NIST Frequency Measurement and

Analysis System: Operator’s Manual

Michael A. Lombardi

NISTIR 6610
NISTIR 6610

NIST Frequency Measurement and

Analysis System: Operator’s Manual

Michael A. Lombardi

Physics Laboratory

Time and Frequency Division

325 Broadway

Boulder, CO 80305

August 2001

U.S. Department of Commerce

Donald Evans, Secretary

National Institute of Standards and Technology

Karen H. Brown, Acting Director
i 

Table of Contents 

 INTRODUCTION iii 

 CHAPTER 1 

 INSTALLATION 1 

 List of Parts 1 

 Block Diagram of System 2 

 System Setup 3 

 GPS Antenna 3 

 Uninterruptible Power Supply (UPS) 4 

 Equipment Rack 4 

 Video Monitor 5 

 GPS Receiver 5 

 Printer 6 

 Telephone Modem 6 

 Connecting Oscillators to the System 7 

 Starting the System 9 

 CHAPTER 2 

 LEARNING TO USE THE MENUS 13 

 Selecting a Menu Item 13 

 Exiting a Menu 14 

 Exiting the FMAS software 14 

 CHAPTER 3 

 MAKING MEASUREMENTS 15 

 Automatic Plotting 17 

 Changing Oscillators 19 

 Quick Check of System 20 

 CHAPTER 4 

 ANALYZING DATA 23 

 Loading the Demonstration Data from Disk 24 

 Viewing the Data on Screen 24 

 Printing the Data 26 

Table of Contents 
ii 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 Viewing the Data Statistics 26 

 Plotting the Data 27 

 Editing the Plot on Screen 29 

 Removing Data Points from the Plot 31 

 Plotting a Portion of the Plot 33 

 YAXIS, XAXIS, and MAGNIFY 34 

 Changing the Title of the Plot 35 

 

 CHAPTER 5 

 RETRIEVING YOUR DATA 37 

 Retrieving All of the Data 37 

 Retrieving Some of the Data 38 

 CHAPTER 6 

 UTILITIES 39 

 ShortTerm 39 

 Retrieving Short Term Measurement Data 43 

 Adjusting an Oscillator Using METER 44 

 History 44 

 GPS 46 

 CalibrateTIC 49 

 CHAPTER 7 

 SETUP SOFTWARE 51 

 APPENDICES

 A - An Introduction to Frequency Calibrations 55 

 B - Calibration Reports 87 

 C - Specifications and Compliance Documents 91 

 D - Glossary 93 

 INDEX 99 
iii 

Introduction 

 The NIST Frequency Measurement and Analysis System (FMAS) measures 

and calibrates oscillators and helps you analyze their performance. The system can 

measure and calibrate quartz, rubidium, and cesium oscillators. Each day, it prints a 

calibration plot of the data collected in the last 24 hours. Up to five oscillators can be 

connected to the system and calibrated simultaneously. 

 

 The FMAS is housed in a metal equipment rack. Its components include a 

Global Positioning System (GPS) radio receiver, a time interval counter, a graphics 

printer, a computer, and a color video monitor. The FMAS also includes a backup 

power supply that can keep it running for about 30 minutes during power outages. The 

FMAS will stop operating during longer power outages, but will restart automatically 

when power is restored. 

 NIST provides all of the equipment and supplies you need to operate the 

FMAS. Your only requirements are to supply one or more oscillators and a dedicated 

telephone line (so that NIST can access the system by modem). 

 This manual tells you how to install and operate the system. The first four 

chapters are especially important. Chapter 1 describes how to install the system. 

Chapter 2 describes how to use the menus. Chapter 3 describes how to run the system 

in the measurement mode. Chapter 4 describes how to run the system in the analysis 

mode. 

 Please call NIST whenever you have a question or need assistance. You can 

reach us Monday through Friday from 8 a.m. to 5 p.m. Mountain Time by calling (303) 

497-3212 or (303) 497-3378. 

Introduction to 

NIST Frequency Measurement 

and Analysis System

1 

Chapter 1 - Installation 

 The NIST Frequency Measurement and Analysis System consists of the 

following parts: 

• metal equipment rack 

• rack-mount computer 

• rack-mount keyboard with sliding drawer 

• rack-mount monitor shelf with color monitor 

• telephone modem 

• printer and cable 

• multi-channel time interval counter 

• Global Positioning System (GPS) receiver with antenna 

• Uninterruptible Power Supply (UPS) 

 In addition to the parts listed above, the system is also shipped with all of the 

other items necessary for operation. These include cables, power cords, manuals, 

printer paper, and printer ribbons/ink cartridges. All parts should be unpacked and 

accounted for before installation. If any part is missing, call NIST at (303) 497-3212 

or (303) 497-3378. 

 Figure 1.1 shows a block diagram of the NIST Frequency Measurement and 

Analysis System. 

Chapter 1 

Installation
2 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Figure 1.1. Block Diagram of the NIST Frequency Measurement and Analysis System. 

 
3 

Chapter 1 - Installation 

The following sections tell how to install the NIST Frequency Measurement and 

Analysis System (FMAS). The installation is described in two parts. The first part, 

SYSTEM SETUP, tells how to set up the FMAS. The second part explains how to 

connect oscillators to the FMAS. 

SYSTEM SETUP

 This section describes how to set up the FMAS. We recommend that you 

install the parts in the order listed. 

1. GPS ANTENNA 

 Install the GPS antenna as follows: 

a. Choose a location for the GPS antenna that is as high 

above the ground and as obstruction free as practical. 

GPS reception is line of sight and you will be receiving 

signals from orbiting satellites, so make sure the antenna 

has an unobstructed view of the sky on all sides. 

b. The antenna size and shape might vary, but most are 

small white plastic cones about 5 or 6 inches high (12 to 

15 cm). The antenna has a threaded mounting base that 

attaches to an 18 inch metal pole (46 cm) included with 

the system. Mount the antenna by any convenient 

means, but make sure the pole is mounted vertically. If 

possible, you can affix the pole to an existing roof mount 

using the supplied clamps. 

c. Route the supplied cable down to your system location. 

The system is shipped with 50 feet (15 m) of RG-58 

cable, unless previous arrangements were made with 

NIST. Make sure that all connections are made 

carefully, and do not kink or stress the cable. If you 

require a longer cable length, contact NIST and a custom 

lower loss cable will be sent to you. RG-58 will not 

work properly at the GPS carrier frequency (1575.42 

MHz) when cables longer than 20 m are used. 
4 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 2. UNINTERRUPTIBLE POWER SUPPLY (UPS) 

 Install the UPS as follows: 

a. Set the UPS on the table or desk top where the system will be 

located. Plug the UPS into a secure three-pronged wall outlet. 

Do not use an outlet that will be turned off or used by power 

drills, vacuum cleaners, motors, or other electrical devices. 

b. Turn on the UPS so that the batteries can begin charging. 

3. EQUIPMENT RACK

The equipment rack is shipped with the computer, keyboard, GPS receiver, and 

monitor shelf already installed. The computer has been configured and tested at 

NIST and is ready to use in the system. You do not have to open the computer 

case or install any software. 

a. Remove the equipment rack from its shipping box and gently set 

it on the table or desk top where you will locate the system. The 

equipment rack is heavy, and you may want to have two people 

available to lift it. Handles are provided on both sides of the 

equipment rack. 

b. Note that the front panel of the equipment rack has two locks. 

One lock is for the monitor shelf. The second lock is for a door 

on the computer that covers a floppy drive and the on/off and 

reset buttons for the computer. You will receive a set of 

identical keys with the FMAS and the same key can be used to 

open either lock. Use the keys to open the computer door. 

Make sure the power switch is OFF for now. 

c. Locate the keyboard cable at the back of the equipment rack and 

plug it into the round keyboard jack on the back of the computer. 

The keyboard jack might not be marked, but it is the only jack 

where the keyboard connector will fit. Then pull out the 

keyboard drawer. Make sure that you can open and close the 

drawer without snagging the cable. 
5 

Chapter 1 - Installation 

d. Connect a power cord to the back of the computer, and 

plug the computer into the UPS. Leave the computer's 

power switch (behind the door) OFF for now. 

4. VIDEO MONITOR 

 Install the monitor as follows: 

a. Place the video monitor on the monitor shelf through the 

large opening on the back of the equipment rack. 

Position the monitor so that the entire screen is visible 

even when the door to the monitor shelf is closed. 

b. Plug the monitor directly into a wall outlet, if available. 

Do not plug the monitor into the UPS unless no other 

outlets are available. The UPS will be able to endure 

longer power outages if it does not have to supply power 

to the monitor. 

c. Connect the monitor’s signal cable to the video jack on 

the back of the computer. The video jack is the only jack 

where the monitor connector will fit. 

d. You may turn the monitor ON now. Keep in mind that 

the monitor will last longer if you turn it OFF when you 

leave for the night or when no one is using the system. 

5. GPS RECEIVER 

The FMAS uses a GPS receiver that is embedded inside the rack mount 

computer system. This receiver draws power from the computer’s 

power supply, and is controlled by the computer’s software. Its antenna 

connector is mounted on the computer’s back panel. 

To install the receiver, simply connect the cable from the GPS antenna 

to the TNC connector marked ANTENNA on the back of the computer. 
6 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

6. PRINTER 

Install the printer as follows: 

a. Remove the printer from the packaging and set it on the 

desk or table top. 

b. Load the printer with paper. It uses standard 8.5 × 11 

inch cut sheet paper. A package of this paper is 

supplied with the system. 

c. Connect the printer cable (a round cable) to the jack on 

the back of the printer. 

d. Connect the other end of the printer cable to the jack on 

the back of the computer. The jack is not marked, but it 

will be the only jack that fits. 

e. Plug the power cord from the printer into the UPS. 

f. Turn the printer ON. 

7. TELEPHONE MODEM

The modem is already installed inside the computer. You simply need 

to plug the modem into a telephone wall jack. 

a. Plug the supplied phone cable into the telephone wall 

jack. 

b. Plug the other end of this cable into the phone jack 

marked "WALL" on the back of the computer. 

Sometimes there will be two modem jacks and neither 

jack is labeled. In that case, either jack will usually 

work. 

 This completes the installation. The next section explains how to connect your 

oscillators to the measurement system. 
7 

Chapter 1 - Installation 

CONNECTING OSCILLATORS TO THE SYSTEM

 This section explains how to connect your oscillators to the FMAS. All 

connections are made using cables (typically RG-58, 50 Ω impedance) with standard 

BNC connectors. Several cables of this type were supplied with the system. 

 The first thing required is to select one of the oscillators in your laboratory as 

your primary oscillator. Your primary oscillator should meet the following criteria: 

• It should be the best oscillator available that is a 

permanent part of your laboratory. For example, 

select an oscillator that will remain permanently at 

your site and not one that will be sent to another 

location. 

• Your oscillator must provide a frequency output of 

either 1, 5, or 10 MHz. 

• Your oscillator should be connected to a stand-by 

power supply so that power outages do not interrupt 

the signals. 

• Your oscillator should run 24 hours a day, 7 days a 

week, so that NIST can continually monitor its 

performance and send you a monthly calibration 

report, and so that you can continuosly establish 

traceability to the United States national frequency 

standard. 

• The signal from your primary oscillator at 50 Ω 

should be at least 200 mV peak-to-peak (about 70 

mV RMS) and should not exceed 3.5 V peak-to-peak 

(about 1.2 V RMS). It is unlikely that the signal will 

be too small, but it could be too large. If the signal 

is too large, you need to use an attenuator to reduce 

it to an acceptable level. 

 Oscillators are connected to the BNC connectors on the back panel of the 

measurement system. The time interval counter already installed inside the system can 
8 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

accept either a 1, 5, or 10 MHz input signal from your primary oscillator as a time 

base. 

Connect your oscillators to the system as follows: 

Primary Oscillator (Channel 1) - Connect a cable from your primary oscillator to the 

BNC connector labeled PRIMARY. This signal serves as the counter's time base, and 

will also be measured via GPS. 

 Channel 1 measures the frequency uncertainty of your primary oscillator using 

GPS as a reference frequency. Since GPS signals are monitored by NIST and since the 

measurement system is linked by modem to NIST, your calibrations are traceable to 

the national frequency standard. You will receive a monthly calibration report that 

documents traceability (see Appendix B). 

Secondary Oscillators (Channels 2, 3, 4, and 5) - Since your primary oscillator is 

traceable to NIST, you can use it as a reference frequency for calibrating other 

oscillators, thus completing their traceability chain. As many as four other oscillators 

may be calibrated simultaneously on channels 2, 3, 4, and 5. To connect additional 

oscillators to the system, simply connect their output to channel 2, 3, 4, or 5 on the 

back panel of the measurement system. The input signal can be any frequency from 1 

Hz to 120 MHz in 1 Hz increments. 

IMPORTANT NOTE: All signals that you connect to the 

system should not exceed 3.5 V peak-to-peak (about 1.2 

V RMS). You should check the signal level of every 

oscillator before connecting it to the system. If the level 

exceeds 3.5 V peak-to-peak, use an attenuator to reduce 

the signal amplitude. Signals that are too large can 

cause interference with adjacent counter channels, or 

cause the counter to make trigger errors. In either case, 

the system will make measurement errors and reduce the 

quality of your data. 
9 

Chapter 1 - Installation 

STARTING THE SYSTEM

 To start the system, turn the computer's power switch ON. If the video monitor 

is not ON, turn it ON at this time. Before the system can make measurements, it has to 

make sure that the GPS signal is usable. After a few seconds, a screen will appear like 

the one shown below: 

Figure 1.2. The GPS acquisition screen.

 The screen shows the status of the GPS signal acquisition process. The receiver 

automatically acquires the signal. There are several conditions that must be met 

before the system decides that GPS is usable for frequency calibrations. The receiver 

must be tracking more than three satellites. The timing stability must be less than 80 ns 

(this internal check should not be confused with the true stability of the receiver which 

is generally much smaller). The value for PDOP must not exceed 3.0. PDOP is an 

acronym for Position Dilution of Precision, which is a measure of the quality of the 

position fix. The lower the PDOP, the more "spread out" the satellites are in the sky 

and the better the position fix. And finally, the Position Hold Mode must be turned on. 

This indicates that your antenna’s position has been surveyed with an uncertainty of 
10 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

less than 10 m. At this point, the system is ready to use for frequency calibrations. 

The system will wait 10 s and then load the measurement software. 

 The first time you install the system, it may take as long as 30 min to complete 

the process of GPS signal acquisition. The acquisition process will be repeated only if 

you restart the system, or if a power outage occurs that lasts longer than about 30 min 

(about what the UPS can handle). However, after the first time, the acquisition 

process should take a few minutes or less since the system has already stored most of 

the data that it needs. If the position of the antenna changes for any reason, you should 

restart the system. This will force the system to again go through the signal 

acquisition process and resurvey the new position of the antenna. 

Figure 1.3. The channel name input screen. 

 Once the measurement software is loaded, the screen shown in Figure 1.3 

should appear on the monitor. If this screen does not appear, go through the previous 
11 

Chapter 1 - Installation 

steps in this chapter again and make sure the system has been properly installed. If 

you need help, call NIST at (303) 497-3212 or (303) 497-3378. 

 

 Once the screen shown in Figure 1.3 appears, enter a name and input 

frequency, and select a reference frequency (described at end of chapter) for each 

channel that has a signal connected. For example, if you connected signals to channels 

1, 2, and 3 on the counter, enter a channel name and input frequency, and select a 

reference frequency for channels 1, 2, and 3. You can move around the input screen 

by pressing the <UP ARROW> key to go up, or the <DOWN ARROW> key to go 

down. Channel names, input frequencies, and the reference frequency selection are 

discussed below. 

Channel Names - Give a clear, simple name to each channel. For example, if your 

primary oscillator is a Model X Rubidium and you are comparing it to GPS, then enter 

a name like: 

 Model X Rubidium vs GPS 

 On channel 2, you might be measuring a Model Z Quartz oscillator. You can 

use a name like: 

Model Z Quartz (Serial Number 101) 

 Clear, simple titles are very helpful when you look at your data in the future. 

You can enter names up to 50 characters long, which allows you to be quite 

descriptive. You can include such information as the oscillator's serial number, 

property number, and so on. When you finish entering a channel name, press 

<ENTER>. If you make a mistake and want to erase the channel name, press the 

<DEL> key to erase the channel name. 

 

Input Frequencies - The input frequency is the nameplate or nominal frequency of the 

oscillator you will be measuring. In other words, this is the frequency that the 

oscillator is supposed to produce. For example, if you are measuring a quartz 

oscillator that is supposed to produce a 5 MHz output, the input frequency to the 

system is 5 MHz. The FMAS will then measure the difference, or frequency offset 

between the the nameplate frequency and the actual frequency produced by the 

oscillator. 

 The input frequency must be entered in hertz, so 5 MHz will be entered as 

5000000 Hz. The FMAS is capable of measuring any frequency from 1 Hz to 120 

MHz in increments of 1 Hz. This allows you to measure nearly any frequency that you 

are likely to encounter in the calibration laboratory. 
12 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Reference Frequency - The FMAS can compare each input frequency either to your 

primary oscillator (the oscillator connected to channel 1), or to GPS. The primary 

oscillator is designated as P and GPS is designated as G, so enter either a P or a G in 

this column. Keep in mind that the primary oscillator on channel 1 can be compared 

only to GPS, so you are not allowed to change the G in the channel 1 column. 

