NBS Measurement Services:

Traceable Frequency Calibrations

How to Use the NBS Frequency Measurement System

in the Calibration Lab

NBS

Special

Publication

250-29

George Kamas and Michael A. Lombardi

U.S. Department of Commerce

National Bureau of Standards
Center for Basic Standards

The Center for Basic Standards develops and maintains the scientific competences and laboratory facilities nec- essary to presen/e and continue to refine the base physical quantities upon which the Nation's measurement

system is constructed. The Center is responsible for the standards of voltage, current, resistance and

impedance, temperature, vacuum, leak rate, pressure, mass, length, time and frequency. The Center collaborates closely with the standardizing laboratories of other countries to ensure that measures used in international

trade are fully compatible. The Center also performs research at the frontiers of physics in order to improve

measurement capability and quantitative understanding of basic physical processes that underlie measurement

science.

Electricity Division

Maintains and improves the national standards of electrical measurement; conducts experiments to realize the

electrical units in terms of the mechanical units; develops stable electrical standards and improved methods for the dissemination of the units of electrical measurement; determines fundamental physical constants related to

electrical units; develops instrumentation for precise electrical measurements directly referenced to the national

standards; implements techniques for their use and puts them into practical applications; assists in applying scientific and technological development to measurement problems, especially in other areas of NBS research.

Temperature and Pressure Division

Maintains and improves the national standards of temperature and pressure; conducts research towards the development of new concepts for these standards; calibrates practical standards for the U.S. scientific and technical community in terms of the primary standards; develops methods and devices to assist user groups in the

assessment and enhancement of the accuracy of their measurements of temperature and pressure; prepares

and promulgates evaluations and descriptions of temperature and pressure measurement processes; coordi- nates these standards and measurement methods nationally and internationally; and determines the accuracy of the national standards of temperature and pressure with respect to fundamental thermodynamic relations. Length and IVIass Division

Develops and uses the competence necessary to implement a more accurate measurement system for length

and mass; applies research, utilizing the most advanced technology available, to the improvement of our present

standards and devises new standards and new methods for their transfer. Time and Frequency Division

Maintains, develops and improves the national standards for the unit of time, the unit of frequency, and time

scales based on these standards; carries out research in areas of importance to the further fundamental improvement of frequency standards and their applications focusing on microwave and laser devices, atomic and

molecular resonances, and the measurement of fundamental physical phenomena and constants; adapts time

and frequency standard devices and concepts to special scientific and technological demands; develops time

and frequency measurement methods in the radiofrequency, microwave, infrared, and visible radiation regions;

coordinates the NBS time scales with international time scales and the time scales maintained by the United

States Naval Observatory; collaborates with national and international organizations in the development of improved methods for the measurement of time related quantities; operates time and frequency dissemination ser- vices, such as radio stations and broadcasts, for the purpose of traceability to the national standards of time and

frequency.

Quantum Physics Division

Engages in research in atomic and molecular physics at the forefront of the field, and performs basic, highly accurate measurements and theoretical analyses that help establish a reliable foundation for scientific and technological measurement and data.

Quantum Metrology Division

Engages in forefront research in quantum metrology; contributes to new determinations of the fundamental

physical constants, and the extension and refinement of the electromagnetic scale and devises, where possible,

tests of basic symmetries and invariances.
NBS MEASUREMENT SERVICES:

Traceable Frequency Calibrations

How to Use the NBS Frequency Measurement

System in the Calibration Lab

George Kamas and Michael A. Lombardi

Time and Frequency Division

Center for Basic Standards

National Measurement Laboratory

National Bureau of Standards

Boulder, CO 80303

January 1988

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
Library of Congress Catalog Card Number: 87-619905

National Bureau of Standards Special Publication 250-29

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-29, 42 pages (Jan. 1988)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

For sale by the Superintendent of Docxjments, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

Calibrations and related measurement services of the National

Bureau of Standards provide the means for makers and users of

measuring tools to achieve levels of measurement accuracy that

are necessary to attain quality, productivity and competitiveness. These requirements include the highest levels of accuracy

that are possible on the basis of the most modern advances in

science and technology as well as the levels of accuracy that are

necessary in the routine production of goods and services. More

than 300 different calibrations, measurement assurance services

and special tests are available from NBS to support the

activities of public and private organizations. These services

enable users to link their measurements to the reference

standards maintained by NBS and, thereby, to the measurement

systems of other countries tliroughout the world. NBS Special

Publication 250, NBS Calibration Services Users Guide, describes

the calibrations and related services that are offered, provides

essential information for placing orders for these services and

identifies expert persons to be contacted for technical

assistance

NBS Special Publication 250 has recently been expanded by the

addition of supplementary publications that provide detailed

technical descriptions of specific NBS calibration services and,

together with the NBS Calibration Services Users Guide, they

constitute a topical series. Each technical supplement on a particular calibration service includes:

o specifications for the service

o design philosophy and theory

o description of the NBS measurement system

o NBS operational procedures

o measurement uncertainty assessment

error budget

systematic error

random errors

o NBS internal quality control procedures
The new publications will present more technical detail than the

information that can be included in NBS Reports of Calibration.

In general they will also provide more detail than past

publications in the scientific and technical literature; such

publications, when they exist, tend to focus upon a particular

element of the topic and related elements may have been published

in different places at different times. The new series will

integrate the description of NBS calibration technologies in a

form that is more readily accessible and more useful to the

technical user.

The present publication, SP 250-29, NBS Measurement Services:

Traceable Frequency Calibrations--How to Use the NBS Frequency

Measurement System in the Calibration Lab, by Kamas and Lombard!,

is one of approximately 20 documents in the new series published

or in preparation by the Center for Basic Standards. It

describes calibration technology and procedures utilized in

connection with NBS Service Identification Numbers from 53110 to

53150 listed in the NBS Calibration Services Users Guide.

Inquiries concerning the contents of these documents may be

directed to the author or to one of the technical contact persons

identified in the Users Guide (SP-250).

Suggestions for improving the effectiveness and usefulness of the

new series would be very much appreciated at NBS. Likewise,

suggestions concerning the need for new calibration services,

special tests and measurement assurance programs are always

welcome

Joe Simmons, Acting Chief

Office of Physical Measurement Services

Helmut Hellwig, Acting Director

Center for Basic Standards

iv
CONTENTS

Page

Abstract 1

1. INTRODUCTION 2

1.1 What the New Service Does for the Calibration

Laboratory 3

1.2 How the Frequency Measurement System Works 6

1.3 The Signals Connected to the Measurement System... 7

2. THE THEORY OF FREQUENCY MEASUREMENTS 9

2.1 Why Frequency Calibrations Are Unique 10

2.2 Traceability for Frequency Calibrations 11

2.3 Frequency Calibration Options Available to the

Laboratory 13

2.4 Oscillator Specifications 13

2.5 What Does a Frequency Calibration Measure? 15

2.6 The Characteristics of Different Kinds of

Oscillators 17

2.7 Setting Up the Frequency Lab 19

2.8 Record Keeping for Frequency Calibrations 21

3. THE NBS FREQUENCY MEASUREMENT SYSTEM 22

3 . 1 The Daily Operation of the NBS Frequency

Measurement System 2 3

4. CONTROL OF THE NBS FREQUENCY MEASUREMENT SERVICE 27

4.1 Hardware Support 27

4.2 Training 27

4.3 Data Verification 28

4.4 Data Monitoring 29

4.5 Reporting Traceability 3 2

5. ASSESSMENT OF MEASUREMENT ERRORS USING THE NBS

FREQUENCY MEASUREMENT SYSTEM 34

6. BIBLIOGRAPHY 3 6

V

TRACEABLE FREQUENCY CALIBRATIONS

HOW TO USE THE NBS FREQUENCY MEASUREMENT SYSTEM

IN THE CALIBRATION LAB

George Kamas and Michael A. Lombard

Time and Frequency Division

National Bureau of Standards

Boulder, CO 803 03

A practical approach to making traceable frequency calibrations

in the calibration laboratory is discussed. This approach

emphasizes obtaining traceable data, keeping appropriate records,

and selecting an oscillator and a radio signal to use for

calibrations. The NBS Frequency Measurement System is used to

illustrate this approach, and to discuss the decisions to be made

when setting up a calibration lab for frequency measurements.

