byron E. , ed. , Time and Fre- quency : Theory and Fundamentals , Nat. Bur. Stand. Monograph 140 (May

1974). (Order from: Superintendent

of Documents, Government Printing

Office, Washington, DC 20402, SD Catalog No. C13.44: 140.

Bagley, A. , "Frequency and Time Mea- surements," Handbuch der Physi k , Vol. XXIII, (Springer-Verlag Co.,

Berlin, 1966).

Czech, J. , Osci 1 loscope Measuri ng Technique : Principles and Appl i ca- tions of Modern Cathode Ray Osci 1 lo- scopes (Springer-Verlag Co. , New

York, 1965).

[11] Tolman, J., V. Ptacek, A. Soucek and

R. Stecher, "Microsecond Clock Com- parison by Means of TV Synchronizing

Pulses," IEEE Trans. Instr. and

Meas., Vol. IM-16, pp. 247-254

(September 1967).

[12] Soucek, A., "Travel Time Stability

on TV Relay Links," Radio and Television, No. 5, pp. 29-31 (1969).

[13] Davis, D. D. , J. L. Jespersen and

G. Kamas, "The Use of Television

Signals for Time and Frequency

Dissemination," Proc. IEEE (Letter),

Vol. 58, No. 6, pp. 931-933 (June

1970).

[14] Davis, D. D. , "Transmission of Time/

Frequency Signals in the Vertical

Interval," Synopses of Papers (108th

Tech. Conf. & Equip. Exhibit, Soc. Mot. Pict. & Tele. Eng. , New York,

NY, October 4-9, 1970), Paper No. 43 (October 1970).

Evans, J., "Time and Time Measurement," Electronics World, pp. 25-29

(July 1966).

[15] Davis, D. D. , "Frequency Standard

Hides in Every Color TV Set," Electronics (May 10, 1971).

Willrodt, M. J., "Frequency and

Frequency Measurement," Electronics

World, pp. 25-28 (October 1966).

[16] Davis, D. D. , "Calibrating Crystal

Oscillators with TV Color-reference

Signals," Electronics (March 1975).

Hironaka, N. and C. Trembath, "The [17] Use of National Bureau of Standards

High Frequency Broadcasts for Time

and Frequency Calibrations," Nat.

Bur. Stand. Tech. Note 668 (May

1975).

Viezbicke, P. P. , "NBS FrequencyTime Broadcast Station WW, Fort Collins, Colorado," Nat. Bur. Stand. [18]

Tech. Note 611 (October 1971).

Howe, Sandra L. , "NBS Time and Fre- quency Dissemination Services," Nat. Bur. Stand. Spec. Publ. 432 (January

1976).

Davis, D. D. , "A Microprocessor Data Logging System for Utilizing TV as a Time/Frequency Transfer Standard. ," (Proc. 8th Annual Precise Time and

Time Interval (PTTI) Applications

and Planning Meeting, Goddard Space

Flight Center, Greenbelt, MD, Nov. 30 - Dec. 2, 1976, pp. 167-181).

Hamilton, W. F. and D. W. Hanson,

"A Synchronous Satellite Time Delay

Computer," Nat. Bur. Stand. Tech. Note 638 (July 1973). (Order from

U.S. Government Printing Office,

Superintendent of Documents, Washington, DC 20402. SD Catalog No. C13. 46:638. Price: $.45.

Milton, J. B. , "Standard Time and

Frequency: Its Generation, Control,

and Dissemination by the National Bureau of Standards," Nat. Bur. Stand. Tech. Note 656 (June 1974).

[19] Cateora, J. V., D. W. Hanson and

D. D. Davis, "Automatic Path Delay

Corrections to GOES Satellite Time

Broadcasts," Nat. Bur. Stand. Tech. Note 1003 (February 1978).

217
[20] Cateora, J. V., D. D. Davis and D. W. Hanson, "A Satellite-Controlled

Digital Clock," Nat. Bur. Stand.

Tech. Note 681 (June 1976).

[26] Siegmann, "An Introduction to Lasers

and Masers," Chapter 2, McGraw-Hill

(1971).

[21] Smith, H. M. , "International Time

and Frequency Coordination," Proc. IEEE, Vol. 60 (1972).

[22] Howe, D. A., "Frequency Domain Sta- bility Measurements: A Tutorial

Introduction," Nat. Bur. Stand.

Tech. Note 679 (1976).

[23] Allan, D. W. , "The Measurement of

Frequency and Frequency Stability of

Precision Oscillators," Nat. Bur. Stand. Tech. Note 669 (1975).

[27]

[28]

Gerber, E. A. and R. A. Sykes, "A Quarter Century of Progress in the

Theory and Development of Crystals

for Frequency Control and Selection," Proc. 25th Annual Symp. on Frequency Control, Ft. Monmouth, NJ (1971).

Ramsey, N. F. , "History of Atomic

and Molecular Control of Frequency

and Time," Proc. 25th Annual Symp. on Frequency Control, Ft. Monmouth,

NJ (1971).

[24] Barnes, J. A., et al , "Characterization of Frequency Stability," IEEE Trans. Instr. and Meas. , Vol. IM-20

(1971).

[29] Audoin, C. and J. Vanier, "Atomic

Frequency Standards and Clocks," J. Phys. E: Scientific Instruments,

Vol. 9 (1976).

[25] Allan, D. W. , H. Hellwig and D. J. Glaze, "An Accuracy Algorithm for an Atomic Time Scale," Metrologia, Vol.

11 (1975).

[30] Hellwig, H. , "Clocks and Measurements of Time and Frequency," WESCON

Technical Papers, Session 32 (1976).

218
GLOSSARY

The following definitions are offered as

j an aid to the reader. However, they may not

' conform to the definitions as recommended by

' the CCIR or other appropriate organizations.

J Further, we have not attempted to include all possible unfamiliar terms. Many terms are fully explained in the text and have not been

repeated here.

! ACCURACY - how well something agrees with the

definition or standard. In the case

of frequency, an oscillator is said

to be accurate if it agrees with the

I master oscillator at the National Bureau of Standards in Boulder,

Colorado. There is often confusion

about the definition of the terms resolution, precision, and accuracy.

AGING - the process whereby crystals change

their characteristics. A quartz

crystal of 100 kHz may age until its

frequency becomes 100.01 kHz. The

process of aging is not well understood.

AMBIGUITY - the properties of something that

allows it to have more than one possible meaning. In the case of a clock, if it displays 3 hours, 5 minutes, it could be morning or

night. The clock is then said to be

ambiguous to the hour. Ambiguity is overcome in time codes by sending

the day number. This makes that

particular time code ambiguous to the year.

ANALOG FREQUENCY METERS - that class of

instruments that uses needle

movements rather than lighted digits

to indicate frequency. Accuracies

obtained with these meters is often

less than with their digital coun- terparts.

AT CUTS - one of the many ways in which bulk

quartz material can be sliced to produce frequency controlling

crystals. The AT cut is usually

used for frequencies above 500 kHz and has excellent frequency versus temperature characteristics.

ATOM DETECTION - a process used in atomic

clocks to detect atoms.

ATOMIC TIME (TA) - the time obtained by counting cycles of an atomic clock as opposed to time based on the earth's

rotation. Atomic time is extremely

uniform.

AUTOMATIC GAIN CONTROL (AGC) - a circuit used

in communications receivers so that

both weak and strong stations can be

heard with nominally equal volume.

BALANCE WHEEL - a small wheel in watches under

control of the hairspring. The

oscillating balance wheel determines

the rate of the watch.