 It is conventional to compare channels 2 through 5 to the primary oscillator, if 

the primary oscillator is a significantly better performer than the other oscillators you 

are measuring. For example, if your primary oscillator is a cesium, and the oscillators 

on channels 2 through 5 are rubidium or quartz devices, you should select P on 

channels 2 through 5. However, if all of your oscillators are rubidiums, you should use 

GPS as the reference on all channels, since this makes more sense than comparing one 

rubidium oscillator to another. 

 After you have entered the necessary information for every channel in use, 

press the <ESC> key to save the channel names and descriptors to disk. You are now 

ready to go on to the next chapter. 
13 

Chapter 2 - Learning to Use the Menus 

 This section tells how to use the FMAS menus. At this point, the MAIN 

MENU should be on screen as shown below: 

 All FMAS menus work the same way. Each menu is located inside the box at 

the top of the screen. The top line of the menu lists the menu items. One menu item is 

always highlighted. The second line of the menu describes the function of the 

highlighted menu item. In the above menu, for example, the menu item LOAD is 

highlighted. The words "Load data from disk" are printed on the second line. This 

tells you that LOAD is used to load data from disk. 

 You can move to a different menu item by using the right and left arrow keys. 

You should try this now. Each time you press either <RIGHT ARROW> or <LEFT 

ARROW>, a different menu item is highlighted. For example, if LOAD is 

highlighted, pushing <RIGHT ARROW> causes VIEW to be highlighted. Whenever 

you move to a different menu item, the text changes on the second line of the menu. 

 The menu is circular. For example, if you are on the left most menu item and 

press <LEFT ARROW>, you will move to the right most menu item. Likewise, if you 

are on the right most menu item and press <RIGHT ARROW>, you will move to the 

left most menu item. Try this by holding down either <RIGHT ARROW> or <LEFT 

ARROW> and watching the menu wrap around. 

 You can also move to the left most menu item by pressing <HOME>, or to the 

rightmost menu item by pressing <END>. 

Selecting a Menu Item

 There are two ways to select a menu item. The first way is to locate the item 

you want using the arrow keys, and to then press <ENTER>. 

Chapter 2 

Learning to Use the Menus
14 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 The second way is easier and faster. Notice that each menu item begins with a 

different letter. You can select a menu item by typing its first letter. For example, you 

could select LOAD by pressing the <L> key. 

Exiting a Menu

 Nearly every menu contains an item called QUIT. When you select QUIT, it 

returns you to the previous menu. If you make a mistake, and wind up with the wrong 

menu on screen, you can select QUIT and go back to where you were. Pressing the 

<ESC> key does the same thing as selecting QUIT. 

Exiting the FMAS software

 To exit the FMAS software, select QUIT from the MAIN MENU, and then 

select YES from the next menu. You are then returned to DOS. To run the FMAS 

software again, type FM2000 and press <ENTER>. 

 You may want to use DOS commands while you are running the FMAS. For 

example, you might want to copy a disk, format a disk, or view a directory. To do so, 

select DOS from the MAIN MENU. When you select DOS you will return to DOS. 

When you are ready to return to the FMAS, type EXIT and press <ENTER>. 
15 

Chapter 3 - Making Measurements 

 To begin making measurements, select MEASURE from the MAIN MENU. In 

a few moments, the measurement screen will appear. The top part of the measurement 

screen is a bar graph that gives you a quick indication of how each oscillator is 

performing. The bars represent channels 1 through 5 (from top to bottom). Figure 3.1 

shows a sample measurement screen: 

Figure 3.1. A sample measurement screen. 

 The bars are color coded. Each bar corresponds to one of the channel names 

shown at the bottom of the measurement screen. The channel names represent 

channels 1 through 5 (from top to bottom). Each channel name is next to a colored 

box. The box colors match the bar colors. For example, the top bar is orange. It 

represents the performance of the oscillator connected to channel 1. The top box is 

also orange. It marks the channel name for channel 1. 

Chapter 3 

Making Measurements
16 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 When you first start the system, the measurement screen shows a bar and a 

channel name for every channel in use. However, the bars extend only to the number 

"5" on the far left side of the screen. After the system has been running for between 1 

and 2 hours, the bars will extend further (assuming that the oscillator's performance is 

better than 1 × 10-5) and show the true performance of the oscillator. 

 The bars show the estimated frequency offset of each oscillator. The numbers 

at the top of the screen (from 5 to 13) indicate frequency offset values ranging from 1 

× 10-5 to 1 × 10-13. For example, if the bar extends exactly to 10, the frequency offset 

is 1 × 10-10. There are 10 divisions between the numbers. If the bar extends halfway 

between 10 and 11 (5 divisions past 10), the frequency offset is 5 × 10-11. The longer 

the bar, the smaller the frequency offset of the device under test. 

 The range of the system is 1 × 10-5 to 1 × 10-13. If the performance of an 

oscillator is worse than 1 × 10-5, the bar will still extend to the "5". If the performance 

of an oscillator is better than 1 × 10-13, the bar will extend only to the "13". Each bar 

also contains either a plus sign "+" or a minus sign "–". A plus sign means the 

oscillator is high in frequency. A minus sign means the oscillator is low in frequency. 

 When the system has been on for less than 24 hours, the bars are based on all 

of the available data. For example, if the system has been on for 12 hours, the bars 

show the oscillator's performance over the last 12 hours. However, when the system 

has collected data for more than 24 hours, only the most recent 24 hours are used. 

 The FMAS stores a new data point to disk every hour at 0 minutes and 0 

seconds UTC. For example, the FMAS stores data at 17:00:00 UTC, 18:00:00 UTC, 

and so on. Whenever the system stores data, it also updates the bars to reflect the 

oscillator’s performance over the last 24 hours. Stable oscillators will perform at the 

same level for long periods of time (many days), and so the length of the bars might 

not change very often. Even so, you should glance at the bars periodically to 

determine whether your oscillators are performing as expected. 

 Figure 3.1 shows the measurement screen of an FMAS with five channels 

connected. The top bar (for channel 1) extends all the way to the right. It shows that 

the frequency offset of UTC(NIST) is 1 × 10-13 or less. Channel 2 shows a quartz 

oscillator with a frequency offset of about 1 × 10-8. Channels 3 through 5 show 

devices whose frequency offset is measured in parts in 1010, 1011, and 1012, 

respectively. 

 
17 

Chapter 3 - Making Measurements 

 The screen area below the bar graph contains information about the 

measurements being made. The small red box contains the current date and the current 

time (UTC). This information is received by satellite from the GPS broadcast. As 

discussed earlier, the channel names are located inside the blue box (below the date 

and time values). 

 The "Difference" column shows the difference between the last two counter 

readings. This difference in expressed in nanoseconds. For example, if the last two 

counter readings are 11736.32 and 11735.34, the difference is 0.98 ns (11736.32 

minus 11735.34). 

 The "Count" column shows the most recent counter reading for each channel. 

The FMAS reads each counter channel in numerical order. The arrow moves each 

time a measurement is made, and points to the channel currently being read. 

 The FMAS has a number of powerful data analysis features that are described 

in Chapter 4. To use these features, you need to stop the measurements. However, 

you should always leave the system running in the measurement mode whenever you 

are not using the analysis software. The FMAS was designed to run 24 hours per day, 

7 days per week. Every 24 hours the FMAS stops taking readings and plots the data 

collected in the past 24 hours. It then starts taking readings again. The only time you 

are required to stop the measurements is when you change the frequency inputs 

connected to the system. Automatic plotting and changing the frequency inputs are 

discussed in the next two sections. 

AUTOMATIC PLOTTING

 The FMAS produces a daily calibration plot for every oscillator connected to 

the system. If two oscillators are connected there are two daily plots; if four are 

connected there are four daily plots, and so on. The plots are normally printed every 

day at 1600 UTC (to change the plot time, see Chapter 7). After the plots are printed, 

the FMAS starts to take readings again. Each plot shows the data recorded from the 

oscillator during the past 24 hours. 

 You do not have to wait for the daily plot time to print a plot of the last 24 

hours. You can print plots at any time by pressing <CONTROL-P>. If you have more 

than one channel, but want to print only the plot for Channel 1, press <CONTROL-P>, 
18 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

then press <ESC> while the plot is being printed. The FMAS will then go back to 

taking readings without printing the other plots. 

 A typical daily calibration plot is shown in Figure 3.2. The plot’s title is 

centered above the plot. The plot’s title is the channel name. The y-axis of the plot is 

labeled MICROSECONDS. The x-axis intercepts the y-axis at 0 μs. The scale of the 

plot is indicated by the numbers at the top and bottom of the y-axis. The plot has the 

number 100 printed at the top and bottom of the y-axis. This means that the areas 

above and below the x-axis each represent 100 μs. The total range of the y-axis is 200 

μs (from –100 to +100). 

Figure 3.2. A sample daily calibration plot.

 There are three lines of text below the plot. The top line shows the time and 

date when the first and last data points were recorded. Since the plot normally covers 

a period of 24 hours, the time of day should be the same for the first and last data 

points, but the date will be advanced by one day. 

 The second line shows the tick mark interval and the channel from which the 
19 

Chapter 3 - Making Measurements 

data was recorded. For a 24 hour plot, the tick marks are always spaced 1 hour apart. 

For example, the left edge of Figure 3.2 represents 1900 UTC, the first tick mark is 

2000 UTC, the second tick mark is 2100 UTC, and so on. 

 The bottom line shows the number of data points (usually 25 for a 24 hour 

plot), the estimated frequency offset of the oscillator, and the correlation coefficient (r). 

For more information about these values, please see Appendix A. 

 Each calibration plot is printed on a separate piece of paper. You may want to 

keep the plots in a notebook for future reference. 

 Please note that if the printer is turned off, is off-line, or is out of paper, the 

FMAS will not be able to print the plots. It will sound an alarm for 10 s when it tries to 

print each plot. If you do not fix the printer (turn it on or put it on-line) the FMAS will 

go back to making measurements without printing any plots. If you do not want or 

need the daily plots, you can leave the printer turned off. However, you should always 

keep the printer connected to the FMAS, or the system may not work properly. 

CHANGING OSCILLATORS 

 From time to time, you'll need to change, remove, or add oscillators to the 

FMAS. To do so, stop the system by pressing <CONTROL-X>. You are then 

returned to the MAIN MENU. Then complete the following steps: 

• Disconnect the oscillators that you want to remove 

and connect the new oscillators using the procedures 

described on pages 7 and 8. 

• Select NAMES from the MAIN MENU. Type in the 

new channel names and input frequencies as 

described on pages 10 through 12 and press <ESC> 

to save. If a channel is no longer used, delete the 

channel name by pressing the <DEL> key. 

• When you finish, press <ESC> to save the new 

channel names to disk. 

• Restart the measurements by selecting MEASURE. 

 When you restart the measurements, the bar for each new oscillator will be all 

the way to the left since the system does not yet have enough data to show the new 
20 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

oscillator's performance. After 1 to 2 hours, the bars will extend to show the 

performance of the oscillator. If you deleted a channel name, the bar for that channel 

will no longer appear on the measurement screen. 

 Whenever the system prints plots of the last 24 hours the plots will include only 

data from the new oscillator. This feature was included so that the 24 hour plots will 

never "mix" data from two different oscillators. For example, if you change oscillators 

at 1800 UTC and press <CONTROL-P> at 2300 UTC, the plots will include data from 

the last 5 hours only. However, the data from the oscillator that you replaced is still on 

the disk and can be retrieved at any time. For information about retrieving data, please 

see Chapter 5. 

QUICK CHECK OF SYSTEM

 When the system is taking measurements it requires only a small amount of 

operator attention. However, make a habit of regularly visually checking the system to 

make sure that everything is working properly. Once you know what to look for, the 

visual check takes just a few seconds. The things to look for are summarized below: 

• Is the printer ON, ON-LINE, and loaded with paper?

The printer must be on, on-line, and loaded with paper in order 

for the system to plot automatically. 

• Are the counter readings (displayed on screen) stable?

If the counter readings are not stable, they are printed in red. If 

this only happens on channel 1, it could mean that the GPS 

receiver has failed or temporarily lost the signal. If it happens 

on all channels, it could indicate a problem with your primary 

oscillator. If it happens on an individual channel other than 

channel 1, it could mean that the oscillator you are calibrating is 

defective or is producing a signal with a frequency offset larger 

than 1 × 10-5 with respect to its nameplate frequency. If you 

have just turned the oscillator on, this situation may occur for a 

few minutes while the oscillator warms up. 
21 

Chapter 3 - Making Measurements 

• Is any counter channel showing NO SIGNAL?

If you see the NO SIGNAL message printed in place of the 

counter readings, it means that the system cannot detect whether 

an oscillator is connected to the channel. Check the cable 

between the oscillator and the counter (it could be loose or 

disconnected). Or, the signal may be too weak to drive the 

counter. If you think this is the case, please contact NIST for 

assistance. 

• Are the oscillators performing as well as usual? 

A quick glance at the bar graph will indicate whether your 

oscillators are working the way they should. For example, if a 

bar is shorter than usual, you may have a problem with one of 

your oscillators or your GPS reception (if GPS is used as the 

reference on that channel). Contact NIST if you need 

assistance. 

23 

Chapter 4 - Analyzing Data 

 This chapter describes the powerful data analysis features of the FMAS. These 

features are used to analyze the data collected by the system. Some of the features are 

listed below: 

• Up to 3600 data points can be included on one graph (150 days of data). 

• Full-page, high-quality plots are printed. 

• Data from all five channels can be loaded at one time, so that you can quickly 

switch back and forth between channels. 

• Outlying data points can be removed. 

• Phase steps can be removed. 

• Any segment of data can be plotted. 

• The scale of the y-axis can be increased. 

• The plot title can be changed. 

• The position of the x-axis can be changed. 

• All data can be viewed on screen or printed. 

• Data statistics can be viewed or printed. 

 In this chapter, we'll illustrate each analysis feature using demonstration data 

that is provided with the FMAS. The next section tells how to load the demonstration 

data from disk. 

 Before beginning, you need to have the MAIN MENU on your monitor. If the 

system is currently taking readings, press <CONTROL-X> to get the MAIN MENU to 

appear. 

Chapter 4 

Analyzing Data
24 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Loading the Demonstration Data from Disk

 To load the demonstration data, select LOAD from the MAIN MENU. The 

LOAD MENU appears as shown below: 

 The LOAD MENU lists four items: ALL, SELECT, DEMO, and QUIT. ALL 

loads all the data on the hard disk. SELECT loads selected data from the hard disk. 

QUIT returns you to the MAIN MENU. Pressing <ESC> does the same thing as 

selecting QUIT. You may try this now. Select QUIT (or press <ESC>) to return to the 

MAIN MENU. Then return to the LOAD MENU by selecting LOAD again. 

 ALL and SELECT are very useful, and are described in detail in Chapter 5. 

They are used when you use your own data with the system. Right now, however, 

select DEMO to load the demonstration data. The demonstration data are included 

with the system and will be used throughout this chapter. 

 

 When you select DEMO, the counter in the center of the screen counts each 

data point as it is loaded. There are 96 demonstration data points. When all 96 points 

are read from the disk, the system returns to the MAIN MENU. 

 The next section tells how to view the data on screen. 

Viewing the Data on Screen

 Select VIEW from the MAIN MENU. The VIEW MENU appears as shown 

below: 

 QUIT returns you to the MAIN MENU. Pressing <ESC> does the same thing 

as selecting QUIT. 
25 

Chapter 4 - Analyzing Data 

 1[ONE], 2[TWO], 3[THREE], 4[FOUR], and 5[FIVE] refer to the time interval 

counter channels. 1[ONE] refers to channel 1, 2[TWO] to channel 2, and so on. When 

you select a channel, data from that channel appear on screen as shown in Figure 4.1. 

The measurement channel title is listed above the data. The column headings are: 

 Number is the number of the data point. 

 MJD is the Modified Julian Date when the data point was recorded. 

 DATE is the date when the data point was recorded. 

 UTC is the Coordinated Universal Time when the data point was 

 recorded. 

 Phase Shift is the oscillator phase shift (in nanoseconds) since 

 the last data point was recorded. 

 

Figure 4.1. The view data screen.
26 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 One screen can hold up to 15 data points. To move to the next screen, press 

<PgDn>. To move to the previous screen, press <PgUp>. To move to the first data 

point, press <HOME>. To move to the last data point, press <END>. Notice that you 

cannot move forward beyond the last data point, or backwards before the first data 

point. 

 At any time, you can print the data points on screen by pressing <ESC> and 

then selecting COPY. When you are finished using VIEW, select QUIT to return to 

the MAIN MENU. 