The theory of frequency measurement is also discussed, with

emphasis on using time interval counters to obtain the relative

frequency of an oscillator.

Key words: calibration; cesium oscillators; frequency

measurement; Loran-C; quartz oscillators; relative frequency;

rubidium oscillators; time interval counters; traceability ; WWV;

WWVB

1
OVERVIEW OF THIS PAPER:

In order that calibration laboratory personnel can get the

gist of this paper quickly, the next few pages describe the

NBS Frequency Measurement System rather briefly. Later in

the paper, the system is discussed in more detail.

The reader is encouraged to scan these pages as an introduction to the later material. Or, if this is a second or

third reading, these next few pages can serve as a quick

review of the main features of the measurement system.

This paper uses scientific notation when discussing the

accuracy of a frequency source. For example, if the

relative frequency of a source is 1 part in 10 to the 11th,

it will be written as l.OOE-11.

1 . INTRODUCTION

The National Bureau of Standards (NBS) began a new Frequency

Measurement Service in 1984. The service made a state-of-the-art

frequency measurement system available to the public. The system

was developed by NBS using low cost, commercially available

equipment

The service provides NBS traceable frequency calibrations

over a range from l.OOE-05 to l.OOE-12. The method used by the

service involves common-view monitoring of low-frequency radio

signals by the user and by NBS. NBS then provides information to

correct the user's frequency data.

Subscribers to the NBS Frequency Measurement Service receive

several benefits. NBS offers hands-on training and customer

support. Measurements are made continuously (24 hours a day)

and the equipment requires minimal operator attention. Another

benefit to the frequency measurement community is the

standardization of the process. Each user makes measurements

using the same method, and NBS can share the benefits of many

measurements (taken from every user site) with each subscriber.

2
1.1 What the New Service Does for the Calibration Laboratory

Every calibration laboratory needs a good oscillator to use

as a frequency source. The oscillator must be accurate enough to

meet the needs of the lab's customers and be periodically

calibrated so that its accuracy is known.

Oscillators are sensitive to shipment, and to being turned

on and off, so it is impractical to remove them from service and

send them out for calibration. Instead, labs need to backup

their oscillators with batteries, keep them running all the time,

and continuously measure their performance. The more accuracy a

lab requires, the more important this becomes. If the oscillator

is a quartz or rubidium device, the lab needs to keep it adjusted

so that it meets specifications. If the oscillator is a cesium

device, it must be measured to ensure that its performance meets

specifications.

To make frequency measurements in the laboratory, lab

personnel need a measurement system. Subscribers to the new NBS

service receive a complete frequency measurement system which

they install using a detailed instruction manual. The subscriber

needs to supply only two things: an oscillator and a telephone

jack. The oscillator is the lab's primary frequency source. The

phone jack lets the system be linked to NBS by modem. All other

required items are supplied by NBS.

The assembled measurement system is about the size of a

component stereo system and fits easily on a desktop or small

table. It consists of a microcomputer with two disk drives and

monitor, a frequency divider, a time interval counter, a Loran-C

receiver, a printer, a telephone modem, and a time-of-day clock

(see figure 1 on page 4)

The microcomputer controls the measurement system.

Measurements are displayed on the monitor as they are being

taken. The measurements are averaged, and the averaged data are

stored on a floppy disk.

The frequency divider accepts input signals of 1, 5, or 10

MHz and produces lower frequency output signals. For example,

it can divide a 10 MHz signal into a 1 Hz signal. In this way,

signals of the same low frequency are obtained for use with the

time interval counter.

The time interval counter has four channels (four start and

four stop connectors) . Therefore, it can measure the frequency

performance of up to four separate oscillators. The counter

makes measurements with 10 nanosecond resolution (see figure 2,

page 5)

3

Loran-C

Recei VQP

STOP

T ime

Primary

Osci 1 lator

TIME BASE

Frgquency

Di vidor

IntGPval

CountGr

START

Clock

Phone Jack

Disks ComputGP

Printer

FIGURE 2 - DIAGRAM OF THE NBS FREQUENCY MEASUREMENT SYSTEM

5
The Loran-C receiver is fully automatic. When the user

connects the antenna and power supply, the receiver begins

tracking the signal from the nearest Loran-C transmitter. The

antenna is an eight-foot whip, similar to a Citizen's Band (CB)

radio antenna.

The printer produces daily calibration plots and is also

used to send messages to users of the service.

The telephone modem is inside the microcomputer. It gives

NBS access to the user's measurement system.

The time-of-day clock is also inside the microcomputer. It

provides the timekeeping functions needed for the measurements.

1 . 2 How the Frequency Measurement System Works

The NBS Frequency Measurement System measures and records

the performance of from one to four frequency sources. It runs

24 hours a day and restarts automatically after power outages.

The Frequency Measurement System is simple to start and

runs itself once started. It is controlled entirely with

software. The software automates the process of making

frequency measurements and seldom requires operator attention.

The software is "transparent," and users do not necessarily

realize that they are using computer software, but instead feel

that they are using a completely integrated system. From time to

time, NBS refines the software to make it even more reliable,

transparent, and integral to the system.

The system takes and averages time interval readings and

stores (on floppy disk) the data and the time the data were

taken. The time is obtained from the clock inside the

microcomputer. Once each day, the system plots the data recorded

in the previous 24 hours and prints a copy of the plot on the

printer (figure 3, next page). These functions are performed

automatically, without user interaction.

A calibration plot is made for each oscillator being

measured. It shows the phase shift of the oscillator versus time

of day and the estimated relative frequency of the oscillator.

The relative frequency is printed above the plot. Relative

frequency is calculated by fitting a linear least-squares curve

to the data and taking the slope of the fitted line. The system

can show relative frequency offsets as small as l.OOE-12, or as

large as l.OOE-05. This allows the system to measure the full

range of oscillators—from low-quality quartz oscillators to

high-quality atomic oscillators.

6
NATIONAL BUREAU OF

RELATIVE FREQUENCY

STANDARDS FREQUENCY CALIBRATION PLOT

= -3.6aE-10 1000/28

QUARTZ US RUBIDIUM

FIGURE 3 - A SAMPLE CALIBRATION PLOT

After the system plots, it starts taking readings for the

next day. The process continues in this way: recording,

plotting, recording, plotting, and so on. The only operator

attention required is to occasionally change the data disk or to

add paper to the printer.

1.3 The Signals Connected to the Measurement System

The frequency measurement service makes time interval

measurements between two signals of the same frequency. One of

these signals provides a reference frequency, and the other is

the oscillator being measured. The time interval counter

supplied with the frequency measurement service can measure the

performance of four oscillators simultaneously. Channel 1 is

dedicated to the calibration of the primary oscillator in the

calibration laboratory. The performance of the primary

oscillator is checked regularly by NBS for traceability

Loran-C provides the reference frequency for the service.

Each system comes with a radio that receives a signal from a Loran-C station. Loran-C is a radio navigation system maintained

by the United States Coast Guard (Loran stands for LOng RAnge

Navigation)

7
Loran-C signals make an excellent reference frequency for a

number of reasons. They are extremely accurate—to about l.OOE12 per day. This is good enough to check the performance of

cesium oscillators and to calibrate rubidium or quartz

oscillators. The continued accuracy of Loran-C is insured since

the stations are controlled by cesium standards monitored by the

U.S. Coast Guard and the U.S. Naval Observatory, in addition to

NBS.

A second benefit is that Loran-C receivers are inexpensive

and automatic. Finally, each Loran-C station has an effective

range of over 1000 miles (1600 kilometers) . Since numerous

Loran-C transmitters are in operation, this means that the

service can be used in all 50 states.