BURST - the name given to a short signal

consisting of a number of cycles.

The WWV tick could properly be

cal led a burst.

CALIBRATION - a frequency source can be calibrated by measuring its frequency

difference with respect to the

National Bureau of Standards frequency standard. It is also pos- sible to calibrate one frequency

source against another.

CESIUM STANDARD (Cs) - This term is often used

to describe any oscillator that uses cesium to obtain atomic resonance.

CHRONOMETER - a very accurate clock or watch,

usually used on ships for navigation. This name has also been

applied to digital instruments

driven from quartz crystals.

COLOR BAR COMPARATOR - an instrument that is used to compare a user's oscillator

to the television color subcarrier

by putting a colored vertical bar on the television screen.

CYCLE CORRECTION - the correction applied to low frequency radio data that allows

for the chance that the receiver may

be locked to the incorrect carrier

cycle. In the case of Loran-C, one cycle is 10 microseconds. For the WWV tone, one cycle would be 1 mi 1 1 i second.

DATE - a complete description of the time of

an event. The date should properly

include year, day, hour, minute, and

second. It can be given by day

number in a particular year and also

by Jul i an Day.

DECADE COUNTING ASSEMBLY (DCA) - a circuit

used in counters that counts from 0 to 9 before it overflows. This

enables the counter to display units

of tens, hundreds, etc.

219
DIGITAL PHASE SHIFTER - an instrument using

digital techniques which causes a phase shift to occur in a signal.

In the case of 1 MHz, for example,

phase shifts usually occur in one-tenth microsecond steps or less.

DIGITAL SLEWING SYSTEM - a means whereby a continuous phase shift in a signal can be caused by digital control.

DIURNAL PHASE SHIFT - the phase shift (diurnal means daily) associated with sunrise

or sunset on low frequency radio

paths.

DOPPLER EFFECT - an apparent change in frequency caused by motion of either

the transmitter or receiver. In the case of radio waves, a Doppler

effect occurs when the ionosphere

changes its position.

DOUBLE CONVERSION - the conversion in two steps from the radio carrier fre- quency to the final intermediate

frequency. High quality radio

receivers use double conversion to avoid reception of image frequencies.

DRIFT (FREQUENCY DRIFT) - frequency drift is the change in frequency of a frequency source—caused by temperature

or aging, for example.

DUT1 - the number to be added to UTC to obtain

an approximation of a UT1 date. The

value of DUT1 is given on the U. S. , Canadian, and most other similar

broadcasts so that a listener can correct the time as heard to make it

agree with UT1 within 0.1 second.

EMISSION DELAY - a delay deliberately inserted

at the transmitter to time the

various signals being emitted. In Loran-C, it allows a number of

stations (a chain) to transmit on the same frequency.

EPHEMERIS TIME (ET) - an astronomical time

system based on the orbital motion

of the earth. See also Universal

Time.

EPOCH - a date selected as a point of reference, usually in astronomy. This

word is very often used instead of

the term date or time, but should be

discouraged because of ambiguities

in its meani ngs.

FADING - the characteristic of radio waves which causes their received amplitude to decrease. Fading is caused

by a combination of several radio waves that are not in exact phase.

FERRITE LOOP - a loop antenna that uses a rod

made of iron powder. It is commonly

used in small AM broadcast receivers. It can also be used to receive WWVB at 60 kHz.

FLYWHEEL - an oscillator or frequency source used to keep a clock going when its

usual source of frequency is lost. Many digital clocks use an RC oscillator to "flywheel" when the 60 hertz power is disconnected.

FREQUENCY - if T is the period of a repetitive phenomena, then the frequency

is f = 1/T. In SI units, the period

is expressed in seconds and the frequency is in hertz. Note that different symbols can be used (especially in mathematics) to identify a particular frequency. In this book,

both "f" and "n" have been used (see

Chapters 3 and 11). The reader

should take note of what is meant

in each case.

FREQUENCY DISCRIMINATOR - an instrument or circuit which can distinguish frequency changes. FM radio circuits

use discriminators and they are also

used to measure frequency differences.

FREQUENCY - the name given to an instrument

developed by NBS to measure frequency by use of the color television subcarrier.

FREQUENCY-SHIFT KEYING (FSK) - a means of

modulating a radio carrier by

changing its frequency by a small amount.

FREQUENCY SYNTHESIZERS - instruments or circuits that change one input frequency to one or more different

output frequencies. A communications system, for example, could use

a 5 MHz quartz crystal oscillator

and synthesize the carrier or data

frequencies needed for communications.

GREENWICH MEAN TIME (GMT) - related to solar

time at Greenwich, England, which

has been used extensively in the

past. At this writing, Greenwich

Mean Time can be considered equivalent to Coordinated Universal Time

(UTC) which is broadcast from all standard time and frequency radio

stations.

220
GROUP REPETITION RATE - the rate of recurrence

of specified groups of pulses. In the case of Loran-C, the group

repetition rate is altered between

different chains to avoid interference.

GT CUTS - refers to the direction in which a quartz crystal blank is cut. GT crystals have excellent frequency

stability but are usually larger in physical size than the AT cuts.

HARMONIC GENERATOR - an instrument or circuit

that generates harmonics of the

input signal. A harmonic generator

provides a means of using a lower

frequency source to calibrate high

frequency signal generators.

HETERODYNE - the means by which new frequencies are generated by mixing two signals in a nonlinear device such

as a vacuum tube, transistor, or diode mixer. For example, a superheterodyne receiver converts all incoming frequencies by heterodyne

action to a common intermediate

frequency.

HETERODYNE CONVERTER - a frequency converter

that uses the heterodyne process to convert input signals to a range

where the instruments can measure

their frequency.

HYSTERESIS - a lag effect in a body when the

force acting on it is changed.

IMAGE REJECTION - the ability of a superheterodyne receiver to reject an un- wanted signal which is just as far

on one side of its local oscillator

as the desired frequency is on the

other side.

INTERMEDIATE FREQUENCY (IF) - the frequency in

a superheterodyne receiver to which

allE incoming frequencies are converted. The intermediate frequency

is usually a low frequency somewhere

between 450 and 3000 kHz.

IONOSPHERE - the outer part of the earth's

atmosphere. The ionosphere consists

of a series of constantly changing

layers of ionized molecules. Many

radio waves reflect back to earth

from the ionosphere.

JITTER - usually used to describe the small

changes in a signal with time.

Phase jitter

JITTER - usually used to describe the small

changes in a signal with time.

Phase jitter causes a counter to be

triggered either early or late.

JULIAN DAY (JD) - the Julian day number is obtained by counting days with a starting point of noon January 1, 4713 B.C., which was Julian Day

zero. This is one way of telling

what day it is with the least pos- sible ambiguity.

LEADING EDGE - on a narrow pulse like that

from a clock, it is the first-occurring change and can be either nega- tive or positive-going.

LEAP SECOND - a one-second interval either

inserted or deleted in a clock to keep Coordinated Universal Time in step with earth time, UT1 . This is usually done only once a year and is intended to assist navigators.

LINE-10 - the tenth line out of 525 that make

up a television picture. Line-10

has been adopted as a measurement

point for time transfer.

LISSAJOUS PATTERNS - the patterns obtained on

a cathode ray tube by driving both

the horizontal and vertical deflection plates with particular signals.

Lissajous patterns are used for

frequency calibration and phase measurement.

LONG-TERM STABILITY - describes the frequency

change of an oscillator that occurs with time. Long-term usually refers

to changes over times longer than one second.