Printing the Data 

 COPY is used to print out all of the data for any channel. A total of 50 data 

points are printed per page. If you want this printout, select COPY and then select a 

channel number (1, 2, 3, 4, or 5). You can calculate the length of the printout by 

rounding ahead to the nearest number that is evenly divisible by 50, and then dividing 

by 50. In this case, we have 96 data points. The first number greater than 96 that is 

evenly divisible by 50 is 100. Since 100 divided by 50 is 2, the printout will be 2 

pages long. 

 When you finish using COPY, select QUIT to return to the MAIN MENU. 

Viewing the Data Statistics

 STATISTICS is used to view or print statistics for any channel. When you 

select a channel, you'll see a screen similar to Figure 4.2. 

 The statistics for the selected oscillator (in this case, a cesium) are printed 

inside the box. By looking at the box, we can see that 96 data points were recorded. 

The first data point was recorded on September 18, 1994 at 1700 UTC, and the last 

data point was recorded on September 22, 1994 at 1600 UTC, a period of 3 days, 23 

hours, and 0 minutes. During this time, the data range was 382.27 ns. The frequency 

offset of the cesium was +3.47 × 10-13, and the correlation coefficient (r) is +0.45 (see 

Appendix A for more information about these values). 

 By selecting 2[TWO], 3[THREE], 4[FOUR], or 5[FIVE] you can look at the 

statistics for the other channels. Whenever you want to print the statistics shown on 

screen, select COPY. 
27 

Chapter 4 - Analyzing Data 

Figure 4.2. The view statistics screen.

 The menu includes another item called FILTER that is used to smooth the data 

by removing phase steps and bad data points. When you select FILTER, the data are 

filtered and the statistics will change. If the filter is in use, the data filter indicator in 

the statistics box will be set to ON. FILTER will not do anything to data that is 

already smooth, but you may find it useful in certain instances. In the next section, 

we'll plot the data and look at some filtered plots. You will then have a better 

understanding of what FILTER does. 

 When you finish using STATISTICS, select QUIT to return to the MAIN 

MENU. 

Plotting the Data

 Select PLOT when you want to plot the data. The PLOT MENU appears 

listing eight items. The first five items are 1[ONE], 2[TWO], 3[THREE], 4[FOUR], 

and 5[FIVE]. These items work just as they did in the VIEW, COPY, and 

STATISTICS menus, except now they plot the data instead of performing the other 
28 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

functions. 1[ONE] plots the data for channel 1, 2[TWO] plots the data for channel 2, 

and so on. QUIT also works like before. Selecting QUIT returns you to the MAIN 

MENU. 

 The other menu items, EDIT and COPY, won't work unless a plot is on screen. 

If you try using them without a plot on screen, you'll receive a message on the second 

line of the menu telling you that there is no plot to EDIT or no plot to COPY. 

 We'll discuss EDIT a little later. As you can probably guess, COPY lets you 

print out a copy of the plot currently on screen. In order for COPY to work, the printer 

must be connected to the computer, turned on, on-line, and loaded with paper. If you 

cannot get a plot to print out, please check the status of your printer. If the printer is 

working properly, the menu disappears when you select COPY and a larger version of 

the plot appears on screen before it is copied to the printer. After the plot is printed, 

the menu and the original plot return to the screen. 

 For now, let's get started by selecting 1[ONE] to plot the demonstration data 

for channel 1. The plot is shown in Figure 4.3. 

Figure 4.3. Demonstration data for channel 1.
29 

Chapter 4 - Analyzing Data 

 Notice that the plot in Figure 4.3 contains a thick line showing the slope of the 

data obtained by doing a linear least squares curve fit. This line is printed on channel 

1 only. 

 The format of the plot is identical to the daily plots printed by the system. The 

title of the plot is the title that you entered using NAMES from the MAIN MENU. 

You may change the title if you wish (we'll discuss that feature later on). 

 The y-axis of the plot is labeled MICROSECONDS. The above plot has a yaxis scaled for 0.4 μs (from plus to minus 0.2 μs). 

 There are three lines of text below the plot. The top line shows the time and 

date when the first and last data points were recorded. The second line shows the tick 

mark interval and the channel from which the data was recorded. Figure 4.3 uses data 

recorded from Channel 1 of the time interval counter. The spacing between any two 

ticks on the x-axis represents 2 hours. The tick mark interval can range from 1 to 96 

hours, depending upon the number of data points. The third line below the x-axis 

shows the number of data points plotted (96), the frequency offset of the oscillator 

(+3.47 × 10-13), and the correlation coefficient (r) for the data (+0.45). This is the 

same information printed on the statistics screen in Figure 4.2. 

Editing the Plot on Screen

 The plot we just looked at is a good example of the results you can obtain when 

everything is working properly. The plot compared a GPS signal to a high-quality 

cesium oscillator. The fluctuations are quite small (tens of nanoseconds). Most of the 

time, the FMAS will produce good looking plots like this one. However, in some 

cases you may see a plot with a phase step or some outlying data points. To see an 

example of this type of plot, select 4[FOUR] to plot the demonstration data for channel 

4. The plot shown in Figure 4.4 will appear on your monitor. You can see that most 

of the data in Figure 4.4 are probably good. However, the bad data prevent us from 

seeing the good data. 

 We can solve this problem by using FILTER. To do so, select EDIT to get to 

the EDIT MENU and then select FILTER. When FILTER is selected, the data are 

replotted and the outlying data points and the phase step are automatically removed. 

The filtered plot for channel 4 is shown in Figure 4.5. 

 
30 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Figure 4.4. A plot with a phase step and some outliers. 

Figure 4.5. The Figure 4.4 graph after selecting FILTER.
31 

Chapter 4 - Analyzing Data 

 With the bad data removed, the filtered plot shows the true performance of the 

rubidium oscillator. Notice that the filtered plot has only 91 points, as opposed to 96 

on the original plot. Five data points were filtered out. Also, the new plot shows a 

frequency offset of +1.04 × 10-10. This is the true frequency offset of the oscillator (it 

formerly was +6.47 × 10-10). Also, removing the bad data reduced the scale of the plot 

from 4000 to 40 μs. 

 FILTER works well with most types of data. However, if the plot has so much 

bad data that it is hard to separate bad data from good data, FILTER may not help. 

Also, FILTER won't do anything to data that have a range of less than 1 μs, or data 

that are perfectly linear (r equals 1 or –1). If this is the case, you'll get the following 

message on the second line of the menu, "The system cannot FILTER this data". 

 FILTER works best with plots where most of the data are good (like the plot we 

just looked at), and where only a few points need to be filtered. If this is the case, 

FILTER removes the bad data and shows you the true performance of your oscillator. 

If the plot is filtered, you can tell by looking at the channel label below the x-axis. For 

example, you'll see the words "Channel 4" for a regular plot, and "Channel 4 Filtered" 

for a filtered plot. 

 As you probably have noticed, there are a number of other editing features on 

the EDIT menu (a total of eleven). We've already covered four of them. To review, 

COPY lets you make copies of the plot on screen. FILTER removes the bad data from 

the plot on screen. RESET restores the plot on screen to its original status. QUIT 

returns you to the PLOT MENU. The other seven menu items let you change, or edit, 

the plot on screen. These items are explained on the following pages. We'll begin with 

a discussion of LO and HI. Before trying LO and HI, select RESET to get the original 

plot back on screen. 

Removing Data Points from the Plot

 LO and HI remove data points from the plot. LO removes the lowest (or 

minimum) value from the plot. HI removes the highest (or maximum) value from the 

plot. To see how LO and HI work, select LO. A new plot appears on screen with the 

minimum data point removed. However, the plot still contains another "spike" caused 

by the highest valued data point. Select HI to remove this data point. You will now 

see the plot shown in Figure 4.6. 
32 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Figure 4.6. The figure 4.4 plot with the outliers removed. 

 Notice that the plot labels now show that the plot has 94 data points (two data 

points were removed). 

 LO and HI are different from FILTER. FILTER adjusts outlying data points 

and phase steps by comparing good data to bad data, and can remove multiple data 

points. LO and HI can remove only one data point at a time, and this point has to be 

either the minimum or maximum point. However, as with FILTER, the data points 

removed by LO and HI are not deleted from the disk or from memory, they are just no 

longer plotted. If you select RESET the original plot returns to the screen. Also, if 

you try using LO and HI on a filtered plot, you'll get an error message on the second 

line of the menu. 

 

 When you are finished using LO and HI, select RESET to restore the plot on 

screen to its original status. The next section tells how to plot a selected portion of the 

plot on screen. 
33 

Chapter 4 - Analyzing Data 

Plotting a Portion of the Plot

 PORTION lets you plot a selected portion of the plot on screen. The following 

example tells how to plot just the first 10 (out of 96) data points, using the data from 

Channel 1. 

 Select PORTION. You'll be prompted (on the second line of the menu) to enter 

the numbers for the first and last data points that you want to plot. Enter 1 for the first 

data point (and press <ENTER>), and enter 10 for the last data point (and press 

<ENTER>). A new plot appears containing only the first 10 data points, as shown in 

Figure 4.7. Notice that the data point label below the plot has changed from 96 to 10. 

Figure 4.7. Plotting a portion of the channel 1 demonstration data.

 PORTION is useful for "blowing up" a selected segment of the plot. Try 

plotting other segments of the data. You have 96 data points in memory. The first 

data point can be any number from 1 to 95. The second data point can be any number 

from 2 to 96, as long as it is greater than the number for the first data point. The 

system won't accept numbers outside these ranges. 

 
34 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 You can use FILTER to adjust any data portion that is plotted on screen. You 

can also get a copy of any portion by selecting COPY. You can't, however, use 

PORTION to plot a portion of a filtered plot. If you try this, you'll receive an error 

message on the second line of the menu. When you finish using PORTION, select 

RESET to restore the plot to its original status. You may now go on to the next 

section. 

YAXIS, XAXIS, and MAGNIFY

 YAXIS lets you change the scale of the y-axis. The current scale of the y-axis 

is 0.4 μs (from plus to minus 0.2 μs). To change the scale of the y-axis, select YAXIS. 

A prompt appears on the second line of the menu asking you to enter the new scale (in 

microseconds). Type 4 and press <ENTER>. The data are replotted, and the scale is 

increased from 0.4 to 4 μs. You can see that the variations in the Channel 1 data are 

very small. When a 4 μs scale is used, the data is almost a straight line. 

 You may find changing the scale to be useful. You can even set all your plots 

to the same scale so you can lay them side by side for comparison. The scale can only 

be increased, however. If you enter a scale smaller than the original scale, the data are 

still plotted with the original scale. At any point, you can return to the original plot by 

selecting RESET. 

 One problem with using YAXIS is that the data are often plotted on top of the 

x-axis when you increase the scale. This makes it difficult to see the data. XAXIS lets 

you change the position of the x-axis so that you can see the data better. To illustrate 

this, select XAXIS now. The data are now plotted with the x-axis on the bottom of the 

plot instead of in the middle, and are now easier to see. This plot is shown in Figure 

4.8. You can use COPY to make copies of plots with the x-axis at the bottom position. 

 XAXIS is a toggle function. If you select it again, the x-axis returns to the 

middle position. 

 MAGNIFY is another useful feature. When you select MAGNIFY, the data are 

plotted using the range of the data as the y-axis scale. This allows you to "magnify" 

the data and show as much detail as possible. Like XAXIS, MAGNIFY is a toggle 

function. If you select it again, the y-axis scale returns to its original value. 
35 

Chapter 4 - Analyzing Data 

Figure 4.8. A plot with the x-axis moved to the bottom position.

 

 Keep in mind that YAXIS, XAXIS, and MAGNIFY change only the 

appearance of the data as plotted on screen. The actual data (and the frequency offset) 

remain the same. 

Changing the Title of the Plot

 The last item on the EDIT MENU that we need to discuss is TITLE. TITLE 

lets you change the title of the plot. The new title appears on the plots on screen, and 

also on the plots that are printed. 

 To illustrate this, select TITLE now. A prompt appears on the second line of 

the menu asking you to enter a new plot title. Type in a new title and press <ENTER>. 

The new title replaces the old one in the plot on screen, and in any copies of the plot 

that you make. If you select RESET, the original plot is put back on screen along with 

its original title. 

 When you finish editing plots, select QUIT to go back to the PLOT MENU. 

Select QUIT again to go back to the MAIN MENU. 

37 

Chapter 5 - Retrieving Your Data 

 In Chapter 4, we explained how to use the data analysis features of the FMAS. 

We used the demonstration data in all of our examples. Of course, you can use the 

same features on your own data. In this chapter, we'll explain how to retrieve the data 

collected by your system. 

 To retrieve your data, stop the system from making measurements (if 

necessary) by pressing <CONTROL-X>. Then, select LOAD from the MAIN MENU. 

The LOAD MENU appears, listing four items: 

 As usual, QUIT returns you to the MAIN MENU. Pressing <ESC> does the 

same thing as selecting QUIT. DEMO loads the demonstration data that we used in 

Chapter 4. The other items (ALL and SELECT) are explained below. 

Retrieving All of the Data

 ALL loads all the data that the system has available. When you select ALL, 

this message appears on the second line of the menu: 

Are you sure? 

 If you decide not to use ALL, press <ENTER>. Otherwise, enter a "Y" or "y" 

and press <ENTER>. The counter in the center of the screen counts each data point as 

it is loaded. When all the data points are loaded, the system returns to the MAIN 

MENU. 

 The FMAS has a maximum storage capacity of 3600 data points. If more than 

3600 data points are available, ALL stops loading data when it gets to 3600. 

Chapter 5 

Retrieving Your Data
38 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Figure 5.1. The file load window. 

Retrieving Some of the Data

 SELECT loads selected data from the disk. When you choose SELECT, a box 

appears that allows you to "tune in" the data that you want to see. You can load any of 

the data listed in the box. For example, if data are available from 09/18/96 to 

10/07/96, you can load data recorded within that time period. A picture of the "tuning 

box" is shown in Figure 5.1. 

 Use the <UP ARROW> and <DOWN ARROW> keys to scroll through the lists 

of available dates one day at a time. Use <PgUp> to move back 30 days. Use 

<PgDn> to move ahead 30 days. If you decide to exit the SELECT function and go 

back to the LOAD MENU, press <ESC>. 

 When you scroll through the dates in the tuning box, notice that three dates are 

always shown. The currently selected date is highlighted. The two dates adjacent to 

the currently selected date are shaded. Locate the start date for the data you want to 

load, and press <ENTER>. Then, locate the end date for the data you want to load and 

press <ENTER> again. After a few seconds, the system will load the data you 

selected. A counter will briefly appear in the center of the screen as the system loads 

each data point. The system then returns to the MAIN MENU. 
39 

Chapter 6 - Utilities 

 The FMAS has several utility features that you should know about. To use 

these features, select UTILITIES from the MAIN MENU. You'll see the menu shown 

below: 

 As always, select QUIT or press <ESC> to leave this menu and go back to the 

MAIN MENU. The other four menu items, SHORTTERM, HISTORY, GPS, and 

CALIBRATETIC are discussed below. 

ShortTerm

 This feature allows you to look at the short-term performance of an oscillator 

by taking readings for a period from 2 to 3600 s. You can then produce graphs that 

show the frequency offset and stability of the oscillator. When you select 

SHORTTERM, the menu shown below appears on your monitor: 

 

 Three items on the SHORTTERM MENU are used to set up a short term 

measurement run. These items are: 

 PICKCHANNEL - This item is used to select the channel that you want to 

use for the measurement. The default channel number is 1. However, you can take 

measurements from any of the five channels. 

Chapter 6 

Utilities
40 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 LENGTH - This item allows you to select the length of the measurement run 

in seconds. The default length is 10 s, but you can make measurements for any period 

from 2 to 3600 s. 

 GO - After you have selected a channel and the length of the measurement 

run, select GO to start the measurements. 

 When the system begins taking measurements, the message "Measurement Run 

in Progress" will appear. The system will complete a new data point every second. 

When the measurements are finished, this message will change to "Measurement Run 

Completed". The screen will then look similar to the screen shown in Figure 6.1. 

Figure 6.1. The short term measurement screen.

 At this point you can graph the short term frequency offset of the oscillator 

under test by selecting OFFSET. A sample graph is shown in Figure 6.2. The plot 

shows each individual data point recorded during the measurement run. It shows the 

start time for the measurement run, the duration of the run, the number of data points 
41 

Chapter 6 - Utilities 

plotted, and the frequency offset and correlation coefficient (r) of the data. You can 

print a copy of the plot on screen by selecting COPY from the menu. 

 You can also plot the stability of the oscillator under test by selecting 

STABILITY. A stability plot is also shown in Figure 6.3. The technique used to 

estimate stability is the Allan deviation. Unlike an uncertainty plot, the stability plot 

doesn't show how close the oscillator is to the correct frequency. Instead, it shows how 

closely the oscillator produces the same frequency over a time interval. By doing so, it 

can tell us a lot about the quality of the oscillator under test. 

Figure 6.2. A short term frequency offset graph.