Loran-C is an NBS traceable frequency source, because its

signals are received by both the user and NBS. NBS continuously

monitors the Loran-C stations and compares the transmitted phase

to the nation's frequency standard. Through this monitoring, NBS

is able to report any phase shifts in the Loran-C signals and to

measure the accuracy of the signals. Any abnormal phase shifts

are reported to subscribers of the service. NBS also publishes

the daily phase shifts of Loran-C, but these are generally too

small to concern most users.

Channel 1 of the Measurement System is dedicated to the

calibration of the primary oscillator in the laboratory. The

primary oscillator is compared to the Loran-C reference

frequency. Channels 2, 3, and 4 can be used to compare other

oscillators to the primary oscillator (see figure 4, below).

CHANNEL

1

Loran-C (Reference Frequency)

User's Primary Oscillator

CHANNELS

2-4

User's Primary Oscillator

(Reference Frequency)

Other Oscillators in the

Calibration Lab

FIGURE 4 - CALIBRATION PROCEDURE USING THE NBS SYSTEM

8
2. THE THEORY OF FREQUENCY MEASUREMENTS

Frequency measurements are made by comparing signals from

two frequency sources. This is true in music, for example, where

the orchestra tunes its instruments from the oboe. In the same

way, oscillators are measured, adjusted, and calibrated by

comparing their output with the output from a higher quality

frequency source. The higher quality frequency source can be a radio signal (WWV, WWVB, or Loran-C, for example) or another

oscillator.

A simple and direct way to measure frequency is to use a frequency counter. The problem with this method is the time it

takes to make measurements. The resolution of an ordinary

counter is increased by increasing the gate time. For example, a

1 second count of a 1 MHz signal might give a resolution of 1 Hz.

By counting for 10 seconds, the resolution improves to 0.1 Hz.

This resolution was obtained by waiting longer. It would take

many hours to reach the resolution of a precision oscillator

(l.OOE-11, for example). Even if the time were available, this

method is not recommended because it allows only a few readings

per day, and the chances for error become great. A system that

must run continuously for hours to get one reading is very prone

to errors due to power outages and other causes.

A better approach is to measure time interval instead of

frequency. This method is faster. If two oscillators with 1 Hz

outputs are compared using a time interval counter, a high

resolution reading can be obtained every second. For example,

the counter used by the NBS Frequency Measurement System has a 100 MHz time base. This means the measurement resolution

obtained is 1 / 100,000,000 of a second, or 10 nanoseconds.

Therefore, changing from frequency to time interval measurements

results in a dramatic increase in resolution.

Time interval measurements are in time units. The data

taken measure the length of the period (in time units) between

the signals being calibrated. For example, two signals are used

to start and stop a time interval counter. The counter measures

the period between the two signals. If their frequency is the

same, the period will not change. If the frequencies differ,

this period will change, although usually very slowly. It is

exactly as if each signal source were a clock, and the data

recorded tell whether one clock gained or lost time relative to

the other clock.

The technique of measuring time interval to get frequency is

common and is used by all high resolution systems. However, it

requires taking two readings to get one data point. A single

reading of the time interval between two clocks is not useful.

9
It is the second reading, subtracted from the first, that tells

whether time is gained or lost. This gain or loss of time is a

measure of the frequency difference between the two frequency

sources.

2.1 Why Frequency Calibrations Are Unique

Laboratory frequency calibrations are unique for two reasons:

1. Oscillators cannot be shipped to NBS or another

laboratory for calibration, because being turned on

and off causes their frequency to change.

2. Since oscillators cannot be sent out for

calibration, calibrations must be done on-site

using radio signals. These radio signals permit

even small laboratories to perform calibrations at

an accuracy of l.OOE-12.

Making calibrations on-site at high levels of accuracy

requires following some carefully set guidelines. These

guidelines are outlined in the following paragraphs.

Calibration labs must have a high-quality primary

oscillator; either a quartz, rubidium, or cesium. The primary

oscillator must meet the requirements for calibrating equipment

that is now in the lab or expected soon. Quartz oscillators

(based on prices at this writing) cost from $500 to $10,000.

Rubidium oscillators cost from $5000 to $20,000, and cesium

oscillators cost from $30,000 to $50,000.

The primary oscillator must have battery backup to prevent

signal loss during power outages. It must run continuously and

be calibrated daily using radio signals. Also, depending on

location of the calibration laboratory, a distribution amplifier

may be needed to prevent errors in the calibrations due to signal

loss, line loading, or other causes.

Once the primary oscillator is selected, the calibrations

must be made traceable. Most calibration labs provide equipment

so that the primary oscillator can serve as the traceable

laboratory source to calibrate other oscillators. The setup for

making calibrations can be as simple as an oscilloscope, or as

elaborate as a computer-controlled measurement system.

10
2.2 Traceability for Frequency Calibrations

Due to the conditions mentioned above (the need to use

radio signals and the need to perform frequency calibrations onsite) , traceability for frequency calibrations differs in some

respects from other calibration activities.

The radio signal used must either be directly controlled by

NBS, like WWV, WWVB, WWVH, GOES, and GPS, or monitored by NBS.

Signals directly monitored by NBS include the Loran-C navigation

signals. Signals indirectly monitored include the Navy VLF

signals, the Omega navigation signals, and Canadian and European

broadcast signals. All of these signals are useful because they

are referenced to atomic oscillators. Since they are carefully

controlled and monitored by other government agencies, and

intercompared between national laboratories, traceability to NBS

is assured. For example, users of the signals from radio station

CHU in Canada can be assured that these signals can be traced to

NBS, since the relationship between the Canadian and U.S.

national standards is known. The accuracy level of the

traceability is limited, however, by the number of steps in the

traceability path. Users should investigate the limitations so

that no confusion exists over what can or cannot be accomplished

with a given signal.

Without trying to discuss all possible arrangements,

traceability for frequency calibrations works like this: users

of WWVB can obtain a monthly frequency bulletin from NBS that

lists (after the fact) any known perturbations of the broadcast

frequencies (figure 5, next page) . Signal outages and special

announcements are also listed. The U.S. Naval Observatory also

mails or provides computer access to frequency data needed by

defense contractors. Many other countries provide bulletins that

announce corrections in their broadcasts. By using documents

from NBS or elsewhere, the calibration lab can obtain enough

data to establish a traceable path to NBS.

Obtaining traceability is not as difficult as it appears,

since most users calibrate their oscillators at a level higher

than needed. For example, if an oscillator maintains a relative

frequency of l.OOE-08 but calibrations are only required at a

level of l.OOE-06, small losses of accuracy in the traceability

path will not affect the final results.

11
AUGUST

1987 MJD* WWVB LORAN-C (DANA) LORAN-C (FALLON)

1 47008 5. 68 62284 . 27 3942.22

2 47009 5. 69 62284 .33 3942. 17

3 47010 5. 69 62284 . 32 3942.20

4 47011 5. 66 62284 .31 3942 . 13

5 47012 5. 67 (-) (-)

6 47013 5. 69 62284.27 3942.60

7 47014 5. 69 62284.28 3942.48

8 47015 5. 68 62284.25 3942.53

9 47016 5. 67 62284 .30 3942.43

10 47017 5. 66 62284 .31 3942.52

11 47018 5. 68 62284.33 3942.46

12 47019 5. 69 62284 .36 3942.44

13 47020 5. 68 62284 .41 3942.52

14 47021 5. 69 62284.40 3942 .48

15 47022 5. 69 62284.45 3942 . 39

16 47023 5. 69 62284.43 3942 . 38

17 47024 5. 69 62284 . 38 3942.45

18 47025 5.72 62284.45 3942 . 38

19 47026 5. 68 62284.48 3942.41

20 47027 5. 67 62284.45 3942.36

21 47028 5. 66 62284.45 3942 . 37

22 47029 5 . 65 62284.30 3942 . 48

23 47030 5.65 62284 .20 3942 . 58

24 47031 5. 64 62284 .30 3942.53

25 47032 5.66 62284 .41 3942.39

26 47033 5. 64 62284 . 36 3942.47

27 47034 5. 67 62284 . 27 3942.47

28 47035 5.70 62284.39 3942 . 39

29 47036 5.72 62284 . 36 3942 . 38

30 47037 5.75 62284 . 39 3942 . 37

31 47038 5.77 62284.28 3942.44

* Modified Julian Date

FIGURE 5 - DATA FROM NBS TIME AND FREQUENCY BULLETIN

12
2.3 Frequency Calibration Options Available to the Laboratory

Users must decide which traceability path (including the

radio signal and oscillator) meets their needs. Help is

available from most manufacturers and also from the national

laboratories that control the radio signals. NBS assists users

in this area with publications, phone consultation, and training

seminars.