LOOP ANTENNA - an antenna used for radio

reception at many different fre- quencies. It consists of a number

of turns of wire usually in a shielded or pipe-type of enclosure.

A loop antenna operates in the mag- netic field of the signal and has a "figure-8" reception pattern which

enables it to null out undesired

signals.

MASER (MICROWAVE AMPLIFICATION BY SIMULATED

EMISSION OF RADIATION) - a device

that uses the natural oscillations

of atoms and molecules to generate

signals usually in the microwave

region.

221
MAXIMUM USABLE FREQUENCY (MUF) - the highest

frequency that can be used for radio

transmissions without having the

signal escape through the ionosphere. Signals below the maximum

usable frequency will reflect off

the ionosphere.

MEAN SOLAR TIME - time based on a mean solar

day, which is the average length of

all solar days in a solar year. The mean solar second is 1/86,400 of a mean solar day.

MIXER - an electrical circuit that mixes two signals and produces a new signal

that is usually either the sum or the difference of the two inputs.

MODIFIED JULIAN DAY (MJD) - this is equal to the Julian date shifted so its

origin occurs at midnight on November 17, 1858. That is, it differs

from the Julian Day by 2,400,000.5

days exactly.

MULTIPATH - propagation of radio waves by more

than one path from transmitter to receiver. This usually causes either fading or distortion.

NETWORK TELEVISION PROGRAM - any program that

originates at either the East or West Coast networks of the U. S. broadcasters. It does not have to be a live program. It simply has to start out at the network studios.

NULL DETECTOR - an instrument that is used to detect the absence of a signal.

Null detectors are usually more sensitive than peak detectors.

OFFSET (FREQUENCY) - frequency offset is the

difference in the available frequency and the nominal frequency.

It is often deliberate. In the case

of the U. S. television networks at this writing, the offset is about

-3000 parts in 1011. The term frequency offset is used interchangeably with the terms relative or fractional frequency.

OMEGA NAVIGATION SYSTEM - a low frequency (10 to 20 kHz) navigation system usually

used in the hyperbolic mode.

OPTICAL DETECTION - detection of signal or its

properties by optical means.

OVERTONE CRYSTALS - crystals designed to operate at a frequency higher than

their fundamental. Crystal con- trolled oscillators as distinguished

from crystal oscillators often use overtone crystals.

PERIOD MEASUREMENT - a measurement of frequency by measuring the duration of

the periods in time units of the

signal. Period measurements are always more accurate than frequency measurements unless the frequency

being measured is as high as the

frequency counter's time base. For example, a counter with a 1 MHz time

base can be used in the period mode

for greater accuracy for frequencies

up to 1 MHz.

PHASE DETECTOR - a circuit or instrument that

detects the difference in phase

(sometimes in degrees or microseconds) between two signals. Phase

detection is useful in high resolution frequency detection.

PHASE ERROR MULTIPLIERS - a scheme whereby the

difference in phase between two signals is enhanced by multiplication and mixing techniques.

PHASE JUMP - the name given to sudden phase

changes in a signal

PHASE LOCK - a servo mechanism technique for

causing one signal to follow

another. They can be but do not have to be at the same frequency.

Phase locking in terms of frequency

sources is analogous to a mechanical

servo where one shaft or wheel

fol lows another.

PHASE SIGNATURE - a deliberate phase offset to identify a signal. WWVB as broad- cast is deliberately phase shifted

at 10 minutes after the hour. This

aids the user by letting him know

that he really is tracking WWVB and not some other signal.

POWER LINE FREQUENCY - In the United States

the power lines are held to a nominal 60 hertz. Variations in this of about + 50 ppm occur when

the power system adjusts the fre- quency to regulate power flow.

PORTABLE CLOCK - any clock which is designed

to be tranported. Usually it is an atomic clock with suitable batteries.

222
PRECISION - the reproducibility of a given measurement. For example, a clock

pulse could be consistently measured

to a precision of a few nanoseconds.

Not to be confused with accuracy. The same clock that is measured to a precision of a few nanoseconds could

be in error by several seconds.

PRESCALING - the process whereby a frequency

out of range of a measurement in- strument can be scaled down to within the range of that instrument.

PROPAGATION DELAY - the delay experienced by a signal as it traverses a radio path.

Q

- the quality of a signal or detector or device. Q of coils is defined as the ratio of the inductive reactance

to the resistance. It is also equal

to the ratios of bandwidth of a resonator to its central frequency.

QUARTZ - a brilliant, crystalline mineral,

mainly silicon dioxide.

RECEIVER DELAY - the delay experienced by the

signal going from the antenna to the

detector or output device, speaker,

or oscilloscope terminal. Typical

receiver delays in high frequency

radio receivers are of the order of

a few mi 1 1 i seconds.

REPRODUCIBILITY - the degree to which an oscillator will produce the same output frequency from one time to another after it has been properly

al igned.

RESOLUTION - the degree to which a measurement

can be determined is called the

resolution of the measurement. A time interval counter might only

resolve tenths of seconds or a few

mi 1 1 i seconds.

RUBIDIUM (Rb) OSCILLATOR - an oscillator based

on a resonance of rubidium gas.

SELECTIVITY - the ability of radio receiver

circuits to separate one signal from

another.

SENSITIVITY - the characteristic of a radio

receiver that allows it to detect

weak signal s.

SHAPER - a circuit that provides uniform

pulses from a variety of inputs. An example would be a circuit to change

sine waves into sharp pulses.

SHORT-TERM STABILITY - a description of the

frequency fluctuations caused by

random noise in an oscillator. To properly state the short-term

stability, the number of samples,

averaging time, repetition time, and

system bandwidth must be specified.

SIDEREAL TIME (SIGH DEER 1 EEE AL) - time based

on observations of stars rather than

the sun. Sidereal time is used by astronomers and a sidereal day is equal to about 23 hours, 56 minutes,

and 4 seconds of solar time. Be- cause it is based on observations of

stars, it is more accurately determined than solar time.

SIGNAL-TO-NOISE RATIO (SNR) - the ratio of the

strength of a radio signal to that

of the noise. This is often a more

useful term than simply signal

strength.

SQUARING CIRCUIT - a circuit designed to produce square waves from an input

waveform, quite often a sinusoid.

This reduces the indecision about

where zero crossings will occur.

STANDARD - a universally accepted reference.

The NBS frequency standard is a cesium oscillator located at the

National Bureau of Standards in Boulder, Colorado. Many frequencies

generated and used in the United

States are referenced to this

standard.

STANDARD FREQUENCY BAND - radio bands allocated expressly for the purpose of

distributing standard frequency

and/or time. For example, all of

the transmissions of WWV occur in standard frequency bands centered on

2.5, 5, 10, and 15 MHz.

STATE SELECTION - the process whereby atoms or molecules are separated, from atoms

or molecules in a different state.

SUBCARRIER - an additional signal inserted

along with the main carrier in radio

or television transmission. The

television color subcarrier is a signal added to the television

format at about 3.58 MHz from the

main carrier.

SUNSPOTS - dark spots sometimes seen on the

surface of the sun which are related

to magnetic and radio propagation

disturbances on earth.

SUNSPOT CYCLES - cycles in the number of

sunspots.

223
SYNCHRONIZATION - in the context of timing,

synchronization means to bring two clocks or data streams into phase

so they agree. They need not be on time.