 To illustrate the difference between frequency offset and stability, consider a 10 

MHz oscillator that is high in frequency by 1 part in 10 million (1 × 10-7). This means 

that the oscillator is actually high in frequency by 1 Hz, and is producing 10 000 001 

Hz instead of 10 000 000 Hz. If we measure this same frequency offset day after day, 

we know that the oscillator is stable enough to produce frequency closer to the correct 

value. In this case, we should set the oscillator closer to its correct frequency to take 

advantage of its full potential. A stability plot provides a quick estimate of the inherent 

stability of an oscillator, even if the oscillator has a large frequency offset. 
42 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Figure 6.3. A short term stability graph.

 Stability graphs use a logarithmic scale. The values along the x-axis (called tau 

values) represent the length of the averaging period in seconds. Each division 

represents an averaging period 10 times longer than the previous division. For 

example, a tau value of 2 represents an averaging period of 100 s (102). A tau value of 

3 represents an averaging period of 1000 s (103

). The data points (rectangles) 

represent measurement periods whose length is a power of 2. For example: 

 20 = 1 s 

 21

 = 2 s 

 22

 = 4 s 

 23 = 8 s 

 24

 = 16 s 

 25 = 32 s 

 26 = 64 s 

 27

 = 128 s 

 28

 = 256 s 

 29 = 512 s 

 210 = 1024 s 

 
43 

Chapter 6 - Utilities 

 We need at least two samples to estimate stability. This means that to estimate 

stability at 4 s, we need to measure for at least 8 s (two 4 s samples). To estimate 

stability at 1024 s we must measure for at least 2048 s (two 1024 s samples). Keep 

this in mind when creating stability graphs. 

 The values along the y-axis represent the result of the measurement. A value 

of –10 means that the oscillator has a stability of 1 × 10-10. This stability value should 

never be confused with the frequency offset value. For example, an oscillator with a 

frequency offset of 1 × 10-8 might be stable to 1 × 10-12 at 1000 s. This means that even 

though the oscillator is producing a frequency with a large offset, the frequency offset 

isn’t changing very much. 

 A typical stability graph (such as Figure 6.3) shows the stability values 

improving as the measurement period increases in length. During this period, phase 

noise from the oscillator, the reference, and the measurement system is being removed 

by averaging. This improvement continues until the oscillator noise floor is reached, 

and no further gains are made by averaging. When the noise floor is reached, the 

stability values level out, and then eventually start to get worse. Most precision 

oscillators reach their noise floor at an averaging period of about 1000 s. Obviously, 

we want the measurement system and the reference to have lower noise floors than the 

device under test for the selected averaging period. For this reason, stability 

measurements should be made only on channels 2 through 5 using your primary 

oscillator as the reference. Although GPS has excellent long term stability, short term 

stability graphs tend to be dominated by GPS phase noise, since it might take hours or 

days of averaging for GPS to reach its noise floor. 

 Keep in mind that the FMAS is designed for long term measurements. Its noise 

floor is too high to accurately measure the stability of the best oscillators at short 

averaging times (such as 1 s). However, at longer averaging times it can measure the 

stability of nearly any oscillator, since its noise floor is near 1 × 10-13 at 100 s, and near 

1 × 10-14 at 1000 s. 

Retrieving Short Term Measurement Data

 Each time a short term measurement run is completed, the data are 

automatically saved. You can retrieve these files and review the graphs at a later time. 

To do so, select FILE. A menu appears listing the available file names. After you 

select the file you want, it is automatically loaded into memory. You can then use 

OFFSET and STABILITY to plot the data, and COPY to copy the plots to the printer. 

 
44 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 The first five digits of the short-term filenames represent the Modified Julian 

Date (MJD). This is followed by a hyphen. The next two digits represent the UTC 

hour when the data were recorded. This is followed by a period. The next two digits 

represent the UTC minute when the data were recorded. And finally, the last digit in 

the filename is the number of the channel from which the data were taken. 

 To illustrate this, consider the filename "49629-16.293". This file was 

recorded on MJD 49629 at 16 hours and 29 minutes UTC. The data were taken from 

channel 3. 

 

 When you are finished making short-term measurements, you can return to the 

UTILITIES MENU by selecting QUIT or by pressing <ESC>. 

Adjusting an Oscillator Using METER

 The FMAS also includes a "frequency meter" that is a useful tool for adjusting 

oscillators. To use this feature, select METER from the UTILITIES MENU. The 

frequency meter will appear on screen. 

 The frequency meter estimates and displays the current frequency offset of the 

oscillator. The possible frequency offset values range from 1 × 10-7 to 1 × 10-10. The 

yellow line on the meter indicates the current value. By adjusting the oscillator, you 

can try to get the yellow line as close to the center of the meter (1 × 10-10) as possible. 

 METER is mostly useful for adjusting quartz oscillators, or rubidium 

oscillators that are well off in frequency. Keep in mind that adjustments must be made 

slowly and carefully, and oscillators will not always "stay" at the frequency where you 

set them. Also, oscillators should have gone through their full warm-up period before 

you even try to make adjustments. 

 When you finish using METER, press any key to return to the menu. 

History

 HISTORY is a powerful feature of the FMAS that allows you to look at the 

performance history of your primary oscillator. HISTORY produces plots similar to 

those that you receive as part of your monthly calibration report (see Appendix B). 

 
45 

Chapter 6 - Utilities 

 Each day, the FMAS records the frequency offset of the primary oscillator over 

the last 24 hours. This information is stored, and one file is kept for each month that 

the system has been running. For example, if your system has been running for three 

months, you will have three files. By using HISTORY, you can plot the information 

in these files, and see how your primary oscillator has performed over past months. 

 When you select HISTORY, a menu appears with three items; PLOT, COPY, 

and QUIT. QUIT and COPY work as explained earlier. QUIT returns you to the 

previous menu (the UTILITIES MENU in this case), and COPY prints a copy of the 

plot on screen if one is available. PLOT produces a list of all of the available 

frequency files. The files are named using the month and year when they were 

created. For example, "10-1996" is the file for October 1996. 

 The menu sorts the files so that the most recent file is at the top of the list. The 

most recent file is already highlighted. To select this file, simply press <ENTER>. To 

select another file, use <DOWN ARROW> to move down, <UP ARROW> to move 

up, and so on. When the file you want is highlighted, press <ENTER>. You will then 

see a plot similar to Figure 6.4: 

Figure 6.4. A sample HISTORY graph. 
46 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 Figure 6.4 shows the performance of a primary frequency standard (cesium 

oscillator) during the month of October 1996. Each marker represents the performance 

of the cesium oscillator for one day. The x-axis shows the days of the month. Each 

tick mark represents one day. If no data are recorded on a given day (for example, if 

the system is turned off), no marker will be present. 

 The y-axis shows the frequency offset of the primary oscillator. The middle of 

the y-axis represents a frequency offset of ±1 × 10-13 or better. In this example, the 

frequency offset of the primary oscillator was better than ±1 × 10-12 for all but two 

days of the month. 

 The history plot can accommodate just about any kind of oscillator. If an 

oscillator is high in frequency, the markers will appear in the top half of the plot. 

Conversely, if an oscillator is low in frequency, the markers will appear in the bottom 

half of the plot. The plot shows values as large as ±1 × 10-8. If an oscillator is high or 

low in frequency by more than that, the markers will appear along the top or bottom 

line of the plot. 

 The important thing to look for on the history plots is stability. For example, if 

your primary oscillator is a low cost quartz unit, it might not be capable of 

performance much better than 1 × 10-9. However, if your system and oscillator are 

operated properly, your measurements should be able to reproduce this value day after 

day, and your history plot should show a nearly straight line of markers. 

GPS

 The GPS menu item shows the current status of GPS reception. When you 

select this option you'll see a screen similar to the one shown in Figure 6.5. This 

screen was included as a diagnostic tool that can assist in troubleshooting GPS 

reception problems. NIST will routinely check your GPS reception by modem. If you 

think you have a problem with your data (or if you are interested), you might want to 

check it also. Please contact NIST when you have questions, need assistance, or 

notice any condition that indicates a problem. The text below Figure 6.5 annotates 

each value shown on the GPS status screen. 

 
47 

Chapter 6 - Utilities 

Figure 6.5. The GPS status screen.

Tracking Status Byte - This byte provides diagnostic information about the receiver. 

It will normally have a value of 8, which means that more than three satellites are being 

successfully received, the almanac is good, and the receiver is in position hold mode. 

For brief periods of time, a value other than 8 might be shown (usually a 32), but it 

should normally be at 8 if the receiver is working properly. 

Satellites (Visible/Tracked) - These two values show the number of satellites within 

view of the receiver and the number of satellites actually being tracked. Often, the two 

values will be equal. However, as many as 12 or 13 GPS satellites could theoretically 

be in view, and the receiver can track a maximum of 8 satellites at one time. 

Timing Stability (ns) - This is an internal estimate made by the receiver of the 

stability (standard deviation) of the 1 Hz output signal. It will normally be a value 

smaller than 50. Keep in mind that this is not a measured value, but simply a rough 

estimate provided for diagnostic purposes. The actual stability of the receiver should 

be much better than this estimate. 
48 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

UTC Date and Time - These values should be correct, but the time might be off by a 

fraction of second due to delays in the screen updates. However, if you see something 

unusual (such as the wrong date) it indicates a receiver problem. If this is the case, 

please call NIST for assistance. 

Pulse Status - This indicates whether the 1 Hz output on the receiver is turned on or 

off. It should always be set to ON if the receiver has completed the acquisition process 

and is working properly. 

The bottom part of the GPS status screen shows information about the satellites being 

tracked (the receiver tracks as many as eight satellites). New satellites are constantly 

coming into view and this information is always being updated. Five columns of 

information are provided for each satellite: 

PRN Number - The pseudo random noise (PRN) number of the satellite being 

tracked. Each satellite in the GPS constellation is identified by a unique PRN number. 

The GPS constellation was designed to use 24 satellites, but more than 24 may be in 

orbit at a given time. For this reason, the possible PRN numbers range from 1 to 32. 

Health (S/T) - This box indicates the “health” of the satellite and whether or not it is 

being used by the receiver to produce the timing pulse. An H indicates that the 

satellite is healthy, a U indicates that it is not healthy. An ON indicates that the 

satellite is being used in the timing solution, OFF indicates that it is not. The box will 

normally contain this notation, H/ON, which means that the satellite is healthy and is 

being used in the timing solution. On rare ocassions, satellites do broadcast bad data 

and are marked as unhealthy. However, the receiver is designed to automatically 

remove bad satellites from the timing solution. Therefore, the probability of a U/ON 

message being shown is extremely low. 

Elevation - The vertical angle measured from the horizon up to the satellite. The 

receiver automatically selects those satellites that have the most favorable elevation 

angle. Once a satellite drops below the horizon (below 0o elevation), the receiver 

begins tracking another satellite in its place. GPS was designed so that at least four 

satellites are above the horizon (in view) at all times. If fewer than four satellites are in 

view, it could mean that your antenna has an obstructed view of the sky. 

Offset (ns) - This value (in nanoseconds) represents the time offset of the satellite with 

respect to the average time offset off all of the satellites being tracked. Since all 

satellites are usually in very close agreement with each other, this value typically 

ranges from about –20 to +20 ns. 
49 

Chapter 6 - Utilities 

Carrier to Noise - The signal-to-noise ratio for the signal being tracked. A number 

of 45 or higher is considered to be very good, with 51 the maximum value. A number 

less than 40 indicates a possible reception problem, but a 0 could simply mean that the 

receiver is still processing data for the specified satellite and has yet to produce a value. 

If everything is working properly, you should see numbers of 40 or higher for all 

satellites. Lower numbers could mean that you are using a cable with excessive signal 

loss. In this case, either a lower loss cable should be used, or the length of the cable 

should be shortened. Please contact NIST if you need assistance. 

 When you finish viewing the GPS status box, press <ESC> to return to the 

UTILITIES MENU. 

CalibrateTIC

 The time interval counter inside the FMAS is automatically calibrated each 

time the system starts and is recalibrated every 24 hours while the system is running. If 

you select the CalibrateTIC menu item, it automatically calibrates the counter and 

displays the calibration values on screen. Although there is usually no reason to do 

this, it was included as a diagnostic tool that NIST can use to see whether the counter is 

working properly. 

51 

Chapter 7 - Setup Software 

 The software used by the FMAS was installed at NIST before it was shipped to 

your site. We have included a setup program with the software called FMSETUP. 

You may never need to use FMSETUP, but we’ll discuss it here in case you are 

interested. 

 To use the setup software, type FMSETUP from the DOS prompt and 

<ENTER>. You will see a screen similar to the one in Figure 7.1. 

Figure 7.1. The FMSETUP screen. 

 The screen contains 12 boxes of information. You can use the arrow keys to 

highlight any of these boxes and then change the information in the box. Be sure to

consult with NIST before changing any setting. The 12 boxes of information are: 

Chapter 7 

Setup Software
52 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Pathname for Software - This is where the file FM2000.EXE (or a 

file with a similar name) is located. This executable file controls the 

entire system. 

Pathname for Phase Data - This is where the system stores the data it 

records when you use MEASURE to make measurements. When the 

system saves data to disk each hour, the data are stored in this directory. 

Pathname for Demo Data - This is where the system stores the 

demonstration data used in the examples in Chapter 4. 

Pathname for Frequency Data - This is where the system stores the 

daily frequency offset values for the primary oscillator. These data can 

be retrieved with HISTORY (see Chapter 6) and are used by NIST to 

produce the monthly calibration reports. 

Pathname for Short Term Data - This is where the system stores the 

data it records when you use SHORTTERM to make measurements. 

Plot Time (UTC hour, 0-23) - This setting determines the UTC hour 

when the system produces the daily phase plot. This is normally set to 

16. This means that the system will plot automatically at 1600 UTC 

every day. If you change this number to 17, the system will plot at 1700 

UTC every day, and so on. You may set the plot time to any hour that 

you choose. However, you should not make a habit of changing the plot 

time. Once you find a setting that is convenient for you, we recommend 

that you leave the plot time at that setting. 

Is Tape Drive Present? (Y or N) - This setting remains from earlier 

versions of the FMAS which used a hard drive/tape drive combination, 

and periodically backed up the contents of the hard drive to the tape 

drive. Since the FMAS now uses a solid state flash disk to store data, 

this value should always be set to “N”. 

Is Counter Present? (Y or N) - This setting specifies whether the 

system has a time interval counter present. Of course, the FMAS 

includes a time interval counter inside the computer system, so the 

normal setting is "Y". However, by changing the setting to "N", the 

software will run on a computer without a time interval counter 

installed. Obviously, measurements can't be made on a system without 
53 

Chapter 7 - Setup Software 

a counter, but the plotting and data analysis features of the software will 

still be usable. 

Is Receiver Present? (Y or N) - This setting specifies whether the 

system has a GPS receiver present. Since a GPS receiver is part of the 

standard FMAS equipment, the normal setting is "Y". If the setting is 

"N", most of the features of the system will still work. However, the 

time on screen will no longer be synchronized to GPS (it will come 

from the computer’s clock), and the GPS option on the UTILITIES 

menu will be unusable. Of course, oscillator to oscillator comparisons 

will still be usable. 

Use Channel 1 Filter? (Y or N) - The FMAS has a FILTER function 

(discussed in Chapter 4) that can remove phase steps and outliers from 

the data. This option allows you to automatically use this feature on the 

channel 1 data. This means that when the system plots, it will 

automatically produce graphs that have the filter turned on. The bars on 

the measurement screen will still indicate the performance of your 

primary oscillator with the filter turned off, so you can be alerted to 

possible problems. This feature was designed to only remove phase 

steps caused by a disturbance of the measurement system, and not phase 

changes that indicate a frequency change in your primary oscillator. 

Use GPS on Channels 2-5? (Y or N) - This setting pertained to an 

earlier version of the FMAS software and is now ignored. GPS can 

now be used on channels 2 through 5 without changing the value here. 

Printer - This setting tells the system what kind of printer is 

connected. The standard printer supplied with the FMAS is either a dot 

matrix with a cut sheet feeder (use the C setting), or an inkjet printer 

with a cut sheet feeder (use the L setting). Both types of printer allow 

each plot to be printed on an individual sheet. However, the system 

also supports a dot matrix using roll paper (R setting), or a laser printer 

(L setting). There is also a memory-resident printer driver that might 

need to be changed, so simply changing this setting might not be all that 

is required when changing printers. Contact NIST if you need 

assistance. 

 When you finish using FMSETUP, press <ESC> to save the changes you have 

just made and go back to DOS. To go back to DOS without saving any changes, press 

<END>. 

55 

Appendix A - An Introduction to Frequency Calibrations 

 Frequency is the rate of occurrence of a repetitive event. If T is the period of a 

repetitive event, then the frequency f = 1/T. The International System of Units (SI) 

states that the period should always be expressed in units of seconds (s), and the 

frequency should always be expressed in hertz (Hz). The frequency of electrical 

signals often is measured in units of kilohertz (kHz) or megahertz (MHz ), where 1 

kHz equals one thousand (103

) cycles per second and 1 MHz equals one million (106) 

cycles per second. 