The decision must be based on the level of calibration

accuracy required by the laboratory. If the required accuracy is

very high, or if it can be expected to increase in coming years,

then consider only those radio signals which support that level

of accuracy. For example, WWV can be used to calibrate an

oscillator of modest performance, but a high-performance

oscillator may require using signals from WWVB or Loran-C.

The choice of radio signal is also limited by the location

of the laboratory, reception problems with certain radio signals,

available equipment, and past experience. Another factor is

whether the radio signal lends itself well to automation. It may

be better to use a signal that is easy to automate, even if the

initial cost of the equipment is high. If low-cost equipment is

purchased, it may require a daily manual calibration procedure.

In these instances, the labor costs will soon exceed the amount

of money that was originally saved.

It is a good idea to talk with manufacturers and try the

equipment before buying. There is also the option of talking

with the NBS staff. They will often know from experience which

services have worked best in a particular area.

2.4 Oscillator Specifications

It is important to know the specifications of the oscillator

to be calibrated. It is especially important to know the

specifications of the primary oscillator, since it is used to

calibrate other oscillators that are brought into the

laboratory.

The previous sections have discussed the technique to use

when making frequency calibrations. To review, the lab should

have a primary oscillator which is the best oscillator available

to the laboratory. It should be battery-backed, kept running at

all times, and be traceable to NBS. The primary oscillator can

then be used to calibrate other oscillators. If carefully

controlled and tended, the primary oscillator will provide

excellent service and can be used even if the radio signal is

temporarily lost.

13
with these criteria as a starting point, here is a review

of some typical oscillator specifications:

ACCURACY : Accuracy is the result obtained from a

calibration; it cannot be purchased. The

manufacturer specifies the frequency of the

oscillator after turn-on. The specification states

what the accuracy of the oscillator will be after a

specified warm-up period. This means that the

factory has calibrated the oscillator, and the

specification tells you what the unit will do after

it has been turned off, shipped, and then turned

back on.

STABILITY : At any given moment, an oscillator

generates an output signal whose frequency depends

on a number of factors: temperature, time since

turn-on, line voltage, vibration and shock, and so

on. Some time later its frequency will change,

again due to a number of causes. The difference

between its frequency at one moment in time and

another moment is called stability. In other

words, how stable is its frequency? This

specification is usually given for a number of time

periods: 1 second, 10 seconds, minutes, hours, or

even days, months, or years. Stability is an

important factor in oscillator cost. More stable

oscillators cost more. Among the variety of quartz

crystal oscillators available, the cost may range

from a few hundred dollars for an oscillator with a

stability of l.OOE-06 per day to over $5000 for a

stability of l.OOE-11 per day.

AGING : This specification is used for quartz

oscillators. It tells the buyer what kind of

change in frequency can be expected due to the

passage of time and the resulting change of the

quartz crystal inside the oscillator. Again, highquality devices (with a slower aging rate) cost

more. Rubidium oscillators also change their

frequency over time, but for different reasons.

Usually this change is small—l.OOE-11 per day is

typical

Another factor in the purchase of a primary oscillator is

its adjustment range and resolution. Contact the vendor and ask

for a demonstration. It does little good to pay a high price

for an oscillator that cannot be adjusted with reasonable

14
effort. A simple screwdriver adjustment is much harder to set

than a multi-turn dial. Keep in mind the need to make very small

changes when adjusting an oscillator!

To summarize, each user wants as good an oscillator as the

budget allows. The most important specification is stability,

and it may be that only atomic oscillators can satisfy some

applications. However, read the other sections of this paper

before making a final decision. It is sometimes better to have

two modest oscillators rather than one very expensive one,

especially when dependability is important and repair costs are

high.

2.5 What Does a Frequency Calibration Measure?

Frequency calibrations measure the relative frequency of the

oscillator. The time interval over which measurements are

performed is important. For the topic covered by this paper

(laboratory calibrations of a primary oscillator) , a time period

of 1 day (24 hours) has been chosen. Shorter time intervals are

a special case and are not addressed here.

From this starting point, two things need to be considered:

the mathematics of calibrations, and the names and symbols to be

used. In each case, this paper follows international practices

of the time and frequency industry and the major calibration

laboratories

A measurement becomes a calibration if it compares the

oscillator's long-term frequency to the national frequency

standard, UTC(NBS) . UTC(NBS) stands for Coordinated Universal

Time at NBS. Traceability to NBS means that an oscillator was

compared to UTC(NBS) over a traceable path (using WWVB, Loran-C,

etc . )

The desired result of a calibration is to obtain the

relative frequency of the oscillator. The international symbol

for relative frequency is F. Relative frequency is defined by

the following equation:

Relative frequency = F = [f (actual) - f (nameplate)

f (nameplate)

15
The nameplate frequency (usually 1, 5 or 10 MHz) is what

the frequency of the oscillator should be. If an oscillator

operated at its nameplate value, it would be a perfect frequency

source. In the real world, however, there is always some

frequency error, or a difference between the actual frequency and

the nameplate frequency. This frequency error is what a

calibration tries to measure. In much of the literature, this is

referred to as delta F (delta means a small difference)

The equation finds the size of the frequency error.

Dividinq delta F by the nameplate value normalizes the equation.

This lets the operator ignore the actual oscillator frequency and

concentrate on the frequency error.

For example, if a 1 000 000 Hz (1 MHz) oscillator has an

actual output frequency of 1 000 001 Hz, it is in error by 1 Hz.

The 1 Hz error represents one cycle out of a possible million, or

l.OOE-06. In the equation, the numerator is 1 and the

denominator is 1 000 000. This means that F = l.OOE-06. F is

the relative frequency. In this example, then, the frequency

error relative to the nameplate value is l.OOE-06. In other

words, the oscillator frequency is in error by one part in a

million.

The sign of F changes from + to - if the oscillator output

is lower than the nameplate. This again follows international

convention and is the practice followed by many manufacturers.

F is a number and has no dimensions like hertz or percent. Being

a numeric, it does not depend on the oscillator frequency. For

example, if a 10 MHz oscillator had an actual frequency output of

10,000,010 Hz, it would also have an F of l.OOE-06. Now, if this

10 MHz oscillator was compared with the 1 MHz oscillator

mentioned earlier, the relative frequency between them would be

zero (as expected) ! This notation is used so that oscillators

can be compared, while ignoring their nameplate frequency.

Relative frequency, or F, is a way to talk about an

oscillator's performance in a way that is easy to understand; the

oscillator is either higher or lower in frequency than it is

supposed to be. Measuring relative frequency requires dealing

with very small quantities. This is because, when calibrating

high-quality oscillators, the difference between the actual

frequency and the nameplate frequency is very small.

To summarize, the result of a frequency calibration is to

obtain the value of F for a primary oscillator for a time period

of 1 day (24 hours) . The primary oscillator then becomes a

traceable frequency source.

16
2.6 The Characteristics of Different Kinds of Oscillators

Quartz, rubidium, and cesium oscillators are the three main

types of oscillators used in calibration laboratories. They are

discussed in this section.