TCXO (TEMPERATURE COMPENSATED CRYSTAL OSCILLATOR) - a crystal oscillator that

contains special components to minimize the effect of temperature on the crystal frequency.

TICK - an electrical signal, usually a pulse,

designed for use in resetting of

dividers, etc. The WWV tick heard

on its radio broadcast consists of 5 cycles of a 1 kHz sine wave.

TIME BASE - in a frequency counter that oscillator which provides the timing

signals for measurement and control.

The accuracy of the counter is directly related to the time base.

TIME INTERVAL COUNTER - a counter designed to measure the time between events,

zero crossings for example.

TIME OF COINCIDENCE (LORAN) - that time when

the Loran signal is coincident with

an exact second of Coordinated

Universal Time. These times of

coincidence will occur about every

10 or 15 minutes and have been

tabulated by the U. S. Naval Observatory.

TRANSFER STANDARDS - the name given to a signal that is used to perform a calibration when it can be referenced to

a standard. An example would be the

use of a VLF frequency source to transfer frequency to a location

based on its calibration at NBS.

TRIGGER ERROR - The error associated with

false triggering caused by phase

noise or jitter on a signal.

TTL INTEGRATED CIRCUITS - a family of integrated circuits that uses transistortransistor logic. At this writing

TTL has become the most popular

family of integrated circuits and is characterized by fast rise times.

Unless deliberately modified, output

levels are nominally zero to 5 volts

positive.

UNIVERSAL TIME (UT) FAMILY - Universal Time is given in several ways. Apparent

solar time is first corrected by the

equation of time to mean solar time,

UTO. It is then again corrected for

migration of the earth's poles to obtain UT1 . This is further cor- rected for periodicity of unknown

origin to obtain UT2.

VFO - variable frequency oscillator as distinguished from a fixed frequency

oscillator such as a crystal.

WAVE ANGLE - the angle above the surface of

the earth at which a signal is transmitted from or received by an antenna.

WAVEMETER - a meter for measuring frequency. Wavemeters usually work by having a coil that collects part of the

energy from a transmitter or a transmission line and displays resonance on a meter movement.

ZERO BEAT - that condition between two signals

when no beat is heard or seen be- tween the signals; that is, the

frequencies are equal.

224
INDEX

AT CUT IN CRYSTALS t 201

ACCURACY

ATOMIC RESONATORS ..... 206

CALIBRATION

GOES Satellite Time Code 176,185

High Frequency Radio Broadcast 77,78,79,80

Loran-C 171,172

Low Frequency/Very Low Frequency Radio Broadcast . . . . 107,130

Television

Frequency 131 ,145,149

Time 147,151

TRANSIT Satellite . . 187,189

CESIUM 208,211

COUNTER 43

DEFINITION 193

HYDROGEN MASER, ATOMIC 209,211

QUARTZ CRYSTAL OSCILLATOR 199,211

RUBIDIUM , 209,211

ADJUSTABLE RATE DIVIDERS 64

AGING, QUARTZ CRYSTAL 201

ALGEBRA OF TIME AND FREQUENCY MEASUREMENTS 27-32

AMBIGUITY

PHASE CHART, WWVB 108

± 1 COUNT 34,44

TELEVISION LINE-10 148,151

ANALOG FREQUENCY METER, DIRECT-READING 58

ANTENNAS

GOES Satellite 180 HF (High Frequency) ........... 82

LF/VLF (Low/Very Low Frequency). ........ 107

TO REDUCE INTERFERENCE AT LF/VLF 113

APPARENT SOLAR TIME ]7

ASTRONOMICAL OBSERVATORIES

ASTRONOMICAL TIME ]7

225
ASTRONOMY, USE OF TIME SCALES FOR 22

ATOMIC

BEAM, CESIUM 207

CLOCK

Cesium 198,207

World's First 18 FREQUENCY SOURCES 205-211

HYDROGEN MASER . . 209

OSCILLATORS 205

Cesium 207

Hydrogen 209

Rubidium 208

RESONATOR 206 RUBIDIUM OSCILLATOR 208 TIME (TA) 18 TIME SCALES (See TIME SCALES)

ATOMIC RESONANCE, ACHIEVING 204

AUTOMATIC TRANSFER OSCILLATOR 37

AVERAGER, SIGNAL 65

BIH (BUREAU INTERNATIONAL DE L'HEURE)

ROLE IN INTERNATIONAL COORDINATION OF TIME AND FREQUENCY ... 23 TIME SCALE, TAI(BIH) 18

BAND, RADIO FREQUENCY 10

BASE UNITS OF MEASUREMENT 5

BASIC STANDARDS 5

BEAM, CESIUM 207

BEAT FREQUENCY METHOD 97

BROADCAST STATIONS (See RADIO BROADCASTS, Using for Calibration)

BULLETIN, NBS TIME AND FREQUENCY

MONITORING DATA FOR TRACEABILITY TO NBS GOES Satellite 180 Loran-C 171 Omega . 108,127

Television

Color Subcarrier Frequency ....... 134,138

Line-10 146

226
WWVB 108,127

WWV/WWVH 95

BULLETIN, USNO TIME SERVICES

MONITORING DATA FOR TRACEABILITY TO USNO Loran-C 170,171

NAA/NLK 127 Omega 127 Television Line-10 .......... 151 TRANSIT Satellite 187

BUREAU INTERNATIONAL DE L'HEURE (See BIH)

CGPM (GENERAL CONFERENCE ON WEIGHTS AND MEASURES) 18

CABLE, DISTRIBUTION OF TIME AND FREQUENCY VIA 11

CALIBRATION

ACCURACY

GOES Satellite .... HF Broadcasts (Time and Frequency)

LF/VLF

Television

Loran-C .....

Frequency 131 ,143,145,149

Time 146,147,151

TRANSIT Satellite 187,189

DEFINITION 8

CALIBRATION TECHNIQUES

ELECTRONIC FREQUENCY COUNTER, USE OF FREQUENCY

HF (High Frequency) Broadcasts.

Beat Frequency Method

Oscilloscope Methods

Time Comparison of Clocks.

LF/VLF (Low/Very Low Frequency) Broadcasts

Television

Color Subcarrier .... Line-10

FREQUENCY COMPARISONS

OSCILLOSCOPE

PHASE LOCK

TIME

HF Broadcasts (WWV/WWVH, CHU) . WWV Time Code

WWVB

Satellite

Television

Loran-C ....... ....... Line-10 ..... Time Code

176,185

69,77-80,105

107,125,126,130

171 ,172

33

96-101

9798

TOO 113-126

131-143

145-152

5048

66

86-92

92168

175-189

145-152

127

227
CARRIER FREQUENCIES

GOES SATELLITE

HF (HIGH FREQUENCY) BROADCAST STATIONS . LORAN-C

LF/VLF (LOW/VERY LOW FREQUENCY) BROADCAST STATIONS

OMEGA

TRANSIT SATELLITE ......

CATHODE-RAY OSCILLOSCOPE

CESIUM BEAM FREQUENCY OSCILLATORS ....

CHAIN, LORAN-C

CHRONOMETER

CLOCK

AGING

ATOMIC

Cesium

First ....... .......

AsMaster

Coordination

Definition

Frequency....... Source ...... ...... .....

Secondary

Portable

Primary ....... ....... . . . .

CODE, TIME (See TIME CODE)

COLOR

BAR COMPARATOR, TV SUBCARRIER, TV TELEVISION SIGNALS FOR FREQUENCY CALIBRATION .