 Average frequency over a time interval can be measured very precisely. Time 

interval is one of the four basic standards of measurement (the others are length, mass, 

and temperature). Of these four basic standards, time interval (and frequency) can be 

measured with the most resolution and the least uncertainty. In some fields of 

metrology, one part per million (1 × 10-6) is considered quite an accomplishment. In 

frequency metrology, measurements of one part per billion (1 × 10-9) are routine, and 

even one part per trillion (1 × 10-12) is commonplace. 

 

 Devices that produce a known frequency are called frequency standards. These 

devices must be calibrated so that they remain within the tolerance required by the 

user’s application. Let’s begin our discussion with an overview of frequency 

calibrations. 

Overview of Frequency Measurements and Calibration

 Frequency calibrations measure the performance of frequency standards. The 

frequency standard being calibrated is called the device under test (DUT). In most 

cases, the DUT is a quartz, rubidium, or cesium oscillator. In order to perform the 

calibration, the DUT must be compared to a standard or reference. The standard 

should outperform the DUT by a specified ratio in order for the calibration to be valid. 

This ratio is called the test uncertainty ratio (TUR). A TUR of 10:1 is preferred, but 

not always possible. If a smaller TUR is used (5:1, for example) then the calibration 

will take longer to perform. 

 Once the calibration is completed, the metrologist should be able to state how 

close the DUT's output is to its nameplate frequency. Often called the nominal 

Appendix A 

An Introduction to Frequency Calibrations
56 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

frequency, the nameplate frequency is labeled on the oscillator’s output. For example, 

a DUT with an output labeled "5 MHz" is supposed to produce a 5 MHz frequency. 

The calibration measures the difference between the actual frequency and the 

nameplate frequency. This difference is called the frequency offset. There is a high 

probability that the frequency offset will stay within a certain range of values, called 

the frequency uncertainty. The user specifies an uncertainty requirement for the 

frequency offset that the DUT must meet or exceed. In many cases, users base their 

requirements on the specifications published by the manufacturer. In other cases, they 

may "relax" the requirements and use a less demanding specification. Once the DUT 

meets specifications, it has been successfully calibrated. If the DUT cannot meet 

specifications, it fails calibration and is repaired or removed from service. 

 The reference used for the calibration must be traceable. The International 

Organization for Standardization (ISO) definition for traceability is: 

The property of the result of a measurement or the value of a standard 

whereby it can be related to stated references, usually national or 

international standards, through an unbroken chain of comparisons all 

having stated uncertainties [1]. 

 In the United States, the "unbroken chain of comparisons" should trace back to 

the National Institute of Standards and Technology (NIST). In some fields of 

calibration, traceability is established by sending the standard to NIST (or to a NISTtraceable laboratory) for calibration, or by sending a set of reference materials (such as 

a set of artifact standards used for mass calibrations) to the user. Neither method is 

practical when making frequency calibrations. Oscillators are sensitive to changing 

environmental conditions and especially to being turned on and off. If an oscillator is 

calibrated and then turned off, the calibration could be invalid when the oscillator is 

turned back on. In addition, the vibrations and temperature changes encountered 

during shipment can also change the results. For these reasons, laboratories should 

always make their calibrations on-site. 

 Fortunately, we can use transfer standards to deliver a frequency reference 

from the national standard to the calibration laboratory. Transfer standards are devices 

that receive and process radio signals that provide frequency traceable to NIST. The 

radio signal is a link back to the national standard. Several signals are available, 

including NIST radio stations WWV, WWVH, and WWVB, and radionavigation 

signals from LORAN-C and GPS. Each signal delivers NIST traceability at a known 

level of uncertainty. The ability to use transfer standards is a tremendous advantage. It 

allows traceable calibrations to be made simultaneously at a number of sites as long as 
57 

Appendix A - An Introduction to Frequency Calibrations 

each site is equipped with a radio receiver. It also eliminates the difficult and 

undesirable practice of moving frequency standards from one place to another. 

 Once a traceable transfer standard is in place, the next step is developing the 

technical procedure used to make the calibration. This procedure is called the 

calibration method. The method should be defined and documented by the laboratory, 

and ideally a measurement system that automates the procedure should be built. ISO/

IEC Guide 17025, General Requirements for the Competence of Testing and 

Calibration Laboratories, states: 

The laboratory shall use appropriate methods and procedures for all 

tests and/or calibrations within its scope. These include sampling, 

handling, transport, storage and preparation of items to be tested and/

or calibrated, and, where appropriate, an estimatation of the 

measurement uncertainty as well as statistical techniques for analysis of 

test and/or calibration data. 

 In addition, Guide 17025 states: 

The laboratory shall use test and/or calibration methods, including 

methods for sampling, which meet the needs of the client and which are 

appropriate for the test and/or calibrations it undertakes .... When the 

client does not specify the method to be used, the laboratory shall select 

appropriate methods that have been published either in international, 

regional, or national standards, or by reputable technical 

organizations, or in relevant scientific texts or journals, or as specified 

by the manufacturer of the equipment [2, 3]. 

 Calibration laboratories, therefore, should automate the frequency calibration 

process using a well documented and established method. This helps guarantee that 

each calibration will be of consistently high quality, and is essential if the laboratory is 

seeking ISO registration or laboratory accreditation. 

 Now that we’ve provided an overview of frequency calibrations, we’ll take a 

more detailed look at the topics introduced. We'll begin by looking at the 

specifications used to describe a frequency calibration. Then, we’ll discuss the various 

types of frequency standards and transfer standards. We’ll conclude with a discussion 

of how the NIST Frequency Measurement and Analysis Service provides a complete 

solution to the frequency calibration problem. 
58 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

The Specifications: Frequency Offset and Stability

 In this section, we'll look at the two main specifications of a frequency 

calibration, frequency offset and stability. We'll define frequency offset and stability 

and show how they are measured. Keep in mind during this discussion that frequency 

offset is often referred to simply as accuracy (or frequency accuracy), and that 

stability is nearly the same thing as frequency uncertainty. 

Frequency Offset

 When we make a frequency calibration, our measurand is a DUT that is 

supposed to produce a specific frequency. For example, a DUT with an output labeled 

5 MHz is supposed to produce a signal at a frequency of 5 MHz. Of course, the DUT 

will actually produce a frequency that isn't exactly 5 MHz. After we calibrate the 

DUT, we can state its frequency offset and the associated uncertainty. 

 Measuring the frequency offset of a DUT requires comparing it to a reference. 

This is normally done by making a phase comparison between the frequency produced 

by the DUT and the frequency produced by the reference. There are several calibration 

methods (described later) that allow us to do this. Once we know the amount of phase 

deviation and the measurement period, we can estimate the frequency offset of the 

DUT. The measurement period is the length of time over which phase comparisons are 

made. Frequency offset is estimated as follows, where ∆t is the amount of phase 

deviation, and T is the measurement period: 

 To illustrate, let's say that we measure +1 μs (microsecond) of phase deviation 

over a measurement period of 24 hours (h). The unit used for measurement period (h) 

must be converted to the unit used for phase deviation (s). The equation then 

becomes: 

 

 

 

 The smaller the frequency offset, the closer the DUT is to producing the same 

frequency as the reference. An oscillator that accumulates +1 μs of phase deviation/

day has a frequency offset of about –1 × 10-11 with respect to the reference. Table A.1 

lists the approximate offset values for some standard units of phase deviation and some 

standard measurement periods. 

T

t f offset − ∆ ( ) =

11 1.16 10

86,400,000,000

1 ( ) − = = − × − ∆ = ss

T

t f offset

μ

μ
59 

Appendix A - An Introduction to Frequency Calibrations 

Table A.1. Frequency offset values for given amounts of phase deviation.

 The frequency offset values in Table A.1 can be converted to units of frequency 

(Hz) if the nameplate frequency is known. To illustrate this, consider an oscillator with 

a nameplate frequency of 5 MHz that is high in frequency by 1.16 × 10-11. To find the 

frequency offset in hertz, multiply the nameplate frequency by the dimensionless offset 

value: 

 (5 × 106

) (+1.16 × 10-11) = 5.80 × 10-5 = +0.000 058 0 Hz 

 The nameplate frequency is 5 MHz, or 5 000 000 Hz. Therefore, the actual 

frequency being produced by the frequency standard is: 

5 000 000 Hz + 0.000 058 0 Hz = 5 000 000.000 058 0 Hz 

 To do a complete uncertainty analysis, the measurement period must be long 

enough to insure that we are measuring the frequency offset of the DUT, and that other 

sources are not contributing a significant amount of uncertainty to the measurement. In 

other words, we must be sure that ∆t is really a measure of only the DUT’s phase 

deviation from the reference and is not being contaminated by noise from the reference 

or the measurement system. This is why a 10:1 TUR is desirable. If a 10:1 TUR is 

Measurement Period Phase Deviation Frequency Offset 

1 s 1 ms 1.00 × 10-3 

1 s 1 μs 1.00 × 10-6 

1 s 1 ns 1.00 × 10-9 

1 h 1 ms 2.78 × 10-7 

1 h 1 μs 2.78 × 10-10 

1 h 1 ns 2.78 × 10-13 

1 day 1 ms 1.16 × 10-8 

1 day 1 μs 1.16 × 10-11 

1 day 1 ns 1.16 × 10-14 
60 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

maintained, many frequency calibration systems are capable of measuring a 1 × 10-10

frequency offset in 1 s [4]. 

 Of course, a 10:1 TUR is not always possible, and the simple equation we gave 

for frequency offset is often too simple. When transfer standards such as LORAN-C 

or GPS receivers are used (discussed later), radio path noise contributes to the phase 

deviation. To get around this problem, a measurement period of at least 24 hours is 

normally used when calibrating frequency standards using a transfer standard. This 

period is selected 

because changes in 

path delay between 

the source and 

receiver often have a 

cyclical variation that 

averages out over 24 

hours. In addition to 

averaging, curvefitting algorithms and 

other statistical 

methods are often 

used to improve the 

uncertainty estimate 

and to show the 

confidence level of 

the measurement [5]. 

 Figure A.1 

shows two graphs of 

phase comparisons 

between a DUT and a 

reference. The top 

graph shows no 

discernible phase 

noise. This indicates 

that a TUR of 10:1 or 

better is being maintained. The bottom graph shows a small amount of phase noise, 

which could mean that a TUR of less than 10:1 exists and that some uncertainty is 

being contributed by the reference. 

 To summarize, frequency offset is the quantity of greatest interest to a 

calibration laboratory because it tells us how close a DUT is to its nameplate 

frequency. You will probably notice that the term frequency accuracy (or just 

Figure A.1. Simple phase comparison graphs. 
61 

Appendix A - An Introduction to Frequency Calibrations 

accuracy) often appears on oscillator specification sheets instead of the term frequency 

offset. Frequency accuracy and frequency offset are equivalent terms that refer to the 

result of a measurement at a given time. Frequency uncertainty indicates the limits 

(upper and lower) of the frequency offset. ISO defines uncertainty as a: 

Parameter, associated with the result of a measurement, that 

characterizes the dispersion of values that could reasonably be 

attributed to the measurand [1]. 

In other words, the frequency uncertainty shows us the possible range of values 

(or limits) for the frequency offset. It is now standard practice to use a 2σ uncertainty 

test. This means that there is a 95.4 % probability that the frequency offset will stay 

within the stated range during the measurement period. The range of values is obtained 

by both adding the frequency uncertainty to and subtracting it from the average (or 

mean) frequency offset. Therefore, frequency uncertainty is sometimes stated with a 

“plus or minus” sign (±1 × 10-12) to show the upper and lower limits of the offset. If 

the “±” symbol is omitted, it is still implied. The largest contributor to the frequency 

uncertainty is usually the stability of the device under test. Stability is the topic of the 

next section. 

Stability 

 Before beginning our discussion of stability, we should mention an important 

distinction between frequency offset and stability. Frequency offset is a measure of 

how closely an oscillator produces its nameplate frequency, or how well an oscillator 

is adjusted. It doesn't tell us about the quality of an oscillator. For example, a stable 

oscillator that 

needs 

adjustment 

might produce a 

frequency with a 

large offset. An 

unstable 

oscillator that is 

well adjusted 

might 

temporarily 

produce a 

frequency very 

close to its 

nameplate value. 

 Figure A.2. Comparison of unstable and stable frequencies. 
62 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 Stability indicates how well an oscillator can produce the same frequency over 

a given period of time. It doesn't indicate whether the frequency is "right" or "wrong," 

only whether it stays the same. Also, the stability doesn't necessarily change when the 

frequency offset changes. You can adjust an oscillator and move its frequency either 

further away from or closer to its nameplate frequency without changing its stability at 

all. Figure A.2 illustrates this by displaying two oscillating signals that are of the 

same frequency between t1 and t2. However, it’s clear that signal 1 is unstable and is 

fluctuating in frequency between t2 and t3. 

 Stability is defined as the statistical estimate of the frequency fluctuations of a 

signal over a given time interval. Short-term stability usually refers to fluctuations 

over intervals less than 100 s. Long-term stability can refer to measurement intervals 

greater than 100 s, but usually refers to periods longer than 1 day. A typical oscillator 

specification sheet might list stability estimates for intervals of 1, 10, 100, and 1000 s 

[6, 7]. 

 Stability estimates can be made in the frequency domain or time domain, and 

statistical tools exist to easily convert from one domain to the other. Time domain 

estimates are more widely used, since time interval counters are often used to measure 

frequency. To estimate stability in the time domain, we must start with a set of 

frequency offset measurements yi that consists of individual measurements, y1, y2, y3, 

and so on. Once this data set is obtained, we need to determine the dispersion or 

scatter of the yi as a measure of oscillator noise. The larger the dispersion, or scatter, 

of the yi , the greater the instability of the output signal of the oscillator. 

 Normally, classical statistics such as standard deviation (or variance, the 

square of the standard deviation) are used to measure dispersion. Variance is a 

measure of the numerical spread of a data set with respect to the average or mean value 

of the data set. However, variance works only with stationary data, where the results 

must be time-independent. This assumes the noise is white, meaning that its power is 

evenly distributed across the frequency band of the measurement. Oscillator data is 

usually nonstationary, since it contains time-dependent noise contributed by the 

frequency offset. For stationary data, the mean and variance will converge to 

particular values as the number of measurements increases. With nonstationary data, 

the mean and variance never converge to any particular values. Instead, we have a 

moving mean that changes each time we add a new measurement. 

 
63 

Appendix A - An Introduction to Frequency Calibrations 

For these reasons, a non classical statistic is used to estimate stability in the time 

domain. This statistic is often called the Allan variance, but since it is actually the 

square root of the variance, its proper name is the Allan deviation. By recommendation 

of the Institute of Electrical and Electronics Engineers (IEEE), the Allan deviation is 

used by manufacturers of frequency standards as a standard specification for stability. 

The equation for the Allan deviation is: 

where M is the number of values in the yi series, and the data are equally spaced in 

segments τ seconds long. Note that while classical deviation subtracts the mean from 

each measurement before squaring their summation, the Allan deviation subtracts the 

previous data point. Since stability is a measure of frequency fluctuations and not of 

frequency offset, successive data points are differenced to remove the time-dependent 

noise contributed by the frequency offset. 

 

Table A.2. Using phase measurements to estimate stability (1 s data intervals).

∑

−

= + − − = 11

2

1 ( ) 2( 1)

1 ( )

M

i

y i i y y M

σ τ

Phase 

Measurements 

(ns) 

Phase 

Deviation (ns), 

∆t 

Frequency 

Offset 

∆t/1 s (yi) 

First 

Differences 

(yi+1 - yi)

First Difference 

Squared 

(yi+1 - yi)

2

3321.44 (-----) (-----) (-----) (-----) 

3325.51 4.07 4.07 × 10-9 (-----) (-----) 

3329.55 4.04 4.04 × 10-9 –3 × 10-11 9 × 10-22 

3333.60 4.05 4.05 × 10-9 +1 × 10-11 1 × 10-22 

3337.67 4.07 4.07 × 10-9 +2 × 10-11 4 × 10-22 

3341.72 4.05 4.05 × 10-9 –2 × 10-11 4 × 10-22 

3345.78 4.06 4.06 × 10-9 +1 × 10-11 1 × 10-22 

3349.85 4.07 4.07 × 10-9 +1 × 10-11 1 × 10-22 

3353.91 4.06 4.06 × 10-9 –1 × 10-11 1 × 10-22 

3357.96 4.05 4.05 × 10-9 –1 × 10-11 1 × 10-22 
64 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 Table A.2 shows how stability is estimated. The first column is a series of 

phase measurements recorded at 1 s intervals. Each measurement is larger than the 

previous measurement. This indicates that the DUT is offset in frequency from the 

reference and this offset causes a phase deviation. By differencing the raw phase 

measurements, we obtain the phase deviations shown in the second column. The third 

column divides the phase deviation (∆t) by the 1 s measurement period to get the 

frequency offset. Since the phase deviation is about 4 ns/s, it indicates that the DUT 

has a frequency offset of about 4 × 10-9. The frequency offset values in the third 

column form the yi data series. The last two columns show the first differences of the 

yi and the squares of the first differences. Since the sum of the squares equals 2.2 ×

10-21, the equation (where τ = 1 s) becomes: 

 

 

 Using the same data, the Allan deviation for τ = 2 s can be computed by 

averaging pairs of adjacent values and using these new averages as data values. For τ

= 4 s, take the 

average of each 

set of four 

adjacent values 

and use these new 

averages as data 

values. More data 

must be acquired 

for longer 

averaging times. 