Quartz Oscillators - Quartz oscillators vary in form, price,

and stability. For example, every frequency counter has an

internal quartz oscillator that it uses as a time base. The

counter's measurements are made relative to its time base, so the

time base should be periodically calibrated. More expensive time

bases require calibration less often and usually give better

measurements between calibrations.

The quartz oscillators used in frequency counters suffer

from being turned off and on between use. Some counters provide

a standby feature that allows the oscillator to stay on and

warmed up even though the other circuits in the counter are off.

Few counters, however, allow for battery backup of their time

base oscillators, and battery backup is required if the time

base is used as a primary oscillator. Therefore, if a quartz is

chosen as the primary oscillator, it will probably be a freestanding unit. These units range in price from a few hundred to

many thousands of dollars.

Both time-base and free-standing oscillators have adjustment

knobs or screws that are turned to put the oscillator on the

correct frequency. Quartz oscillators change their frequency due

to the aging of the quartz crystal itself. Since aging is a

continuous process, adjustments must be made on a regular

schedule. Other factors (like room temperature and voltage)

also affect the data. Needless to say, the laboratory manager

needs to study the situation and try to prevent frequency changes

due to outside causes.

After a quartz oscillator has been calibrated for a period

of time, an adjustment schedule should be planned. Some

operators like to make adjustments on Friday so the oscillator

can settle down over the weekend. Since it is recommended that

data be taken every day, by the following Monday several days of

data can be studied to see how well the adjustment was made. In

this way, the lab can keep a lower quality oscillator adjusted

and not have to buy a more expensive unit. It is good practice

to have two oscillators running all the time. The second

oscillator serves as a backup, adds confidence to the calibration

process, and makes it easier to troubleshoot the calibration

system when errors occur.

17
Rubidium Oscillators - Rubidium oscillators overcome some

disadvantages of quartz oscillators. A rubidium oscillator

contains a quartz oscillator whose frequency is forced to follow

certain characteristics of a rubidium gas cell. The result is a

very stable frequency source that normally changes its frequency

much more slowly than a quartz oscillator without rubidium

control. Since rubidium oscillators are more stable, they give

better results with fewer adjustments than quartz oscillators.

The tradeoff is in the price. Rubidium oscillators cost more

initially. However, the price of a rubidium oscillator should be

compared to the price of the quartz oscillator, plus the labor

costs involved in keeping the quartz oscillator adjusted.

Cesium Oscillators - By definition, cesium oscillators are a

primary frequency source. This means that, when operated

correctly and not broken, a cesium oscillator will be very close

to its correct frequency without any calibration. However, it is

extremely important to know that a cesium oscillator is operating

correctly, especially at the limit of its performance capability.

Since the expected level of performance is so high, checking the

performance is a very important part of operating a cesium

oscillator.

Review of oscillator characteristics:

QUARTZ Performance is in the range of l.OOE-06 to

l.OOE-11 per day. Requires more adjustment

than rubidium or cesium, but its low cost

allows the option of having more than one.

RUBIDIUM Cost more than quartz oscillators, but need

less adjustment and usually perform better.

Frequency change should be around l.OOE-11

per day.

CESIUM The primary frequency standard, a cesium

can be operated without calibration if

correct procedures are followed and if the

device is not broken. However, a cesium

must be calibrated if documented traceability is required. Cesiums are the most

expensive type of oscillator to purchase

and repair. The beam tube inside the

oscillator must be replaced every 3 to 5

years, at a cost of $10 000 or more.

18
Should oscillators be adjusted, or should they be allowed to

run and their frequency error measured? In other words, if an

oscillator's frequency error is known, should it be adjusted at

all, or should an allowance for the error be made when it is used

to calibrate other oscillators? The answer to this question

depends on the needs of the laboratory. If the oscillator is

used to feed an in-house distribution system, it might be better

to adjust it. On the other hand, if its output is used only in

the calibration lab and the error can be allowed for in future

calibrations, adjustments can be made less often. Remember, each

adjustment carries the risk of maladjustment. Also, some

oscillators are difficult to adjust. Sometimes it becomes a

trial and error process that may take several days to complete.

A good practice is to start calibrations without adjustment

for a period of several weeks. Then, try an adjustment to see

how well it goes. If the results are satisfactory, set up a

tentative schedule; once a week, for example. Try that for a

while and change the interval if necessary. If adjustments prove

difficult, space them out further. In the worst case, if the

adjustments fail to meet the needs of the laboratory, consider

using a different oscillator.

2 . 7 Setting Up the Frequency Lab

Once the radio receiver and oscillator are selected, set up

the laboratory for calibrations. Remember, the primary

oscillator needs battery backup and should operate in a stable

environment. The radio needs access to the roof for its antenna

and may also need access to a ground connection. Several antenna

types are currently offered. VLF and Loran-C receivers generally

operate with short vertical whips, but HE services like WWV need

long wires for good, noise-free reception. Mount the antenna in

an area free from obstructions and interference.

Arrange the equipment in the lab so that signals are

available for checking other oscillators. Distribution

amplifiers can be used to isolate the primary oscillator from

possible disturbing influences, like long cable runs and power

transients. Remember, a system that makes measurements with high

resolution needs to be carefully planned.

There are several types of frequency measurement systems

which the calibration lab manager can use, instead of designing a new system. Manufacturers offer VLF, LF, and Loran-C receivers

that make measurements and record the data on chart paper. This

equipment usually uses the primary oscillator as a receiver

input. The receiver then makes the comparison and draws the

chart. Front panel indicators assist in operating the system.

19
Receiver options include different types of antennas, some

distribution capability, and even a way to discipline the primary

oscillator to make it agree in frequency with the radio signal.

The NBS Frequency Measurement System mentioned throughout

this paper is another option. It has most of the components

necessary to perform frequency calibrations in the lab. It

includes a Loran-C receiver and a multiple input counter for

doing four measurements at one time. Channel 1 is used for the

primary oscillator (measured versus Loran-C) , and three

additional channels are provided to accommodate other oscillators

under test. For example, a lab using quartz oscillators could

use the first two channels for their primary quartz oscillator

and the backup. Then two additional channels are available for

other oscillators. The system includes a printer that makes a

daily calibration plot of all four channels, and a computer and

disk drives to store the data.

Frequency measurement systems should run 24 hours a day if

possible. It is not a good idea to start and stop the oscillator

due to its tendency to change frequency. This is mainly a

problem with quartz oscillators; rubidium and cesium oscillators

have a minimal restart effect. However, all oscillators benefit

from running continuously. It may also take a considerable

amount of time for an oscillator to stabilize (a quartz

oscillator may take a full week)

Radio signals are affected by events along their path (like

bad weather and electrical interference) . Therefore, data from a radio signal must be averaged when compared to a primary

oscillator (which may be accurate to better than l.OOE-11 per

day) . This is why 24 hours has been chosen as the period for

calibration labs to use. Comparisons between two oscillators can

be made more quickly, because no radio path noise is involved.

After using an oscilloscope to reset a counter time base for

example, the time base can be used immediately. However, the

time base is subject to the same recovery time as any quartz

oscillator, and a change in frequency can be expected for the

first few days.

It is one thing to reset the frequency dial on an oscillator

and quite another to expect the frequency to remain at the set

value. This is why HF radio signals (like WWV) limit the

accuracy obtained. The initial setting can be very accurate

(even with the noise present on HF) , but an adjusted oscillator

takes some time to settle. This is also a consideration with

equipment that synchronizes or disciplines an oscillator to a

radio signal (WWVB, for example) . In some cases, the oscillator

may follow a radio signal which has noise on it.

20
2.8 Record Keeping for Frequency Calibrations

Enough records should be kept to satisfy the needs of the

laboratory. For example, users of the NBS Frequency Measurement

System can keep the daily plots in a notebook for future

reference and diagnostic purposes. They can also keep the data

on computer disk, as well as the monthly traceability report sent

by NBS.

Users of an integral radio-oscillator-recorder can keep the

chart records provided to support claims of traceable

calibrations. If the signal being received is listed in an NBS

or USNO publication, those documents can also be made part of

the calibration data file.