COMMERCE, U. S. DEPARTMENT OF

COMPARATORS, FREQUENCY

COMPARISONS, FREQUENCY

OSCILLOSCOPE

HF (HIGH FREQUENCY) WWVB

CONVENTIONS OF TIME AND FREQUENCY MEASUREMENT

228
CONVERSIONS

PREFIX

PARTS PER... TO PERCENTS

9

9

CONVERTERS, HETERODYNE 35

COORDINATED UNIVERSAL TIME, UTC . . . . 19

COORDINATION OF TIME AND FREQUENCY ACTIVITIES, INTERNATIONAL ... 6 BIH 23 NBS 23 USNO 34 VIA LORAN-C 171

CORRECTIONS, UT1 (See UT1 CORRECTIONS)

COUNTERS, ELECTRONIC FREQUENCY 33 ACCURACY 43

ERRORS 34,43,44,45

FUNCTIONAL PARTS 33 MEASUREMENTS USING

Frequency 34

Period 39 Phase 42 Pulse-Width Determination ........ 43

Time Interval ........... 40

PRINTOUT AND RECORDING 48

COVERAGE

GOES SATELLITE 176

HF (HIGH FREQUENCY) BROADCASTS 69 LORAN-C

Groundwave ........... 166 Skywave ............ 167

TRANSIT SATELLITE 187 WWVB HI

CRYSTAL, QUARTZ, OSCILLATORS (See QUARTZ CRYSTAL OSCILLATORS)

CYCLE PHASE SHIFT, RECEIVER 108

DATE 4 - 17

DEAD TIME ^

DECADE COUNTING ASSEMBLY IN COUNTERS 33

229
DEFENSE, DEPARTMENT OF 24

DEFINITIONS

ACCURACY 193 CLOCKS 191 DATE 4 FREQUENCY 4,10,193

FREQUENCY STANDARD 191 HERTZ 193 LEAP SECOND 19 PRECISION 223

Q 197,223

REPRODUCIBILITY 193

SECOND 4,193

STABILITY 193 SYNCHRONIZATION 21 TIME 4 TIME SCALE 17 STANDARD 5

DELAY

CABLE 12 DIFFERENTIAL PATH 146 NBS TELEPHONE TIME-OF-DAY . 105 RADIO PROPAGATION PATH (See Propagation Path Delay Measurement)

RECEIVER TIME 86

TELEPHONE LINE 12 TELEVISION SIGNALS 146

DELAYED TRIGGER METHOD OF TIME CALIBRATION 90

DETECTOR, PHASE 62

DIGITAL FRAME SYNCHRONIZER 142,151

DIGITAL OFFSET COMPUTER, TV 138

DIRECT FREQUENCY MEASUREMENT

USING ELECTRONIC COUNTER 35 USING OSCILLOSCOPE ..... 50

DIRECT-READING ANALOG FREQUENCY METER 58 ELECTRONIC AUDIO 58 RADIO FREQUENCY 59

230
DIRECT-TRIGGER METHOD OF TIME CALIBRATION 88

DISCRIMINATOR, FREQUENCY 60

DISSEMINATION TECHNIQUES

GOES SATELLITE

HF (HIGH FREQUENCY) BROADCASTS

LORAN-C

LF/VLF (LOW/VERY LOW FREQUENCY) BROADCASTS

TV Color Subcam'er Frequency ........ 131-143

Line- 10 Time and Frequency ........ 145-152

TRANSIT SATELLITE 187-189

DISTANCE, GREAT CIRCLE CALCULATIONS 101

DISTRIBUTION OF TIME AND FREQUENCY

CABLE

HF (HIGH FREQUENCY) RADIO BROADCASTS

LF/VLF (LOW/VERY LOW FREQUENCY) RADIO BROADCASTS

TELEPHONE

TELEVISION

Frequency Using TV Color Subcarrier ...... 131-143

Time and Frequency Using TV Line-10 ...... 145-152

TO UNITED STATES 7

DIVIDER

DIGITAL 63 Adjustable Rate 64

Analog ............ 63 FREQUENCY 63

DRIFT, FREQUENCY 11,44

QUARTZ CRYSTAL OSCILLATORS 201

DUTI (UT1) 17 CORRECTIONS ON WWV/WWVH 19,80,94

CORRECTIONS ON WWVB 127 DETERMINATION OF 23,24

ET (EPHERMERIS TIME) 18

ELECTRONIC AUDIO FREQUENCY METER

ELECTRONIC COUNTERS (See COUNTERS, ELECTRONIC FREQUENCY)

175-187

69-105

153-173

107-130

1169-105

107-130

105

231
ENVELOPE (WWVB) 128

EPHEMERIS TIME (ET) 18

EQUATION OF TIME 17

EQUIPMENT FOR MAKING CALIBRATIONS

ADJUSTABLE RATE DIVIDER 64 COUNTER, ELECTRONIC 33 DIRECT-READING ANALOG FREQUENCY METER 58

Electronic Audio .......... 58

Radio Frequency .......... 59 FREQUENCY COMPARATOR 60

FREQUENCY DIVIDER 63

FREQUENCY SYNTHESIZER 60

HETERODYNE FREQUENCY METER 57 OSCILLOSCOPE 48 PHASE DETECTOR . .' ; . 62 PHASE ERROR MULTIPLIER 61 RECEIVER

LF/VLF (Low/Very Low Frequency) . . . . . . . 113

HF (High Frequency) .......... 81 TV Frequency 136,138,147

Time 147 SIGNAL AVERAGER ... . . . ..... 65 WAVEMETER 56

ERROR

COUNTER TIME BASE 34,43

GATE 44

NOISE 34,40

± 1 COUNT AMBIGUITY 34,44

PROPAGATION (See PROPAGATION PATH DELAY MEASUREMENT)

RECEIVER . 86 TIME BASE 34,43

TRIGGER 45

F 28

FCC 8 REQUIREMENTS FOR TV BROADCASTS 141

FADING OF HF SIGNALS 69

FIELD STRENGTH, WWVB Ill

232
FRACTIONAL FREQUENCY (See RELATIVE FREQUENCY)

| FRAME SYNCHRONIZER, DIGITAL 142,151

!

1 FREQUENCY

ACCURACY OF CALIBRATION

GOES Satellite . 176

HF (High Frequency) Broadcasts. . . . . . . . 69,105

LF/VLF (Low/Very Low Frequency) . . . . . . 107,130

Loran-C 171 AGING EFFECTS 11,201

BANDS, RADIO 10 BEAT OR DIFFERENCE 97 CALIBRATION (See CALIBRATION TECHNIQUES, FREQUENCY)

COMPARATORS 60

COMPARISONS 50

HF (High Frequency) .......... 96 Oscilloscope 50,98 WWVB 115

DEFINITION . 4,10,193

DIFFERENCE 28

DISCRIMINATOR 60 DIVIDER 63 DRIFT 44,193

HERTZ 4,10

MEASUREMENT (See CALIBRATION TECHNIQUES)

METER, HETERODYNE 57 MULTIPLIER 60

OFFSETS OF TV NETWORKS 134

RADIO BANDS TO RADIO CARRIER (See CARRIER FREQUENCIES)

SERVICES (see also SERVICES) . SOURCE (See SOURCE, FREQUENCY)

STABILITY

Atomic Resonator Measuring .... Quartz Crystal Oscillator.

SYNTHESIZER ....