Keep in mind that 

the confidence 

level of a 

stabilility estimate 

improves as the 

averaging time 

increases. In the 

above example, 

we have eight 

samples for our τ = 1 s estimate. However, we have only two samples for an estimate 

of τ = 4 s. 

 

Figure A.3. A sample Allan deviation graph. 

11

21

1.17 x 10

2(9 1)

2.2 10 ( ) −

−

= −

×

σ y τ =
65 

Appendix A - An Introduction to Frequency Calibrations 

 The confidence level of our estimate (1σ) can be roughly estimated as: 

 

 

 In our example, where M = 9, our error margin is 33 %. With just 2 samples, 

our estimate may be in error by as much as 70 %. With 104 samples, the error margin 

is reduced to 1 %. 

 

 A sample Allan deviation graph is shown in Figure A.3. It shows the stability 

improving as the measurement period gets longer. Part of this improvement is because 

measurement system noise becomes less of a factor as the measurement period gets 

longer. At some point, however, the oscillator will reach its flicker floor, and from a 

practical point of view, no further gains will be made by averaging. The flicker floor is 

the point where the white noise processes begin to be dominated by nonstationary 

processes such as frequency drift. Most quartz and rubidium oscillators reach their 

flicker floor at a measurement period of 103

 s or less, but cesium oscillators may not 

reach their flicker floor for 105 s (more than 1 day). Figure A.3 shows a sample Allan 

deviation graph of a quartz oscillator that is stable to about 5 × 10-12 at 100 s and is 

approaching its flicker floor [8, 9, 10]. 

 Be sure not to confuse stability with frequency offset when you read a 

specifications sheet. For example, a DUT with a frequency offset of 1 × 10-8 might 

still reach a stability of 1 × 10-12 in 1000 s. This means that the output frequency of the 

DUT is stable, even though it is not particularly close to its nameplate frequency. To 

help clarify this point, Figure A.4 is a graphical representation of the relationship 

between frequency offset (accuracy) and stability. 

Figure A.4. The relationship between frequency 

offset (accuracy) and stability. 

 x 100% 1

M
66 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Frequency Standards 

 Frequency standards all have an internal device that produces a periodic, 

repetitive event. This device is called the resonator. Of course, the resonator must be 

driven by an energy source. Taken together, the energy source and the resonator form 

an oscillator. Two main types of oscillators are used as frequency standards: quartz 

oscillators and atomic oscillators. 

Quartz Oscillators

 Quartz crystal oscillators first appeared during the 1920s and quickly replaced 

pendulum devices as laboratory standards for time and frequency [11]. More than 109

quartz oscillators are produced annually for applications ranging from wristwatches 

and clocks to communications networks and space tracking systems [12]. However, 

calibration and standards laboratories usually calibrate only the more expensive 

varieties of quartz oscillators, such as those found inside electronic instruments (such 

as frequency counters) or those designed as stand-alone units. The cost of a highquality quartz oscillator ranges from a few hundred to a few thousand dollars. 

 The 

quartz crystal 

inside the 

oscillator can be 

made of natural 

or synthetic 

quartz, but all 

modern devices 

are made of 

synthetic 

material. The 

crystal serves as 

a mechanical 

resonator which 

creates an 

oscillating 

voltage due to the 

piezoelectric 

effect. This 

effect causes the 

crystal to expand or contract as voltages are applied. The crystal has a resonance 

frequency that is determined by its physical dimensions and the type of crystal used. 

No two crystals can be exactly alike or produce exactly the same frequency. The 

output frequency of a quartz oscillator is either the fundamental resonance frequency 

Figure A.5. Block diagram of quartz oscillator. 
67 

Appendix A - An Introduction to Frequency Calibrations 

or a multiple of that frequency. Figure A.5 is a simplified circuit diagram that shows 

the basic elements of a quartz oscillator. The amplifier provides the energy needed to 

sustain oscillation. 

 Quartz oscillators are sensitive to environmental parameters such as 

temperature, humidity, pressure, and vibration [12, 13]. When environmental 

parameters change, the fundamental resonance frequency also changes. There are 

several types of quartz oscillator designs that attempt to reduce the environmental 

problems. The oven-controlled crystal oscillator (OCXO) encloses the crystal in a 

temperature-controlled chamber called an oven. When an OCXO is first turned on, it 

goes through a "warm-up" period while the temperatures of the crystal resonator and 

its oven stabilize. During this time, the performance of the oscillator continuously 

changes until it reaches its normal operating temperature. The temperature within the 

oven then remains constant, even when the outside temperature varies. An alternate 

solution to the temperature problem is the temperature-compensated crystal oscillator

(TCXO). In a TCXO, the output signal from a special temperature sensor (called a 

thermistor) generates a correction voltage that is applied to a voltage-variable 

reactance (called a varactor). The varactor then produces a frequency change equal 

and opposite to the frequency change produced by temperature. This technique does 

not work as well as oven control, but is much less expensive. Therefore, TCXOs are 

normally used in small, usually portable units when high performance over a wide 

temperature range is not required. A third type of quartz oscillator is the 

microcomputer-compensated crystal oscillator (MCXO). The MCXO uses a 

microprocessor and compensates for temperature using digital techniques. The MCXO 

falls between a TCXO and an OCXO in both price and performance. 

 All quartz oscillators are subject to aging, which is defined as “a systematic 

change in frequency with time due to internal changes in the oscillator.” Aging is 

usually observed as a nearly linear change over time in the resonance frequency. 

Aging can be positive or negative, and occasionally, a reversal in aging direction is 

observed. Often, the resonance frequency decreases, which might indicate that the 

crystal is getting larger. Aging has many possible causes including: contamination of 

the crystal due to deposits of foreign material, changes in the oscillator circuitry, or 

changes in the quartz material or crystal structure. The vibrating motion of the crystal 

can also contribute to aging. High quality quartz oscillators age at a rate of no more 

than 5 × 10-9 per year. 

 The best quartz oscillators have excellent short-term stability. An OCXO 

might be stable to 1 × 10-13 at 1 s. The limiting factor in the short-term stability is 

often noise from electronic components in the oscillator circuits. However, due to 

aging and other factors, the long term stability of a quartz oscillator is poor. Even the 

best OCXOs require periodic adjustments to stay within 1 × 10-10 of their nameplate 
68 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

frequency. The TCXOs found in test equipment like counters and signal generators 

typically have frequency offsets ranging from 1 × 10-7 to 1 × 10-9. Quartz oscillators 

without temperature control (such as those found in wristwatches, computers, radios, 

and so on) typically have frequency offsets measured in parts per million. 

Atomic Oscillators 

 Atomic oscillators use the quantized energy levels in atoms and molecules as 

the source of their resonance frequency. The laws of quantum mechanics dictate that 

the energies of a bound system, such as an atom, have certain discrete values. An 

electromagnetic field can boost an atom from one energy level to a higher one. Or, an 

atom at a high energy level can drop to a lower energy level by emitting 

electromagnetic energy. The resonance frequency (f) of an atomic oscillator is the 

difference between the two energy levels divided by Planck’s constant (h) [14]: 

 

 All atomic oscillators are intrinsic standards, since their frequency is inherently 

derived from a fundamental natural phenomenon. There are three main types of 

atomic oscillators: rubidium standards, cesium standards, and hydrogen masers 

(discussed individually in the following sections). All three types contain an internal 

quartz oscillator that is locked to a resonance frequency generated by the atom of 

interest. Locking the quartz oscillator to the atomic frequency is advantageous. Most 

of the factors that degrade the long-term performance of a quartz oscillator disappear, 

since the atomic resonance frequency is much less sensitive to environmental 

conditions than the quartz resonance frequency. As a result, the long-term stability and 

uncertainty of an atomic oscillator are much better than those of a quartz oscillator, but 

the short-term stability is unchanged. 

Rubidium Oscillators

 Rubidium oscillators are the lowest priced members of the atomic oscillator 

group. They offer perhaps the best price/performance ratio of any oscillator. They 

perform much better than a quartz oscillator and cost much less than a cesium 

oscillator. 

 A rubidium oscillator operates at the resonance frequency of the rubidium atom 

(

87Rb), which is 6 834 682 608 Hz. This frequency is synthesized from a lower quartz 

frequency (typically 5 MHz) and the quartz frequency is steered by the rubidium 

resonance. The result is a very stable frequency source with the short-term stability of 

quartz, but much better long-term stability. Since rubidium oscillators are more stable 

h

E E f 2 − 1

=
69 

Appendix A - An Introduction to Frequency Calibrations 

than quartz oscillators, they can be kept within tolerance with fewer adjustments. The 

initial price of a rubidium oscillator (typically from $3000 to $8000) is higher than that 

of a quartz oscillator, but since fewer adjustments are needed, labor costs are reduced. 

As a result, a rubidium oscillator might actually be less expensive to own than a quartz 

oscillator when used as a frequency standard. 

 The typical frequency offset of a rubidium oscillator ranges from 5 × 10-10 to 5 

× 10-12. Stability is typically about 1 × 10-12 at one day. Maintaining frequency 

within 1 × 10-11 can be done routinely with a rubidium oscillator but is nearly 

impossible with even the best quartz oscillators. The performance of a wellmaintained rubidium oscillator can approach the performance of a cesium oscillator, 

and a rubidium oscillator is much smaller, more reliable, and less expensive. 

Cesium Oscillators

 Cesium oscillators are primary frequency standards because the SI second is 

based on the resonance frequency of the cesium atom (133Cs), which is 9 192 631 770 

Hz. This means that a cesium oscillator that is working properly should be very close 

to its nameplate frequency without any adjustment, and there should be no change in 

frequency due to aging. The time scale followed by all major countries, Coordinated 

Universal Time (UTC), is derived primarily from averaging the performance of a large 

ensemble of cesium oscillators, although some hydrogen masers also contribute to 

UTC. 

 Cesium oscillators are the workhorses in most modern time and frequency 

distribution systems. The primary frequency standard for the United States is a large 

cesium fountain oscillator named NIST-F1 that has a frequency uncertainty of about 

1.7 × 10-15. Commercially available cesium oscillators use cesium beam technology, 

and are small enough to fit in a standard equipment rack. They differ in quality, but 

their frequency offset should still be no greater than ± 5 × 10-12 after a brief warmup 

period, and is typically parts in 1013. Stability generally reaches parts in 1014 at one 

day, and it might take days or weeks before a cesium oscillator reaches its noise floor. 

 Reliability and cost are both issues to consider when purchasing a cesium 

oscillator. The main component of a cesium oscillator, called the beam tube, has a life 

expectancy of about 3 to 25 years, depending upon the type of tube and the amount of 

beam current used. The beam tube is needed to produce the resonance frequency of 

the cesium atom, and this frequency is then used to discipline a quartz oscillator. 

When the beam tube fails, the cesium oscillator performs like an undisciplined quartz 

oscillator. For this reason, a cesium oscillator needs to be constantly monitored to 

make sure that it is still delivering a cesium-derived frequency. Cost is also a major 

issue. The initial purchase price of a cesium oscillator ranges from $30,000 to 
70 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

$80,000, and the cost of a replacement beam tube is a substantial fraction of the cost of 

the entire oscillator. Laboratories that use cesium oscillators need to budget not only 

for their initial purchase, but for the cost of maintaining them afterwards. 

Hydrogen Masers

 The hydrogen maser is the most elaborate and most expensive commercially 

available frequency standard. Few masers are built and most are owned by 

observatories or national standards laboratories. “Maser” is an acronym that stands for 

"Microwave Amplification by Stimulated Emission of Radiation." Masers use the 

resonance frequency of the hydrogen atom, which is 1 420 405 752 Hz. 

 There are two types of hydrogen masers. The first type, called an active maser, 

oscillates spontaneously and a quartz oscillator is phase-locked to this active 

oscillation. The second type, called a passive maser, frequency-locks a quartz 

oscillator to the atomic reference. The “passive” technique is also used by rubidium 

and cesium oscillators. Since active masers derive the output frequency more directly 

from the atomic resonance, they have better short-term stability than passive masers. 

Both types of maser have better short-term stability than a cesium oscillator. However, 

since the maser’s performance depends upon a complex set of environmental 

conditions, its frequency uncertainty is greater than that of a cesium oscillator. And 

due to their complexity and low volume of production, masers typically cost $200,000 

or more [15,16]. 

 Table A.3 summarizes the characteristics of the different types of oscillators. 

Transfer Standards

 To briefly review, a frequency calibration compares the device under test

(DUT) to a reference. The DUT is usually a quartz, rubidium, or cesium oscillator. 

The reference is an oscillator of higher performance than the DUT or a transfer 

standard that receives a radio signal. All transfer standards receive a signal that has a 

cesium oscillator at its source, and this signal delivers a cesium derived frequency to 

the user. This benefits many users, since not all calibration laboratories can afford to 

buy and maintain a cesium. Even if a laboratory has a cesium oscillator, it still needs 

to check its performance, and the only practical way to do this is by using a transfer 

standard. 

 
71 

Appendix A - An Introduction to Frequency Calibrations 

Oscillator 

Type 

Quartz 

(TCXO) 

Quartz 

(MCXO) 

Quartz 

(OCXO) 

Rubidium Cesium Hydrogen 

Maser 

Primary 

Standard 

No No No No Yes No 

Intrinsic 

Standard 

No No No Yes Yes Yes 

Resonance 

Frequency 

Mechanical 

(varies) 

Mechanical 

(varies) 

Mechanical 

(varies) 

6.834682608 

GHz 

9.19263177 

GHz 

1.42040575 

GHz 

Leading 

Cause of 

Failure 

None None None Rubidium 

Lamp 

(15 years or 

more) 

Cesium 

Beam Tube 

(3 to 25 

years) 

Hydrogen 

Depletion 

(7 years or 

more) 

Stability, σy

(τ), τ =1s 

1 × 10-9 1 × 10-10 1 × 10-12 5 × 10-11 to 

5 × 10-12 

5 × 10-11 to 

5 × 10-12

1 × 10-12 

Noise Floor, 

σy(τ) 

1 × 10-9 

(τ = 1 to 102

s) 

1 × 10-10 

(τ = 1 to 102

s) 

1 × 10-12 

(τ = 1 to 102

s) 

1 × 10-12 

(τ = 103

 to 

105

 s) 

1 × 10-14 

(τ = 105

 to 

107

 s) 

1 × 10-15 

(τ = 103

 to 

105

 s) 

Aging/year 5 × 10-7 5 × 10-8 5 × 10-9 2 × 10-10 None ∼1 × 10-13 

Frequency 

Offset after 

warm up 

1 × 10-6 1 × 10-7 to 

1 × 10-8 

1 × 10-8 to 

1 × 10-10 

5 × 10-10 to 

5 × 10-12 

5 × 10-12 to 

1 × 10-14 

1 × 10-12 to 

1 × 10-13 

Warm-Up 

Time 

< 10 s to 

1 × 10-6 

< 10 s to 

1 × 10-8 

< 5 min to 

1 × 10-8 

< 5 min to 

5 × 10-10 

30 min to 

5 × 10-12 

24 hours to 

1 × 10-12 

Cost $100 $1000 $2000 $3000 to 

$8000 

$30,000 to 

$80,000 

$200,000 to 

$300,000 

Table A.3. Summary of oscillator types. 
72 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 Transfer standards provide traceability. Most transfer standards receive signals 

traceable to the national frequency standard maintained by NIST. Some signals, such 

as those transmitted by HF (high frequency) radio stations WWV and WWVH and the 

LF (low frequency) station WWVB, are traceable because they are directly controlled 

by NIST. Other signals, such as the LORAN-C and Global Positioning System (GPS) 

broadcasts, are traceable because their received signals are regularly compared to the 

NIST standard. Some signals broadcast from outside the United States are also 

traceable. This is because NIST compares its frequency standard to the standards 

maintained in other countries. 

 Some compromises are made when using a transfer standard. Even if the radio 

signal is referenced to a nearly perfect frequency, its performance is degraded as it 

travels along the radio path between the transmitter and receiver. To illustrate, consider 

a laboratory that has a rack-mounted frequency standard that produces a 5 MHz signal. 

Metrologists need to use this signal on their work bench, so they run a length of coaxial 

cable from the frequency standard to their bench. The signal is delayed as it travels 

from the standard to the bench, but since the cable is of fixed length the delay is 

constant. Constant delays don’t change the frequency. The frequency that goes into 

one end of the cable is essentially the same frequency that comes out the other end. 

However, what if the cable length were constantly changing? This would generally 

cause the frequency to fluctuate. Over long periods of time, these fluctuations will 

average out, but the short-term frequency stability would still be very poor. This is 

exactly what happens when you use a transfer standard. The “cable” is actually a radio 

path that might be thousands of kilometers in length. The length of the radio path is 

constantly changing and appears to introduce frequency fluctuations, even though the 

source of the frequency (a cesium oscillator) is not changing. This makes transfer 

standards unsuitable as a reference when making short-term stability measurements. 