Users of WWV who employ an oscilloscope for comparison can

simply keep a daily log of their results. Such a notebook

provides adequate proof of calibration, especially at the

expected level of accuracy (l.OOE-07).

Laboratories that use a cesium standard without radio

comparisons need to keep a notebook of front panel meter

readings. They should also include a history of adjustments

following the manufacturer's instructions.

Calibration labs that need strong evidence of traceability

should draw up a system block diagram showing how signals are

generated, distributed, and calibrated. When changes are made,

they can be noted on copies of the diagram. This method also

keeps the lab manager aware of changes and problem areas and

provides the information needed for future purchases.

The rest of this paper explains, in detail, how the NBS

Frequency Measurement System is used as part of the overall

system in a calibration laboratory, and how users of the system

obtain traceable frequency calibrations.

21
3. THE NBS FREQUENCY MEASUREMENT SYSTEM

This section reviews what has been previously explained by

using the NBS Frequency Measurement System as an example.

The NBS Frequency Measurement System brings together the

commercial hardware and software items needed to perform stateof-the-art frequency calibrations. By following the procedure,

each user gains from having a standardized measurement approach.

The NBS makes data comparisons using telephone modem links. That

way, a user whose data is in question can benefit from another

user's data via the NBS telephone data link. This is especially

important where maximum performance is required. The system

calibrates all kinds of oscillators and even permits calibrating

four oscillators simultaneously.

The NBS Frequency Measurement System is controlled by a

computer with two disk drives, a monitor, and a printer. A

Loran-C radio receives a traceable signal from a Loran-C

transmitter. That signal is compared with the primary

oscillator in the calibration laboratory. Any oscillator can be

used since the system can operate at either 1, 5, or 10 MHz.

Oscillators can be calibrated at levels up to l.OOE-12.

Since Loran-C radio signals are subjected to path noise

(like all radio signals) , the system averages its data. It

records data for 24 hours (one day) . At the end of the day, it

automatically plots the data and restarts itself for the next

days run.

The system restarts after a power outage and generally

operates unattended. The equipment in the NBS Frequency

Measurement System is always turned on. The frequency divider,

the Loran receiver, and the clock card inside the computer are

backed up by batteries. The operator only needs to check the

available disk space and printer paper. Even those conditions

are monitored, and alarms indicate if the disk is full or the

printer is out of paper.

The system's battery-backed frequency divider makes it easy

to compare oscillators since it divides input frequencies of 1,

5, or 10 MHz to a more convenient 1 Hz for comparison purposes.

It also provides a number of additional output signals that are

useful for oscilloscope comparisons or system diagnostics. The

divider also synthesizes a low-frequency Loran signal (about 10

Hz) from the primary oscillator.

The system's time interval counter can measure up to four

channels simultaneously. It has no knobs or controls, and

therefore eliminates measurement errors due to knob settings.

The counter's time base frequency is obtained from the primary

22
oscillator. The time base frequency can be either 1, 5, or 10

MHz, and a lamp indicates if a time base signal is present.

By design, at 1530 UTC the system stops taking data and

makes a paper plot of each channel being used. The plot shows

the number of microseconds that were gained or lost in the 24

hour interval of the measurement. The unit for the vertical axis

is microseconds. The unit for the horizontal axis is time of

day (hours)

By looking at the plot, the operator can quickly assess the

oscillator's performance. The plot also shows the system's

estimate of the oscillator's relative frequency. This estimate

is nearly exact for quartz oscillators with uniform aging. It is

less accurate for high-precision oscillators, since the system

automatically enlarges the plot to show the radio path noise. Of

course, when comparing two oscillators on channels 2, 3, or 4

the estimated relative frequency will again be nearly exact. The

automatic plotting and relative frequency features make the lab

technician's job easier, make the calibrations less confusing,

and reduce the amount of operator attention needed.

3 . 1 The Daily Operation of the NBS Frequency Measurement System

Like any measurement program, frequency measurement requires

the lab technician to monitor the operation of the equipment and

to make sure that the daily plots or charts are in order. The

manuals included with monitoring equipment usually explain which

front panel lamps and meters are important for daily system

checks. Once a lab technician becomes familiar with the items

that need to be checked daily, the daily check should take only a

few minutes.

The NBS Frequency Measurement System was designed to make

daily operation easy. Operators can perform a daily check by

looking at the video screen (figure 6, next page)

23
NBS FREQUENCY MEASUREMENT SYSTEM

START DATE: 11/19/86

DISK SPACE: 63.5%

START TIME: 1538

TIME: 1604

RUBIDIUM VS LORAN 9940

QUARTZ VS RUBIDIUM

34183 . 32

19125. 61

READING 435 OUT OF 1000 DATA POINT 1

34183 .37 19125. 61

34183 .47 19125. 61

34183 .46 19125. 62

34183 . 38 19125. 62

34183 . 38 19125. 63

34183 .48 19125. 63

34183 . 39 19125. 63

34183 .39 19125. 64

34183 .49 19125. 64

34183 .49 19125. 65

FIGURE 6 - SCREEN DISPLAY FOR THE NBS SYSTEM

The readings from each channel are displayed in a separate

column. There is one column for one channel, two columns for two

channels, and so on.

The screen shows the time when the measurement run began.

This is usually just a few minutes after the 1530 UTC plot time.

If the system was stopped and restarted, or if the power was

lost, the restart time is shown on the screen. In addition, the

present time of day is also shown on the screen. If that time is

wrong for any reason, the operator can quickly reset the time

from the keyboard without stopping the system.

The screen also shows the title given to each measurement

channel. A typical title might be:

This title tells anyone reading the plot what the record means,

and identifies the oscillator being measured. All four channels

are shown, and the separate daily plots can be easily filed for

reference

Rubidium vs Loran 994 0

24
The screen also shows the very first reading taken on a particular measurement run. A glance at the screen shows how

many microseconds the oscillator has moved relative to its

comparison source. Channel 1 compares the primary oscillator to

Loran-C. The other channels compare other oscillators to the

primary oscillator.

The most important part of the daily check is to look at the

phase plots printed by the system (figure 7, next page). The

plots show the operator if the oscillator is high or low in

frequency and estimate the oscillator's relative frequency. The

operator can also tell if there were any losses of the signals

being measured; for example, if the Loran-C receiver tracked

properly for the last 24 hours. The operator will soon get a

feel for how well each oscillator is expected to perform and

will quickly be able to notice if an oscillator is broken.

All time of day noted on the NBS Frequency Measurement

System is Coordinated Universal Time (UTC) . Using UTC eliminates

confusion caused by time changes and time zones. The date is

shown both as calendar date (10/01/87, for example) and as a

Modified Julian Date (MJD) . This also helps sort out data and

avoids confusion.

The operator must keep in mind that the quantity being

measured is a very small number obtained by averaging. This

means that even a small error at any time during the daily

measurement run will affect the data. For this reason the

equipment should be left on, sudden temperature changes should be

avoided, and electrical interference (from cleaning equipment,

power devices, generators, etc.) should be minimized.

25
NATIONAL BUREAU OF STANDARDS FREQUENCY CALIBRATION PLOT

RELATIVE FREQUENCY = -2.66E-09 2000/26

^00 XTAL OSC US LORAN 7980

M

IC

R

O

S

E

C

o

N

D

o

100

16 26

MJD=47132

14 = 48

DEC/87-031626 #1

NATIONAL BUREAU OF STANDARDS FREQUENCY CALIBRATION PLOT

RELATIVE FREQUENCY = -3.39E-09 2000/26

^00 XTAL OSC US LORAN 798©

riIC

R

n

S

E

C

o

N

D

S

-200

16 = 26

MJD=47133

14 = 48

DEC-''S7-041626 #1

FIGURE 7 - DAILY CALIBRATION PLOTS
4. CONTROL OF THE NBS FREQUENCY MEASUREMENT SERVICE

An important part of the NBS Frequency Measurement Service is

NBS support. This support is broken down into five categories:

hardware support, training, data verification, data monitoring,

and reporting traceability . Each category is discussed below.