ZERO-BEAT METHOD

I GT CUT IN CRYSTALS ....

| GATE ERROR

GENERATOR

RF SIGNAL

TIME-BASE, CALIBRATING

5

193 206

193

201 6037,97

201

44

54

49

233
GOES SATELLITE

ACCURACY 176,185

ANTENNAS 180 CLOCK CALIBRATION, for ... ' 183

CONTINUITY 187 COVERAGE 176 EQUIPMENT NEEDED 180

INTERFERENCE 186

OUTAGES 186 PERFORMANCE 180

RECEIVER MANUFACTURERS 181 SATELLITE-CONTROLLED CLOCK 181 SIGNAL CHARACTERISTICS 177 SMART CLOCK 182 TIME CODE FORMAT 177

GREAT CIRCLE DISTANCE CALCULATIONS 101

GREENWICH, ENGLAND MERIDIAN 17,22

GREENWICH MEAN TIME (See UK)

GROUNDWAVE

COVERAGE, LORAN-C 166

Accuracy ............ 172 DEFINITION 166

HERTZ 4,10,193

HETERODYNE

CONVERTER 35 FREQUENCY METER 57

HF (HIGH FREQUENCY) BROADCASTS, STANDARD TIME AND FREQUENCY

ACCURACY (See ACCURACY, Calibration)

ANTENNAS 82 CALIBRATIONS USING

Frequency ............ 96-101

Time 86-93

Time Code 93-95

IONOSPHERIC EFFECTS ON RECEPTION 69,102

RECEIVER SELECTION 81 SIGNAL CHARACTERISTICS 82

STATIONS . 69-80

ATA . 71 CHU 70

234
I AM 72 IBF 72 JJY 72 OMA 74

VNG 74 WWV/VH 69,81-105,215

HYDROGEN MASER, ATOMIC 209

IRIG TIME CODE 13

INTERFERENCE

HF RADIO BROADCAST 69 LF/VLF RADIO BROADCAST 113

INTERNATIONAL ATOMIC TIME SCALE (TAI) 18

INTERNATIONAL COORDINATION OF TIME AND FREQUENCY ACTIVITIES (See

COORDINATION OF TIME AND FREQUENCY ACTIVITIES, INTERNATIONAL)

IONOSPHERE, EFFECTS ON HF RECEPTION 69,102

JITTER 40,44

LF/VLF RADIO BROADCAST

ACCURACY (See ACCURACY, Calibration)

ANTENNAS 107 CALIBRATIONS USING

Frequency . . . . . . . . . . . . 113

Time Code WWVB 127 FIELD STRENGTH Ill SIGNAL FORMAT 107

STATIONS 107-126

DCF 77 118

HBG 120 JG2AS/JJF-2 120

Loran-C 153-173

MSF 121 OMA 123 Omega Navigation System . . . . . . . . • 115

RBU, RTZ 124

U.S. Navy Communication Stations (NAA, NDT, NLK, NPM, NSS, NWC) . 121 VGC3, VTR3 . 124 WWVB 107-115,127-130

LEAP SECOND 19

LINE-10 SERVICE, TELEVISION

ACCURACY 146,149

EQUIPMENT NEEDED 147 GETTING TIME-OF-DAY 151

235
REFERENCED TO

NBS 146 USNO 151

LISSAJOUS PATTERNS 50,98

LONG-TERM STABILITY 44

LORAN-C NAVIGATION SYSTEM (100 kHz)

ACCURACY, CALIBRATION

Frequency ............ 171 Time 172 CARRIER FREQUENCIES 161 CHAINS 154-159

CONTROL BY USNO AND U.S. COAST GUARD 153 COVERAGE 166 FREQUENCY CALIBRATION 170 GROUNDWAVE SIGNAL RANGE 166

MASTER STATION 153 PULSE SHAPE 161 SIGNAL CHARACTERISTICS 153,167

SKYWAVE SIGNAL RANGE 167 SLAVE STATION 153 TIME SETTING 168

MASER, ATOMIC HYDROGEN 209

MASTER

CLOCK 7 STATION, LORAN-C 153

MEAN SOLAR TIME 17

MEASUREMENT

TIME INTERVAL (See also CALIBRATION TECHNIQUES) 40 Direct Frequency .......... 35 Period 39

Phase 42 Pulse-Width Determination. . . ...... 43

MERIDIAN OF GREENWICH, ENGLAND ' 17,22

METERDIRECT-READING ANALOG FREQUENCY

Electronic Audio

Radio Frequency

HETERODYNE FREQUENCY. WAVE

5858

59

57

56

236
METHODS OF CALIBRATION (See CALIBRATION TECHNIQUES)

MONITORING DATA AVAILABILITY (See BULLETIN, NBS or USNO)

MULTIPLE-PERIOD AVERAGING 39,45

MULTIPLIER

PHASE ERROR 61 FREQUENCY 60

NBS (NATIONAL BUREAU OF STANDARDS)

ATOMIC CLOCK 6 FREQUENCY STANDARD .... 6,191

NBS-6 6,198

RADIO BROADCASTS

WWV/WWVH 69,81-105

WWVB 107-115,127-130

ROLE (See NBS TIME AND FREQUENCY DIVISION, Responsibilities)

SERVICES

Calibration ........... 23

GOES Satellite Time Code 175-187

Television

Frequency Calibration via Color Subcarrier .... 131-143

Line- 10 Time and Frequency ....... 145-152

WWV/WWVH 69,81-105

WWVB 107-115,127-130

SYSTEM 358 FREQUENCY MEASUREMENT COMPUTER 138 TIME AND FREQUENCY DIVISION 23

Organization ........... 23

Responsibilities . . . . . . . . . . 7,23 TIME CODE

GOES Satellite 177 WWV/WWVH 14,92 WWVB 14,127 TIME STANDARD 6

NATIONAL BUREAU OF STANDARDS (See NBS)

NAVAL OBSERVATORY, U.S. (See USNO)

NAVIGATION

SYSTEM

Loran-C

Omega

TRANSIT Satellite

USE OF TIME SCALES FOR

153-173

115 187-189

22

237
NETWORK TELEVISION BROADCASTS, USES FREQUENCY CALIBRATION 131-143,145-152

TIME CALIBRATION . . . 145-152

NOAA (NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION) 187

NOISE

ERRORS 34,40

QUARTZ CRYSTAL OSCILLATOR 201

NULL DETECTORS 37

OBSERVATORIES, ASTRONOMICAL 23

OMEGA NAVIGATION SYSTEM

FORMAT, BROADCAST

MONITORING DATA NOTICES AND NAVIGATIONAL WARNINGS

OPERATING CHARACTERISTICS

PROPAGATION CHARACTERISTICS

SYNCHRONIZATION CONTROL

ORGANIZATION OF TIME AND FREQUENCY ACTIVITIES, INTERNATIONAL (See

COORDINATION OF TIME AND FREQUENCY ACTIVITIES, INTERNATIONAL)

OSCILLATOR

ATOMIC 6,205

AUDIO 54

CESIUM BEAM 207,211

HYDROGEN MASER . 209,211

QUARTZ CRYSTAL (See QUARTZ CRYSTAL OSCILLATOR)

RUBIDIUM 208,211

TELEVISION NETWORK 132

TRANSFER 37 VARIABLE FREQUENCY (VFO) 37

OSCILLOSCOPE, CATHODE-RAY 48

OSCILLOSCOPE, METHODS OF CALIBRATION USING

DIRECT MEASUREMENT OF FREQUENCY

FREQUENCY COMPARISON

LISSAJOUS PATTERNS ....

PHOTOGRAPHY

PATTERN DRIFT

SWEEP FREQUENCY CALIBRATION .