However, transfer standards are well suited for long term measurements, since you can 

minimize these frequency fluctuations if you average for a long enough measurement 

interval. Eventually, you will recover the performance of a cesium oscillator. 

 Some radio signals have path variations that are so pronounced that they are not 

well suited for high level frequency calibrations. To illustrate this, consider the signal 

broadcast from WWV, located in Fort Collins, Colorado. WWV is a HF radio station 

(often called a shortwave station) that transmits on 2.5, 5, 10, 15, and 20 MHz. WWV 

is referenced to the national frequency standard at NIST, but by the time the signal gets 

to your receiver, much of its potential performance has been lost. Most shortwave 

users receive the skywave, or the part of the signal that travels up to the ionosphere and 

is reflected back to earth. Since the height of the ionosphere constantly changes, the 

path delay constantly changes, often by as much as 500 to 1000 μs. Since there is so 
73 

Appendix A - An Introduction to Frequency Calibrations 

much variability in the path, averaging leads to only limited improvement. Therefore, 

although WWV is traceable to NIST, its frequency uncertainty is limited to parts in 109

when averaged for one day. 

 Other radio signals have more stable paths and much lower uncertainty values. 

Low frequency (LF) radio stations (such as WWVB and LORAN-C) can provide 

traceability to NIST with a frequency uncertainty of 1 × 10-12/day. An LF path is 

much more stable than an HF path, but still experiences a path delay change when the 

height of the ionosphere changes at sunrise and sunset. Currently, the most widely 

used signals originate from the Global Positioning System (GPS) satellites. GPS 

signals have the advantage of an unobstructed path between the transmitter and 

receiver. The frequency uncertainty of GPS is about 2 × 10-13/day. WWVB, LORANC and GPS are described in the next three sections. 

 Table A.4 shows some of the transfer standards available, as well as their 

frequency uncertainty with respect to NIST when averaged for at least 24 h [17]. 

WWVB 

 Many countries broadcast time and frequency signals in the LF band from 30 to 

300 kHz, as well as in the VLF (very low frequency) band from 3 to 30 kHz. Since 

part of the LF signal is groundwave and follows the curvature of the earth, the path 

stability of these signals can be quite good. One such station is NIST’s WWVB, which 

transmits on 60 kHz from the same site as WWV in Fort Collins, Colorado and 

provides coverage to most of North America. 

 Although far more stable than an HF path, the WWVB path length is still 

influenced by environmental conditions along the path and by daily and seasonal 

Transfer Standard Frequency Uncertainty over 24 h 

measurement period (with respect to 

NIST) 

HF receiver (WWV and WWVH) ±5 × 10-9

LF receiver (LORAN-C and WWVB) ±1 × 10-12

Global Positioning System receiver (GPS) ±2 × 10-13

Table A.4. Typical frequency uncertainties of various transfer standards. 
74 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

changes. Path length is important because part of the signal travels along the ground 

(groundwave) and another part is reflected from the ionosphere (skywave). The 

groundwave path is far more stable than the skywave path. If the path is relatively 

short (less than 1000 km), the receiver might continuously track the groundwave signal 

since it always arrives first. For longer paths, a mixture of groundwave and skywave is 

received. And over a very long path, the groundwave can become so weak that it will 

be possible to receive only the skywave. In this instance, the path becomes much less 

stable. 

 The characteristics of an LF path are different at different times of day. During 

the daylight and nighttime hours, the receiver might be able to distinguish between 

groundwave and skywave, and path stability might vary by only a few hundred 

nanoseconds. However, if some skywave is being received, diurnal phase shifts occur 

at sunrise and sunset. For instance, as the path changes from all darkness to all 

daylight, the ionosphere lowers and the path gets shorter. The path length then 

stabilizes until either the transmitter or receiver enters darkness. At this point, the 

ionosphere rises and the path gets longer. 

 WWVB receivers have several advantages when used as a transfer standard. 

They are low cost and easy to use, and the received signals are directly traceable to 

NIST. With a good receiver and antenna system, you can achieve a frequency 

uncertainty of 1 × 10-12 by averaging for one day [18]. 

LORAN-C 

 LORAN-C is a radionavigation system that operates in the LF band. Most of 

the system is operated by the U. S. Department of Transportation (DOT), but some 

stations are operated by other governments. The system consists of groups of stations 

called chains. Each chain has one master station and from two to five secondary 

stations. The stations operate at high power, typically 275 to 800 kW, and broadcast 

on a carrier frequency of 100 kHz using the 90 to 110 kHz band. 

 Since all LORAN-C chains use the same carrier frequency, the chains transmit 

pulses so that individual stations can be identified. Each chain transmits a pulse group 

that includes pulses from all of the individual stations. The pulse group is sent at a 

unique Group Repetition Interval (GRI). For example, the 7980 chain transmits pulses 

every 79.8 ms. When the pulses leave the transmitter, they radiate in all directions. 

The groundwave travels parallel to the surface of the Earth. The skywave travels 

upward and is reflected off of the ionosphere. The pulse shape was designed so that the 

receiver can distinguish between groundwave and skywave and lock on to the more 

stable groundwave signal. Most receivers stay locked to the groundwave by tracking 
75 

Appendix A - An Introduction to Frequency Calibrations 

the third cycle of the pulse. The third cycle was chosen for two reasons. First, it 

arrives early in the pulse so we know that it is groundwave. Second, it has more 

amplitude than the first and second cycles in the pulse, which makes it easier for the 

receiver to stay locked. Generally, a receiver within 1500 km of the transmitter can 

track the same groundwave cycle indefinitely and avoid skywave reception. The 

variations in groundwave path delay are typically quite small (<500 ns/day). However, 

if the path length exceeds 1500 km, the receiver might lose lock, and jump to another 

cycle of the carrier. Each cycle jump introduces a 10 μs phase step, equal to the period 

of 100 kHz. 

LORAN-C Performance

 The frequency uncertainty of LORAN-C is degraded by variations in the radio 

path. The size of these variations depends upon the signal strength, your distance from 

the transmitter, the weather and atmospheric conditions, and the quality of your 

receiver and antenna. Path variations cause the short-term stability of LORAN-C to be 

poor. However, since the path variations average out over time, the long-term stability 

is quite good. This means that we can use LORAN-C to calibrate nearly any frequency 

Figure A.6. LORAN-C compared to cesium oscillator over 96 h interval. 
76 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

standard if we can average for a long enough interval. For this reason, a measurement 

period of at least 24 h is recommended when using LORAN-C to calibrate atomic 

oscillators. Figure A.6 shows the results of a 96 h calibration of a cesium oscillator 

using LORAN-C. The thick line is a least squares estimate of the frequency offset. 

Although the path noise is clearly visible, the slope of the line indicates that the cesium 

oscillator is low in frequency by 3.4 × 10-12. 

Global Positioning System (GPS) 

 GPS is a radionavigation system developed and operated by the U.S. 

Department of Defense (DOD). The system consists of a constellation of at least 24 

earth-orbiting satellites (21 primary satellites and 3 in-orbit spares). Each satellite 

carries its own atomic frequency standards (cesium and/or rubidium oscillators) that 

are referenced to the United States Naval Observatory (USNO) and traceable to NIST. 

The 24 satellites orbit the Earth in six fixed planes inclined 55o from the equator. Each 

satellite is 20 200 km above the earth’s surface and has an orbital period of 11 h, 58 

min, which means a satellite will pass over the same place on earth 4 min earlier each 

day. Since the satellites continually orbit the earth, GPS should be usable anywhere on 

the earth's surface. 

 The GPS satellites broadcast on two carrier frequencies: L1 at 1575.42 MHz, 

and L2 at 1227.6 MHz. Each satellite broadcasts a spread-spectrum waveform, called a 

pseudo-random noise (PRN) code on L1 and L2, and each satellite is identified by the 

PRN code it transmits. There are two types of PRN codes. The first type is a coarse 

acquisition (C/A) code with a chip rate of 1023 chips per millisecond. The second type 

is a precision (P) code with a chip rate of 10230 chips per millisecond. The C/A code is 

broadcast on L1, and the P code is broadcast on both L1 and L2. The C/A code repeats 

every millisecond. The P code repeats only every 267 days, but for practical reasons is 

reset every week. The C/A code is broadcast on L1, and the P code is broadcast on 

both L1 and L2 [19, 20]. GPS reception is line-of-sight, which means that the antenna 

must have a clear view of the sky. If a clear sky view is available, the signals can be 

received nearly anywhere on earth. 

 Each satellite carries either rubidium or cesium oscillators, or a combination of 

both. The on-board oscillators provide the reference for both the carrier and code 

broadcasts. They are steered from USDOD ground stations and are referenced to 

Coordinated Universal Time (UTC) maintained by the USNO. By mutual agreement 

UTC(USNO) and UTC(NIST) are maintained within 100 ns of each other, and the 

frequency offset between the two time scales is always <1 × 10-13. 
77 

Appendix A - An Introduction to Frequency Calibrations 

GPS Receiving Equipment

 Most GPS receivers provide a 1 pulse per second (pps) output. Some receivers 

also provide a standard frequency output (1, 5, or 10 MHz). To use these receivers, 

you simply mount the antenna, connect the antenna to the receiver, and turn the 

receiver on. The antenna is often a small cone or disk (normally about 15 cm in 

diameter) that must be mounted outdoors where it has a clear, unobstructed view of the 

sky. Once the receiver is turned on, it performs a sky search to find out which 

satellites are currently above the horizon and visible from the antenna site. It then 

computes its three-dimensional coordinate (latitude, longitude, and altitude as long as 

four satellites are in view) and begins producing a frequency signal. The simplest GPS 

receivers have just one channel and look at multiple satellites using a sequencing 

scheme that rapidly switches between satellites. More sophisticated models have 

parallel tracking capability and can assign a separate channel to each satellite in view. 

These receivers typically track from 5 to 12 satellites at once (although more than 8 

will be available only in rare instances). By averaging data from multiple satellites, a 

receiver can reduce the frequency uncertainty [20]. 

GPS Performance

 GPS has many technical advantages over LF radio signals. The signals are 

usually easier to receive, the equipment is often less expensive, the coverage area is 

much larger (worldwide), and the performance is better. However, as with all transfer 

Figure A.7. GPS compared to cesium oscillator over 100 s interval. 
78 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

standards, the short-term stability of a GPS receiver is not particularly good and this 

lengthens the time required to make a calibration. As with LORAN-C, a measurement 

period of at least 24 h is recommended when calibrating atomic frequency standards 

using GPS. 

 

 To illustrate this, Figure A.7 shows a 100 s comparison between GPS and a 

cesium oscillator. The cesium oscillator has a frequency uncertainty of 1 × 10-13, and 

its accumulated phase shift during the 100 s measurement period is <1 ns. Therefore, 

most of the phase noise on the graph can be attributed to GPS path variations. 

 Figure A.8 shows a one-week comparison between a GPS receiver and the 

same cesium oscillator used in Figure A.7. The range of the data is well under 100 ns. 

The thick line is the result of a linear least squares fit. Although phase noise caused by 

GPS path variations is visible, we can clearly see the linear trend contributed by the 

small frequency offset of the cesium (<1 × 10-13). 

Figure A.8. GPS compared to cesium oscillator over 1 week interval. 
79 

Appendix A - An Introduction to Frequency Calibrations 

Calibration Methods

 To review, frequency 

standards are normally 

calibrated by comparing them 

to a traceable reference 

frequency. In this section, 

we’ll discuss how this 

comparison is made. To 

begin with, let’s look at the 

electrical signal produced by 

a frequency standard. This 

signal can take several forms, 

as illustrated in Figure A.9. 

The dashed lines represent the 

supply voltage inputs (ranging from 5 to 15 V for CMOS), and the bold solid lines 

represent the output voltage. 

 If the output frequency is an oscillating sine wave, it might look like the one 

shown in Figure A.10. This signal produces one cycle (2π radians of phase) in one 

period. Frequency calibration systems compare a signal like the one shown in Figure 

A.10 to a reference signal of higher quality, and measure and record the change in 

phase between the two signals. If the two frequencies were exactly the same, their 

phase relationship would not change. Since the two frequencies are not exactly the 

same, their phase relationship does change, and the rate of change determines the 

frequency offset of the DUT. Under normal circumstances, the phase changes in an 

orderly, predictable fashion. However, external factors such as power outages, 

component failures, or human errors can cause a sudden phase change, or phase step. 

A calibration system measures the total amount of phase shift (caused either by the 

frequency offset of the DUT or a phase step) over a given measurement period. 

Figure A.9. Oscillator outputs. 

Figure A.10. An oscillating signal. 
80 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 Figure A.11 shows a phase comparison between two sinusoidal frequencies. 

The top sine wave represents a signal from the DUT, and the bottom sine wave 

represents a signal from the reference. Vertical lines have been drawn through the 

points where each sine wave crosses zero. The bottom of the figure shows “bars” that 

indicate the size of the phase difference between the two signals. If the phase 

relationship between the signals is changing, the phase difference will either increase 

or decrease to indicate that the DUT has a frequency offset (high or low) with respect 

to the reference. Earlier, we introduced this simple equation to estimate frequency 

offset: 

 

 In Figure A.11, both ∆t and T are increasing at the same rate and the phase 

difference “bars” are getting uniformly wider. This indicates that the frequency from 

the DUT is stable, but is offset with respect to the reference. 

 

T

t f offset − ∆ ( ) =

Figure A.11. Two signals with a changing phase relationship. 
81 

Appendix A - An Introduction to Frequency Calibrations 

 Several types of calibration systems can be used to compare phase. The 

simplest type of system involves directly counting and displaying the frequency output 

of the DUT with a device called a frequency counter. This method has many 

applications but is unsuitable for measuring high performance devices. The DUT is 

compared to the counter’s time base (typically a OCXO), and the uncertainty of the 

system is limited by the 

uncertainty of the time base, 

typically ±1 × 10-8. Some 

counters allow use of an external 

time base, which can improve 

the results. The biggest 

limitation is that frequency 

counters display frequency in 

hertz and have a limited amount 

of resolution. Detecting small 

changes in frequency may take 

many days or weeks, which 

might make it difficult or 

impossible to use this method to 

adjust a precision oscillator or to 

measure stability. For this 

reason, most high performance calibration systems collect time series data that can be 

used to estimate both frequency uncertainty and stability. We’ll discuss how phase 

comparisons are made using the time interval method [9, 10]. 

 The time interval method uses a device called a time interval counter (TIC) to 

measure the time interval between two signals. A TIC has inputs for two electrical 

signals. One signal starts the counter and the other signal stops it. If the two signals 

have the same frequency, the time interval will not change. If the two signals have 

different frequencies, the time interval will change, although usually very slowly. By 

looking at the rate of change, you can calibrate the device. It is exactly as if you had 

two clocks. By reading them each day, you could determine the amount of time one 

clock gained or lost relative to the other clock. It takes two time interval measurements 

to produce useful information. By subtracting the first measurement from the second, 

we can tell whether time was gained or lost. 

 TIC's differ in specification and design details, but they all contain several basic 

parts known as the time base, the main gate, and the counting assembly. The time base 

provides evenly spaced pulses used to measure time interval. The time base is usually 

an internal quartz oscillator that can often be phase locked to an external reference. It 

must be stable because time base errors will directly affect the measurements. The 

main gate controls the time at which the count begins and ends. Pulses passing through 

Figure A.12. Measuring time interval. 
82 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

the gate are routed to the counting assembly where they are displayed on the TIC's 

front panel or read by computer. The counter can then be reset (or armed) to begin 

another measurement. The stop and start inputs are usually provided with level 

controls that set the amplitude limit (or trigger level) at which the counter responds to 

input signals. If the trigger levels are set improperly, a TIC might stop or start when it 

detects noise or other unwanted signals and produce invalid measurements. 

 Figure A.12 illustrates how a TIC measures the interval between two signals. 

Input A is the start pulse and Input B is the stop pulse. The TIC begins measuring a 

time interval when the start signal reaches its trigger level and stops measuring when 

the stop signal reaches its trigger level. The time interval between the start and stop 

signals is measured by counting cycles from the time base. The measurements 

produced by a TIC are in time units: milliseconds, microseconds, nanoseconds, and so 

on. These measurements assign a value to the phase difference between the reference 

and the DUT. 

 The most important specification of a TIC is resolution, which is the degree to 

which a measurement can be determined. For example, if a TIC has a resolution of 10 

ns, it could produce a reading of 3340 ns or 3350 ns but not a reading of 3345 ns. This 

is because 10 ns is the smallest significant difference the TIC can measure. Any finer 

measurement would require more resolution. In traditional TIC designs, the resolution 

is limited to the period of the TIC’s time base frequency. For example, a TIC with a 

10 MHz time base would be limited to a resolution of 100 ns. This is because 

traditional TIC designs count whole time base cycles to measure time interval and 

cannot resolve time intervals smaller than the period of one cycle. To improve this 

situation, some TIC designers have 

multiplied the time base frequency to get 

more cycles and thus more resolution. For 

example, multiplying the time base 

frequency to 100 MHz makes 10 ns 

resolution possible, and 1 ns counters have 

even been built using a 1 GHz time base. 