4 . 1 Hardware Support

The Frequency Measurement System was designed using

low-cost, reliable hardware. Low-cost hardware made the service

economically feasible. Reliable hardware made the service

possible. Each measurement system is intended to run 24 hours a

day.

Most hardware failures occur soon after the hardware is

installed. To eliminate this problem, each measurement system is

burned-in and tested by NBS for a period of several weeks prior

to shipment.

When parts fail in the field, every effort is made to

minimize system downtime. NBS can usually determine which part

failed by using the telephone modem. A replacement part (that

was burned-in and tested) is then shipped to the user within 24

hours

4.2 Training

As part of the initial service fee, NBS offers a training

seminar to each measurement service subscriber. These seminars

are held at NBS laboratories in Boulder, Colorado. They are

limited to small groups and provide actual hands-on training with

the measurement system. They also provide the user with a

general background in frequency calibrations.

The training does not end when the user leaves Boulder. NBS

provides telephone support for the service, and users may call

any time during normal working hours. NBS is always happy to

help users apply the service to specific applications they have

in their own organizations.

27
4.3 Data Verification

As part of the service agreement, NBS looks at each user's

data with the telephone modem. The system records phase and

frequency data. The two types are explained below:

Phase Data - The phase data show the change in phase of

the frequency being measured in comparison to the reference

frequency. These data are plotted every morning, and a copy of

the plot is printed. The Y-axis shows the change in phase

(measured in microseconds) , and the X-axis shows the time of day.

The X and Y values are stored on disk before they are plotted

and remain on disk after they are plotted.

Phase data are recorded on from one to four channels, and

from one to four phase plots are printed each day. NBS can

transfer any or all of the recorded phase data to the Boulder

laboratories and reproduce the user's phase plots. However,

phase data are generally transferred only if something unusual is

spotted in the frequency data.

Frequency Data - Frequency data are recorded for channel

1 only. The data show the frequency offset of the user's

primary oscillator relative to the Loran-C reference frequency.

The relative frequency is printed on the phase plots for all

channels but is recorded on disk for channel 1 only.

The file containing the frequency values is transferred to

NBS by modem. NBS knows at a glance if the values are valid,

because it has the frequency history of each user's primary

oscillator. For example, NBS may know that a particular

oscillator typically stays within l.OOE-10 relative to Loran-C.

The frequency file lists the following values:

Day 1 1. 03E-10

Day 2 1. 05E-10

Day 3 1.08E-10

Day 4 1. llE-10

Day 5 1. 16E-10

Day 6 8.73E-08

Day 7 1.24E-10

The values for days 1-5 appear to be typical (about l.OOE-10),

based on the known past performance. The slight variations are

normal and due to the aging rate of the quartz crystal. However,

the value for day 6 is markedly different from the other values.

It is obvious that a phase shift occurred on day 6. This shift

28
could have been caused by either one of the signals being

compared (the primary oscillator or Loran-C) , or perhaps by a system malfunction. To find the cause of the phase shift, NBS

transfers the phase data for day 6 and reproduces the phase

plot.

Once the cause of the phase shift is found, NBS does one of

the following things:

a. If a system part has failed, NBS replaces it.

b. If the user's oscillator failed, NBS notifies

the user.

c. If a Loran-C phase shift occurred, NBS reports

the phase shift to the user.

In the above example, items a and b above are unlikely causes of

the phase shift occurring on day 6. The reason they are

unlikely is that the system "recovered" and produced valid data

for day 7. This does not usually happen when parts fail.

Therefore, the most likely cause of the phase shift is Loran-C.

NBS continuously monitors Loran-C and is aware of Loran-C phase

shifts the day they occur. This is the topic of the next

section.

4 . 4 Data Monitoring

NBS monitors the Loran-C stations received by subscribers of

the service. This monitoring is done with the same equipment

used by the subscribers. The stations monitored are listed

below:

GRR* Location

7980 Malone, Florida

8970 Dana, Indiana

9940 Fallon, Nevada

9960 Seneca, New York

* Group Repetition Rate

Since NBS monitors all of these stations, each user has a

reference frequency in common-view to themselves and to NBS.

29
As mentioned earlier, users of the service make phase comparisons between their primary oscillator and Loran-C.

Loran-C is the reference frequency, and the user's primary oscillator is the frequency being measured. NBS does the

opposite; it measures the frequency of Loran-C by comparing it to

the national frequency standard (figure 8, below). The national

frequency standard is the reference frequency, and Loran-C is the

frequency being measured. These comparisons let NBS know the

relative frequency of Loran-C. Therefore, NBS knows if a phase shift in the user's data was caused by Loran-C.

The real strength of NBS monitoring lies in the amount of

data that NBS looks at. NBS keeps at least two receivers (one

backup) locked to each Loran-C station at all times. In

addition, NBS looks at data from users in the field. Most users

are closer to the station that they are monitoring than NBS is,

and their data are often smoother than NBS data. Many of them

use atomic oscillators and monitor Loran-C 24 hours a day, 365

days a year. This makes the NBS Frequency Measurement System

the largest network of Loran-C monitoring in the United States.

The NatioMl mr§m of StaiidaMs Frequency ^asureMnt Service

Lo?an-C 83?9 faster (ta, IMiaiia) us uIC(HBS)

5

I

C

0

-i—i—i-i

0

N

E'

7

\ \ \rv \ \ \ \

S =5 35/12/8? m 46?27)

_6;8? [tic MPks = Z hcursj

im mm 35/14/8?

1 iS:27

I The frequency offset of the oscillator is! t2.41M2 Data Foiiits S?

FIGURE 8 - LORAN-C DATA COMPARED TO UTC(NBS)

30
For example, if NBS data reveal a phase shift in a Loran-C

signal (figure 9, below), it can be proved or disproved if this

phase shift was "real" by looking at data from other users of the

service

Th? NatioMl Bureau of StaMards Fraquensy MeasureMent Service

Loran-C ?940M (FalloTi. Nevada) vs UICiNBS)

H 5888 -

0fc/lH/8? (HJy 46956) Channel i (HJn 4695?) 06/il/S?

09:59 itiG nms - Z hours] ii;i2

The frequensy offset of the oscillator is; -2=5?M8 Data Foirts 21

FIGURE 9 - A LORAN-C PHASE SHIFT

If the phase shift occurred on the signals from Fallon,

Nevada, NBS could look at the data from seven users (at this

writing) who compare the Fallon, Nevada, signals to cesium

oscillators. The location of these users and their distance and

direction from the Loran-C transmitter are listed below:

Location Distance (miles) Direction

Albuquerque, NM 736 East

Los Alamos, NM 728 East

San Diego, CA 478 South

Pomona , CA 383 South

Seal Beach, CA 402 South

Alameda, CA 222 West

Keyport , WA 590 North

As the table illustrates, NBS is effectively tracking the

frequency performance of Fallon from all directions at distances

well within the range of Loran-C broadcasts. NBS monitors other

Loran-C stations equally as well.

31
4.5 Reporting Traceability

Each user of the service receives a monthly frequency

measurement report showing the frequency performance of their

primary oscillator for the previous month (figure 10, next page)

This report certifies that the user's measurements are

traceable

The report also lists any Loran-C phase shifts that

occurred during the month, and any periods when the user's

system was not running.

32
The National Bureau of Standards

Frequency Measurement Report

The data below are shown as received at:

ABC CALIBRATIONS

ANYTOWN, USA

USER: JOHN Q. PUBLIC

The plot shows the frequency performance of your primary oscillator relative to Loran-C.

The data were taken from 09/01/1987 to 09/30/1987.

E-a?