127

118

115

117

117

5050,98

92100

55

238
TIME BASE, CALIBRATING 49 TIME INTERVAL »....,< 56

PTTI (PRECISE TIME AND TIME INTERVAL) 24

PATH DELAY, RADIO PROPAGATION (See PROPAGATION PATH DELAY MEASUREMENT)

PATTERN DRIFT METHOD OF CALIBRATION 100

PATTERNS, LISSAJOUS . 50,98

PERIOD MEASUREMENT 39

PHASE

CHANGES, WWVB, LF/VLF 108

DETECTOR 53,62

ERROR MULTIPLIER 53,61

INSTABILITY OF TV SIGNALS 133

JUMPS, TELEVISION 133 LOCK LOOP 66 LOCK TECHNIQUE 66

MEASUREMENT 42,132

RECORDINGS FOR FREQUENCY CALIBRATION, WWVB 113

RETARDATION, LF/VLF 108

SHIFT IDENTIFICATION, WWVB 114 SHIFT

LF/VLF 108

Cycle 108

TRACKING RECEIVER 66

PHOTOGRAPHY, OSCILLOSCOPE 92

PIEZOELECTRIC EFFECT 199

± 1 COUNT AMBIGUITY 34,44

PORTABLE CLOCK . . . . 5,24

PRECISION, DEFINITION 223

PRESCALING 35

PRIMARY

CLOCK 6 FREQUENCY STANDARD 6,197

239
PRINTER-RECORDER 48

PROPAGATION

CHARACTERISTICS

HF . LF/VLF DIFFERENTIAL PATH DELAY

PATH DELAY MEASUREMENT

HF

Time Code .

PUBLICATIONS (See Bulletin, NBS or USNO)

PULSE-WIDTH DETERMINATION MEASUREMENT 43

Q, RESONATOR

CESIUM 208 DEFINITION. 197 HYDROGEN 209

QUARTZ CRYSTAL 201 REQUIREMENTS 60 RUBIDIUM 208

QUARTZ CRYSTAL OSCILLATOR

ACCURACY 201,211

AGING 11,43,201

CHARACTERISTICS. .

' . . . . . . . . . . 199

COST 202,211 DRIFT 11,43

FREQUENCY STABILITY 201 LONG-TERM DRIFT 44

PERFORMANCE 201 PROPERTIES 199

Q 201 SHORT-TERM STABILITY 44

TEMPERATURE EFFECTS 201

RADIO ASTRONOMY OBSERVATORY 23

RADIO BROADCASTS (USING FOR CALIBRATION)

HF 69-105

LF/VLF 107-130

LORAN-C 153-173

NBS 69-105,107-130

OMEGA NAVIGATION SYSTEM 115

69108

146

87,101

15

240
RADIO FREQUENCY

BAND . METER.

1059

RADIO PROPAGATION DELAY MEASUREMENT (See PROPAGATION PATH DELAY MEASUREMENT)

RECEIVER

CYCLE PHASE SHIFT, LF/VLF. DELAY SELECTION (HF) . TIME DELAY MEASUREMENTS .108

15,86

8187

RELATIVE FREQUENCY

DIFFERENCE.

F

28

28

REPRODUCIBILITY 193

RESONANCE

ATOMIC

DETECTING .

202

204

RESONATOR

ATOMIC

Accuracy . Detecting Resonance Stability . State Selection.

QUARTZ CRYSTAL .

206

204

206

203

199

RESPONSIBILITIES IN TIME AND FREQUENCY COORDINATION (See COORDINATION

OF TIME AND FREQUENCY ACTIVITIES, INTERNATIONAL)

RUBIDIUM GAS CELL OSCILLATOR

S, RELATIVE FREQUENCY DIFFERENCE

208

28

SCALE, TIME (See TIME SCALE)

SCHMITT TRIGGER. 46

SECOND

DEFINITION.

LEAP .

193

19

241
SECONDARY

CLOCK 7 FREQUENCY STANDARD 197

SECONDS TICK, WWV/WWVH 86

SERVICES

NBS GOES Satellite Time Code 175-187

Telephone Time-of-Day ......... 105

Television

Frequency Calibration, Color Subcarrier ..... 131-143

Time and Frequency Calibration, Line-10 ..... 131-143

Time and Frequency Division . . . . . . 23 WWV/WWVH 69,81-105 WWVB . 107-115,127-130

RADIO BROADCAST

HF 69-105

LF/VLF 107-130

Loran-C 153-173

Omega. . 115 USNO 24,142,151,

170,171,187

SHIFT, DIURNAL, LF/VLF 108

SHORT-TERM STABILITY 44

SIDEREAL TIME 17

SIGNAL

AVERAGER 65 CHARACTERISTICS

HF 82

LF/VLF 1 08 Loran-C ............ 153 Television. ........... 133 WWVB 108

SKYWAVE

COVERAGE, LORAN-C

Accuracy . DEFINITION.

HOPS, HF .

SLAVE STATION, LORAN-C ^^^^HH ...... 153

SOLAR TIME 17

167

172

167

102

242
SOURCE, FREQUENCY

ATOMIC OSCILLATOR

Cesium Beam ........... 207

Hydrogen Maser ........... 209

Rubidium . 208

COMPARISONS OF 211 PERFORMANCE . . . . 193 PRIMARY AND SECONDARY STANDARDS 197

QUARTZ CRYSTAL OSCILLATOR 199

RESONATORS 196

STABILITY (See STABILITY)

SPECIFICATIONS, STABILITY 44

STABILITY

ATOMIC OSCILLATOR

Cesium Beam 207,211

Hydrogen Maser 209,211

Rubidium 208,211

DEFINITION 193 FRACTIONAL FREQUENCY 194

INFLUENCES ON . 193,194

QUARTZ CRYSTAL OSCILLATOR 197

SPECIFICATIONS

Long-Term ............. 44

Short-Term. ........... 44

TELEVISION COLOR SUBCARRIER 141

STANDARDS

ATOMIC 6 BASIC. . 5 CESIUM BEAM 207

DEFINITION 5,191

FREQUENCY 6 HYDROGEN MASER 209

NBS TIME AND FREQUENCY 6 PRIMARY (DEFINED) 197 RUBIDIUM 208

SECONDARY (DEFINED) 197

STATE SELECTION, ATOMIC 203

STATE STANDARDS LABS 7

SUBCARRIER, COLOR TELEVISION, 3.58 MHz 131

SUNDIAL

243

17
SWEEP FREQUENCY CALIBRATION 55

SYNTHESIZER, FREQUENCY . 60

SYNCHRONIZATION, SYSTEMS 21

TA (ATOMIC TIME) 18

TAI (INTERNATIONAL ATOMIC TIME) 18

TCXO (TEMPERATURE-CONTROLLED CRYSTAL OSCILLATOR) 201

TABLES

CHARACTERISTICS OF THE MAJOR TIME AND FREQUENCY DISSEMINATION SYSTEMS . 213

CHARACTERISTICS OF THE OMEGA NAVIGATION SYSTEM STATIONS. . . . 116 COMPARISON OF FREQUENCY SOURCES 211 CONVERSION TO HERTZ 10 HOW FREQUENCY IS CALIBRATED 214

HOW TIME IS CALIBRATED 214 IMAGE FREQUENCIES FOR WWV/WWVH 82

LORAN-C CHAINS 154

LORAN-C ECD TABLE: HALF CYCLE PEAK AMPLITUDE vs ECD . . 163

LORAN-C GROUP REPETITION INTERVALS 164 PARTS PER. . . TO PERCENT CONVERSION . 9 PREFIX CONVERSION 9 RADIO FREQUENCY BANDS 10 READINGS OBTAINED WITH VARIOUS SETTINGS OF TIME BASE AND PERIOD

MULTIPLIER CONTROLS ... 40

STANDARD FREQUENCY AND TIME SIGNAL BROADCASTS IN THE HIGH FREQUENCY BAND 77-80

STANDARD FREQUENCY AND TIME SIGNAL BROADCASTS IN THE LOW FREQUENCY BAND 125-126

TECHNIQUES, CALIBRATION (See CALIBRATION TECHNIQUES)

TELEPHONE

DISTRIBUTION OF TIME AND FREQUENCY 11 NBS TIME-OF-DAY SERVICE ... 12,105

TELEVISION

ACCURACY OF CALIBRATION

Color Subcarrier, Frequency ........ 131,138

Line-10

Frequency 145,149

Time 147,151

COLOR

Bar Comparator 136

Subcarrier, 3.58 MHz 132

244
CALIBRATION TECHNIQUES

Frequency

Color Subcarrier . Line- 10 Time, Line-10

DIGITAL FRAME SYNCHRONIZER . DIGITAL OFFSET COMPUTER

FREQUENCY OFFSET

PHASE INSTABILITY OF SIGNAL . SYSTEM 358 FREQUENCY MEASUREMENT COMPUTER

TRACEABILITY OF CALIBRATION

NBS USNO

USNO TV FREQUENCY CALIBRATION SERVICE .

TEMPERATURE

COMPENSATED CRYSTAL OSCILLATOR (TCXO)

EFFECTS ON CRYSTALS

EFFECTS ON TIME BASE OSCILLATORS .

TIME ACCURACY OF CALIBRATION

GOES Satellite Time Code . HF Radio Broadcasts . LF/VLF Radio Broadcasts .

Television

Loran-C ....... Line-10 . . . . . TRANSIT Satellite Navigation System.

ATOMIC

CALIBRATION (See CALIBRATION TECHNIQUES, TIME)

CODE (See TIME CODE)

OF-DAY

DEFINITION

DELAY (See DELAY)

EPHEMERIS

EQUATION OF FREQUENCY DIVISION, NBS INTERVAL

Measurement

Definition. ...... ...... SERVICE DIVISION, USNO

SERVICES (see also SERVICES) . . . . SIDEREAL

SOLAR

STANDARD

TELEPHONE TIME-OF-DAY SERVICE .

131-143

145-151

151 142,151

138

134

133

138

135,146

151 142

201

201 43

184

69,77-80,105

107,125-126,130

172 147,151

189

18

4,86

4

18

1723

440,56

245,23,24

1717

6105

245
TRANSFER

Using WWVB Transmitted Envelope 128

Using Television Line-10 145-152

UNIVERSAL . . . . . 19 ZONES 20

TIME BASE CALIBRATION 49

ERROR 34,43

FUNCTION IN COUNTERS 33

TIME CALIBRATION (See CALIBRATION TECHNIQUES, TIME)

TIME CODE ACCURACY 15 DEFINITION 13 GOES Satellite 177 IRIG 13 RECOVERING

WWV/WWVH 92 WWVB 127 UTILITY OF 15 WWV/WWVH 14,92 WWVB 14,127

Time Transfer Using Transmitted Envelope. . . . . . 128

TIME OF COINCIDENCE (TOC), LORAN-C 170

TIME COMPARISON FOR FREQUENCY CALIBRATION 100

TIME SCALE

ASTRONOMICAL 17 ATOMIC

TA, TAI 18 UTC 19 DEFINITION 17 HISTORY 17 NAVIGATOR'S 17 UNIFORMITY. 17 USES 21

TRACEABILITY

DEFINITION 8

246
ESTABLISHING

NBS Time and Frequency Bulletin

Loran-C .... Television Color Subcarrier

Television Line- 10 Omega. WWVB . USNO

Time Services Bulletin

LF/VLF Broadcasts

Loran-C

Television Line-10

TRANSIT Satellite

TRANSIT SATELLITE NAVIGATION SYSTEM.

TRIGGER LEVEL ERRORS.

UTC (COORDINATED UNIVERSAL TIME) TIME SCALE

RADIO BROADCASTS

UT1 CORRECTIONS

WWV/WWVH .... WWVB DETERMINATION ....

USNO (U.S. NAVAL OBSERVATORY)

MONITORING DATA (See BULLETIN, USNO)

LINE-10 SERVICE 151 PTTI COORDINATION GROUP . 24 ROLE IN INTERNATIONAL COORDINATION OF TIME AND FREQUENCY ... 24 Determination of UT1 24 TIME SERVICES DIVISION 24

TV FREQUENCY CALIBRATION SERVICE 142

UNIVERSAL

TIME, COORDINATED 19 TIME FAMILY 17

U.S. NAVAL OBSERVATORY (See USNO)

VFO (VARIABLE FREQUENCY OSCILLATOR) 37

VLF RADIO BROADCASTS, CALIBRATIONS USING 107-130

134,138

146 127

127

127 170,171

151 187

187

42,45

1977-80,125-126

19,94

127 22,24

247
WAVELENGTH. 10

WAVEMETER 56

WWV/WWVH

ANTENNAS 82

CALIBRATION TECHNIQUES

Frequency 96

Time 86

Time Code 92

CHARACTERISTICS, SIGNAL 82

FORMAT ... 69,215

Time Code 93 TIME-OF-DAY ANNOUNCEMENTS 86

TELEPHONE TIME SERVICE 105

PROPAGATION PATH DELAY 101

WWVB

ANTENNAS 107 CALIBRATION TECHNIQUES

Frequency . . . . . . . . . . . . 113

Time Code 127 CHARACTERISTICS 108 FIELD STRENGTH . . . Ill FORMAT 107 Time Code 127 PHASE SHIFT IDENTIFICATION 114

YEARLY UTC OFFSET (PRIOR TO 1972) 19

ZERO BEAT METHOD OF FREQUENCY COMPARISON 37,97

ZERO OFFSET OF UTC TIME SCALE 19

ZONES, TIME 20

248
NBS-114A (rev. 0-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. SPECIAL PUBLICATION 559

n. "

,-. 4. TITLE AND SUBTITLE 5. Publication Date November 1979

TIME & FREQUENCY USERS' MANUAL

7. AUTHOR(S)

George Kamas and Sandra L. Howe, eds.

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/ Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, city, state, ZIP)

Same as 9.

13. Type of Report & Period Covered

log *«ency Code

. I I. ... Illl II ^I I — 15. SUPPLEMENTARY NOTES

Library of Congress Number 79-600169

Supersedes TN 695

I | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual summary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here.)

This manual has been written for the person who needs information on making time and frequency measurements. It has been written at a level that will satisfy those with a casual interest as well as laboratory engineers and technicians who use time and frequency every

day. It gives a brief history of time and frequency, discusses the

roles of the National Bureau of Standards, the U. S. Naval Observatory,

and the International Time Bureau, and explains how time and frequency

are internationally coordinated. It also explains what time and

frequency services are available and how to use them. It discusses

the accuracies that can be achieved using the different services as well as the pros and cons of using various calibration methods.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by aemicolona) Frequency calibration; high frequency; Loran-C; low frequency; radio broadcasts;

satellite broadcasts; standard frequencies; television color subcarrier; time

and frequency calibration methods; time calibration; time signals.

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