E-tiy + F

R

E E-11 -

ABC PkINhRY use Us LORAH 9940

1

1

u

Y

^—i—i—i—i—i—i—i—i—i—i—r- T—!—I—

r

I I i 1 I i i I i i I i i

E-as --

E-e? -L . 47039 47063

LORAN-C PHASE SHIFTS:

DAYS WHEN DATA WERE NOT PLOITED:

COMMENTS:

FIGURE 10 - TRACEABILITY REPORT SENT TO USERS OF THE SERVICE

33
5. ASSESSMENT OF MEASUREMENT ERRORS USING

THE NBS FREQUENCY MEASUREMENT SYSTEM

Based on empirical data from the NBS Time and Frequency

Division and by others over a period exceeding two decades, the

accuracy of Loran-C (as controlled by the Coast Guard and

monitored by NBS and the United States Naval Observatory) , is

l.OOE-12 per day. Therefore, the NBS Frequency Measurement

Service should meet calibration requirements to this level.

Possible sources of measurement errors are discussed below.

Measurement Errors of the Counter - All counter

measurements are made in the time interval mode.

The user's primary oscillator (often a Cesium or

Rubidium) serves as a reference. Since readings

are taken at intervals of about 1 second maximum

and successive readings are differenced, errors

caused by an inaccurate time base oscillator are

minor. All equipment is tested prior to shipment.

After shipment, further routine tests are made by

having the user perform a self-check of the counter

by using the same source for both the START and

STOP inputs to the counter. Typical results for

these tests are l.OOE-14 per day. These errors are

not significant.

Errors Due to Loran-C Variations - NBS can

verify when propagation errors occur by checking

its own data, data from user's systems, and data

from other major laboratories (principally the

U.S. Naval Observatory). Data obtained by NBS for

several decades and by other major standard

laboratories gives the error due to propagation as

l.OOE-12 for averaging times of several hours. All

NBS Frequency Measurement Calibrations are operated

for 24 hours to minimize the contribution of

propagation errors. All of the United States

Loran-C chains are monitored at Boulder by NBS.

Errors Due to Coast Guard Mandated Shifts of

Loran-C Phase - From time to time (a few times a

year), the U.S. Coast Guard shifts the phase of

the Loran-C transmissions. This is usually done to

cause the frequency (as broadcast) to track the

average frequency of the U.S. Naval Observatory's

oscillators. This frequency, in turn, is reported

to the international control agencies.

34
NBS receives notice of changes in the broadcast

phase of Loran-C and passes them on to users. In

rare cases where the phase is inadvertently

changed, NBS monitoring will detect and verify the

change, and report it to the users.

To summarize, counter errors are not significant, and

propagation errors are minimized by averaging. The phase

control exercised by the Coast Guard is carefully monitored by

NBS and other major laboratories. Therefore, there should be no

significant measurement errors when using the NBS Frequency

Measurement System.

35
6. BIBLIOGRAPHY

Booker, S. R. ; Jones, M. C. The NBS Frequency Measurement System

From a User's Viewpoint. NCSL 1984 Workshop and Symposium; 1984

October; Gaithersburg, MD. 1-11.

Howe, S. L. , ed. NBS Time and Frequency Dissemination Services.

Nat. Bur. Stand. (U.S.) Spec. Publ. 432; 1979 September. 20 p.

Jespersen, J. ; Fitz-Randolph, J. From Sundials to Atomic Clocks:

Understanding Time and Frequency. Nat. Bur. Stand. (U.S.)

Monogr. 155; 1977 December. 177 p.

Kamas, G. ; Howe, S. L. , eds. Time and Frequency User's Manual.

Nat. Bur. Stand. (U.S.) Spec. Publ. 559; 1979 November. 256 p.

Kamas, G. ; Lombardi, M. A. A New System For Measuring Frequency.

NCSL 1985 Workshop and Symposium; 1985 July; Boulder, CO.

224-231.

United States Coast Guard. Loran^-C User Handbook. Dep. of Tran.

(U.S.) COMDTINST M16562.3; 1980 May. 82 p.

36
NBS-n4A (REV. 2-BC) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publ ication Date

RIRI lORRAPHIC DATA

REPORT NO. SHEET (See instructions) NBS/SP-250/29 January 1988

4. TITLE AND SUBTITLE Measurement Services:

TRACEABLE FREQUENCY CALIBRATIONS

HOW TO USE THE NBS FREQUENCY MEASUREMENT SYSTEM IN THE CALIBRATION LAB

5. AUTHOR(S)

George Kamas and Michael A. Lombard!

6. PERFORMING ORGANIZATION (if joint or other thon NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Same as item 6

.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number 87-619905

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bibliography or iiterature survey, mention it here) A practical approach to making traceable frequency calibrations

in the calibration laboratory is discussed. This approach

emphasizes obtaining traceable data, keeping appropriate records,

and selecting an oscillator and a radio signal to use for

calibrations. The NBS Frequency Measurement System is used to

Illustrate this approach, and to discuss the decisions to be made

when setting up a calibration lab for frequency measurements.

The theory of frequency measurement is also discussed, with

emphasis on using time interval counters to obtain the relative

frequency of an oscillator.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

calibration; cesium oscillators; frequency measurement;

Loran-C; quartz oscillators; relative frequency; rubidium oscillators;

time interval counters; traceability ; WWV; WWVB

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

42

15. Price

USCOMM-DC 6043-P80

*U.S. Government Printing Office : 1988 - 201 -597/72499

THE SP 250 SERIES ON NBS MEASUREMENT SERVICES*

SP 250-1 Spectral Radiance Calibrations 003-003-02792 $3.50

SP 250-2 Far Ultraviolet Detector Standards 003-003-02810-0 $4.25

SP 250-3 Radiometric Standards In the Vacuum

Ultraviolet 003-003-02806-1 $6.50

SP 250-4 Fricke Dosimetry in High-Energy Electron Beams

003-003-02816-9 $2.75

SP 250-5 Alpha-Partide Calibrations

SP 250-6 Regular Spectral Transmittance 003-003-02805-3 $3.25

SP 250-7 Radiance Temperature Calibrations 003-00^-02827-4 $2.25

SP 250-8 Spectral Reflectance 003-003-02812-6 $7.00

SP 250-9 Calibration of Beta-Partide-Emitting

Ophthalmic Applicators

003-003-02817-7 $2.00

SP 250-15 Photometric Calibrations

SP 250-16 Calibration of X-Ray and Gamma-Ray

Measuring Instruments

SP 250-17 The NBS Photodetector Spectral Response

Calibration Transfer Program

SP 250-18 Neutron Source Strength Calibrations

SP 250-19 Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

SP 250-20 Spectral Irradiance Calibrations 003-003-02829-1 $5.50

SP 250-21 Calibration of Beta-Partide

Radiation Instmmentation

SP 250-22 Platinum Resistance Thermometer

Calibrations 003-003-02831-2 $17.00

SP 250-23 Liquid-in-Glass Thermometer

Calibration Sen/ice

SP 250-10 Radioactivity Calibrations with the Gamma Ionization Chamber, and Other

Radioactivity Calibration Capabilities

003-00^-02824-0 $2.25

SP 250-1 1 Dosimetry for High-Dose Applications

SP 250-12 Neutron Personnel Dosimetry

003-003-02811-8 $2.50

SP 250-13 Activation Foil Irradiation with Californium

Fission Sources

SP 250-14 Activation Foil irradiation by Reactor Cavity

Rssion Sources

SP 250-24 Standard Cell Calibrations 003-003-02825-6 $2.75

SP 250-25 Calibration Sen/ice for Inductive Voltage Dividers

SP 250-26 Phase Angle Calibrations

SP 250-27 AC-DC Difference Calibrations

SP 250-28 Solid-State DC Voltage Standard Calibrations

SP 250-29 Traceable Frequency Calibrations

SP 250-30 GOES Satellite Time Code Dissemination:

Description and Operation

* Those entries containing a stock number (003-003 ) and price can be purchased from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. GPO will accept checks, money orders, VISA, and Mastercharge. For more information, or to place an order, call (202) 783-3238. Be sure to use the stock number in all orders. Titles without stock numbers are in preparation.
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300