However, a more common way to increase 

resolution is to detect parts of a time base 

cycle through interpolation and not be 

limited by the number of whole cycles. 

Interpolation has made 1 ns TICs 

commonplace, and even 20 ps TICs are 

available [21, 22]. 

Figure A.13. Time interval system. 
83 

Appendix A - An Introduction to Frequency Calibrations 

 A time interval system is shown in Figure A.13. This system uses a TIC to 

measure and record the difference between two signals. Low frequency start and stop 

signals must be used (typically 1 Hz). Since oscillators typically produce frequencies 

such as 1, 5, and 10 MHz the solution is to use a frequency divider to convert them to a 

lower frequency. A frequency divider could be a stand-alone instrument, a small 

circuit, or just a single chip. Most divider circuits divide by multiples of 10, so it is 

common to find circuits that divide by one thousand, one million, and so on. For 

example, dividing a 1 MHz signal by 106 produces a 1 Hz signal. Using low frequency 

signals reduces the problem of counter overflows and underflows and helps prevent 

errors that could be made if the start and stop signals are too close together. For 

example, a TIC might make 

errors when attempting to 

measure a time interval of 

<100 ns. 

 The time interval 

method is probably the most 

common method in use today. 

It has many advantages, 

including low cost, simple 

design, and excellent 

performance when measuring 

long term frequency offset or 

stability. 

The NIST Frequency 

Measurement and 

Analysis System

 We have now 

completed our discussion of 

how frequency calibrations are 

made. We've seen that a 

frequency calibration system 

must include several basic 

components: an oscillator to 

calibrate, a reference (preferably a NIST-traceable transfer standard), and a phase 

comparison device. We'll conclude our tutorial with a brief look at how the NIST 

Frequency Measurement and Analysis System (FMAS) implements the concepts we’ve 

discussed. 

 

Figure A.14. The NIST Frequency Measurement 

and Analysis System. 
84 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 A block diagram of the FMAS is shown in Figure A.14. The FMAS fits in a 

metal equipment rack (75 cm tall), and is computer controlled. Software developed at 

NIST controls all aspects of the calibration process. It makes measurements and stores 

and graphs them automatically. Up to five oscillators can be calibrated at once. The 

system produces calibration graphs that show the performance of each DUT for 

intervals ranging from 2 s to 150 days. 

 By looking at Figure A.14, you can see all the various components of a 

frequency calibration system. The FMAS uses a GPS receiver as a transfer standard. 

The GPS receiver produces a 1 kHz output frequency that provides traceability to 

NIST with an uncertainty of 2 × 10-13. 

 The FMAS makes phase comparisons using the time interval method. It 

includes a TIC with a single-shot resolution of less than 40 ps. The TIC achieves its 

high resolution through an interpolation scheme and includes a built-in multiplexer that 

enables it to switch between five inputs. This allows the FMAS to calibrate up to five 

oscillators simultaneously. The TIC also includes built-in divider circuitry and can 

directly accept an input frequency ranging from 1 Hz to 120 MHz on each of the five 

channels. The TIC is software controlled, so users don't have to set trigger levels. The 

system is connected to NIST through a telephone modem so that NIST personnel can 

troubleshoot the system remotely and analyze the calibration data. 

 As you can see, the FMAS implements many of the concepts discussed in this 

overview. It provides a calibration laboratory with a well-defined and documented 

calibration method, and is a major asset to laboratories seeking ISO compliance or 

laboratory accreditation. 
85 

Appendix A - An Introduction to Frequency Calibrations 

References 

[1] International Organization for Standardization (ISO). International Vocabulary of 

Basic and General Terms in Metrology. Switzerland: ISO; 1993. 59 p. 

[2] International Organization for Standardization (ISO). ISO/IEC Guide 17025, 

General Requirements for the Competence of Testing and Calibration Laboratories. 

Switzerland: ISO; 1999. 26 p. 

[3] American National Standards Institute (ANSI). ANSI/ISO/IEC Guide 17025:2000, 

General Requirements for the Competence of Testing and Calibration Laboratories. 

Boulder, Colorado: NCSL International; 2001. 26 p. 

[4] Lombardi, Michael A. An Introduction to Frequency Calibration: Part I. Cal Lab 

Int. J. Metrology: 17-28. 1996 January-February. 

[5] Taylor, B. N.; Kuyatt, C. E. Guidelines for Evaluating and Expressing the 

Uncertainty of NIST Measurement Results. Nat. Inst. Stand. Technol. Tech. Note 

1297; 1994 September. 24 p. 

[6] IEEE Standards Coordinating Committee 27 on Time and Frequency. IEEE 

Standard 1139-1999, IEEE Standard Definitions of Physical Quantities for 

Fundamental Frequency and Time Metrology - Random Instabilitites. New York, NY: 

IEEE, 1999. 31 p. 

[7] Sullivan, D. B.; Allan, D. W.; Howe, D. A.; Walls, F. L.; ed. Characterization of 

Clocks and Oscillators. Nat. Inst. Stand. Technol. Tech Note 1337; 1990 March. 352 p. 

[8] Jespersen, James. Introduction to the Time Domain Characterization of Frequency 

Standards. Proc. 23th Annu. Precise Time and Time Interval (PTTI) Meeting; 1991 

December; Pasadena, CA. 83-102. 

[9] Stein, S. R. Frequency and Time - Their Measurement and Characterization, 

chapter 12 in Precision Frequency Control, Vol. 2. Gerber, E. A.; Ballato, A.; ed. New 

York, NY: Academic Press; 1985. 191-232. Reprinted in [7]. 

[10] Howe, D. A.; Allan, D. W.; Barnes, J. A. Properties of Signal Sources and 

Measurement Methods. Proc. 35th Annu. Sym. Freq. Cont; 1981. 47 p. Reprinted in 

[7]. 

[11] Marrison, W. A. The Evolution of the Quartz Crystal Clock. Bell Systems Tech. 

27(3): 510-588; 1948. 
86 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

[12] Vig, J. R. Introduction to Quartz Frequency Standards. Army Research and 

Development Technical Report: SLCET-TR-92-1; 1992. 49 p. 

[13] Walls, F. L.; Gagnepain, J. Environmental Sensitivities of Quartz Oscillators. 

IEEE Trans. Ultrasonics, Ferroelectrics, Frequency Control 39(2): 241-249; 1992 

March. 

[14] Itano, W. M.; Ramsey, N. F. Accurate Measurement of Time. Sci. Am. 269(1):

56-65; 1993 July. 

 

[15] Lewis, L. An Introduction to Frequency Standards. Proc. IEEE 79(7): 927-935; 

1991 July. 

[16] Major, F. G. The Quantum Beat: The Physical Principles of Atomic Clocks. New 

York: Springer Verlag; 1998, 475 p. 

[17] Lombardi, Michael A. An Introduction to Frequency Calibration: Part II. Cal Lab 

Int. J. Metrology: 28-34. 1996 March-April. 

[18] Beehler, R.; Lombardi, M. A. NIST Time and Frequency Services. Nat. Inst. 

Stand. Technol. Spec. Publ. 432; 1991 June. 30 p. 

[19] Hoffmann-Wellenhof, B.; Lichtenegger, H.; Collins, J. Global Positioning 

System: Theory and Practice. New York: Springer-Verlag; 1997. 370 p. 

[20] Lombardi, Michael A; Nelson, Lisa. M.; Novick, Andrew N.; Zhang, Victor S. 

Time and Frequency Measurements Using the Global Positioning System. Cal Lab Int. 

J. Metrology: 26-33. 2001 July-September. 

[21] Zhang, Victor S., Davis, Dick D.; Lombardi, Michael A. High Resolution Time 

Interval Counter. Proc. 26th Annu. Precise Time and Time Interval (PTTI) Meeting; 

1994 December; Reston, VA. 191-200. 

[22] Novick, Andrew N., Lombardi, Michael A.; Zhang, Victor S.; Carpentier, 

Anthony. High Performance Multi-Channel Time Interval Counter with Integrated 

GPS Receiver. Proc. 31st Annu. Precise Time and Time Interval (PTTI) Meeting; 

1999 December; Dana Point, CA. 561-568. 
87 

Appendix B - Calibration Reports 

 Calibrations made using the FMAS are traceable to the National Institute of 

Standards and Technology (NIST) under NIST Test Number 76100S. If your system 

is recording data and is connected to a phone line, you will receive a monthly 

calibration report from NIST that documents measurement traceability. The 

calibration report that you receive is in full compliance with ISO Guide 17025. 

 GPS is officially referenced to the United States Naval Observatory (USNO), 

but the time offset between UTC(NIST) and UTC(USNO) is kept within 100 ns by 

mutual agreement, and the frequency offset is <1 × 10-13. Therefore, GPS can be used 

to establish traceability to either agency with very small uncertainties. The current 

time offset between UTC(USNO) and UTC(GPS) is available on the NIST web site: 

http://www.boulder.nist.gov/timefreq/pubs/bulletin/nistusno.htm 

 NIST also establishes traceability through GPS by monitoring each of the 

individual satellites as they fly over the Boulder, Colorado laboratories. This 

information is published daily at: 

http://www.boulder.nist.gov/timefreq/service/gpstrace.htm 

 

 Each calibration report is three pages long. The content of each page is 

described in the following sections. To obtain the lowest possible uncertainties on 

your calibration report, you must operate the FMAS as described in the manual and 

follow good laboratory practices. You must keep the FMAS running 24 hours a day, 

whenever possible. Your primary frequency standard should have an uninterruptible 

power supply and should also run 24 hours a day. 

Page 1

 The top portion of page 1 contains some general information about each 

customer including: the customer’s address, the main point of contact, and the 

manufacturer, model number, and type of the customer’s primary frequency standard 

(called the device under test). If this information is incorrect or if it changes, please 

contact NIST. 

Appendix B 

Calibration Reports
88 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 The rest of page 1 contains a brief description of the calibration method, and 

some general information about the calibration service. This text may change slightly 

from time to time if changes are made to the calibration service. 

 The bottom of page 1 contains a footer (also found on pages 2 and 3) that lists 

the issue date of the report, the report number, the NIST Test number, and the page 

number. The report number is a six-digit number that uniquely identifies the report. 

Page 2

 

 Page 2 contains a table that lists the daily frequency offset values for the device 

under test. Each offset value is the result of a 24-hour calibration. The table also lists a 

confidence level for each measurement, and a status code for each measurement. The 

status codes are assigned by NIST. The possible value for each status code is listed 

below. 

 

Code Description 

0 Valid Calibration - NIST has determined that all components of the 

measurement system were working properly during the calibration and that the 

calibration results are valid. This usually means that the calibration was within 

two standard deviations (2σ) of your average frequency. However, in some 

cases a valid calibration falls outside of the 2σ range by a small amount. 

1 No Data - No data was recorded. The measurement system was either turned 

off or malfunctioning. 

2 GPS Reception Error - NIST has determined that you had a problem with 

GPS reception during the calibration. The cause could be a malfunctioning 

receiver, antenna, cable, or an interference problem. 

3 GPS Broadcast Error - NIST has determined that a GPS satellite in view of 

your location was broadcasting invalid data that adversely influenced the 

calibration. This situation occurs only in rare instances. 

4 Measurement System Error - NIST has determined that the measurement 

system was malfunctioning due to a hardware or software failure. 

5 DUT error - Your primary frequency standard was out of normal tolerance 

during the calibration. 

6 DUT change - Your primary frequency standard was replaced by another 

standard. 
89 

Appendix B - Calibration Reports 

 If a status code other than 0 is assigned, NIST will provide a brief comment 

that explains the problem. If a status code of 0 is assigned it means that NIST has 

reviewed the calibration, found it to be valid, and no further comment is necessary. 

Page 3

 Page 3 contains a plot showing the daily frequency offset values for the device 

under test. The frequency offset values represent the performance of your primary 

standard during each 24 hour calibration. The x-axis shows the days of the month. 

Each tick mark represents one day. The y-axis shows the frequency offset of the 

primary oscillator. The middle of the y-axis represents a frequency uncertainty of 1 ×

10-12 or better. The plot accommodates oscillators that are high in frequency (the top 

half of the plot), or low in frequency (the bottom half). The plot shows frequency 

offset values as large as 1 × 10-8. If the frequency offset is >1 × 10-8, it is plotted along 

the top or bottom line of the plot. 

 Page 3 also contains an uncertainty statement. This statement lists the number 

of days in the month when data was recorded, and the number of days when your 

primary standard was in tolerance. It also lists the following values: 

 Mean Frequency Offset (MFO) - This is your primary standard’s mean (or 

average) frequency offset for the entire month. 

 Uncertainty (U), 2σ - This is the measurement uncertainty assigned to the 

calibration. Measurement uncertainty is contributed by the GPS receiver, by DUT 

aging and frequency drift, and by measurement system noise. NIST monitoring 

indicates that the received GPS signals have an uncertainty of 2 × 10-13 with respect to 

UTC(NIST). Measurement system noise contributes an uncertainty of 2 × 10-15. 

 FOlower and FOupper - These numbers represent the upper and lower bands of 

the performance of your frequency standard. There is a 95.4 % probability that the 

frequency offset of your standard was within the tolerance established by these bands 

at at any given instant during the month covered by the report. For example, if the 

lower band is –1.52 × 10-12 and the upper band is +9.44 × 10-13 there is a 95.4 % 

probability that the frequency offset remained within these limits. 

 Page 3 also lists the signatures and telephone numbers of the preparer of the 

calibration report and the manager of time and frequency services at NIST. Please 

contact these individuals if you have questions about the report. 

91 

Appendix C - Specifications and Compliance Documents 

 This section lists the specifications for the NIST Frequency Measurement and 

Analysis System (FMAS). It also lists the compliance documents that define the 

terminology and techniques used by the FMAS hardware, software, and 

documentation. 

Specifications 

 Number of measurement channels: 5 

 Frequencies directly accepted by the system: 1 Hz to 120 MHz 

 (1 Hz increments) 

 Maximum number of data points per graph: 3600 

 Sampling rate (all measurements): 1 Hz 

 Averaging period (long-term measurements): 1 h 

 Averaging period (short-term measurements): 1 s 

 

 Single shot measurement resolution: < 30 ps 

 Frequency uncertainty using GPS (24 h): 2 × 10-13 

 Frequency uncertainty without GPS (24 h): 2 × 10-15 

 (oscillator to oscillator comparisons)

 Stability during oscillator self test (1000 s): 1 × 10-14

 Auto-trigger sensitivity range (50 Ω): 200 mV to 3.5 V 

 (peak to peak) 

 70 mV to 1.25 V 

 (rms) 

Appendix C 

Specifications and Compliance Documents
92 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

Compliance Documents 

ANSI/ISO/IEC Guide 17025, General Requirements for the Competence of Testing and 

Calibration Laboratories, International Organization for Standardization (ISO), 2000. 

ANSI NCITS 310-1998, Representation of Time for Information Interchange, 

American National Standards Institute (ANSI), 1998. 

ISO/IEC Guide 17025, General Requirements for the Competence of Testing and 

Calibration Laboratories, International Organization for Standardization (ISO), 1999. 

International Organization for Standardization (ISO), International Vocabulary of 

Basic and General Terms in Metrology (VIM), 1993. 

NVLAP Calibration Laboratories Technical Guide, NIST Handbook 150-2, 2001. 

Taylor, Barry N.; Kuyatt, Chris E., Guidelines for Evaluating and Expressing the 

Uncertainty of NIST Measurement Results, NIST Technical Note 1297, 1993. 
93 

Appendix D - Glossary 

The definitions in this glossary have been obtained from several sources, including ISO 

(International Organization for Standardization) and ITU (International Telecommunications 

Union) documents. 

ACCURACY - The degree of closeness to which a measured value agrees with its 

definition at a given point in time. 

AGING - A systematic change in frequency with time due to internal changes in the 

oscillator. 

ALLAN DEVIATION - A widely-used statistical test for estimating the stability of an 

oscillator. Also referred to as the Allan variance. 

ANSI - American National Standards Institute. 

ATOMIC OSCILLATORS - Any oscillator whose resonance frequency is derived 

from an atomic frequency. The most common atomic oscillators are rubidium and cesium 

oscillators. 

ATOMIC TIME (TA) - A time scale based on the resonance frequency of the cesium 

atom. 

BIPM - International Bureau of Weights and Measures. The BIPM is located near Paris, 

France and maintains the International System (SI) of units, and Coordinated Universal 

Time (UTC). 

CALIBRATION - A set of operations that establish, under specified conditions, the 

relationship between values of quantities indicated by a measuring instrument or measuring 

system, and the corresponding standard or known values derived from the standard. 

Practically speaking, a calibration is a comparison between an unknown quantity (produced 

by the device under test) and a known quantity (produced by a reference). 

Appendix D 

Glossary
94 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

CALIBRATION METHOD - The technical procedure used to make a calibration. 

CESIUM OSCILLATOR - An atomic oscillator based on the resonance frequency 

of the cesium atom. Cesium oscillators provide the basis for the international definition of 

the second. 

CHARACTERIZATION - The process of measuring both the frequency 

uncertainty and stability of an oscillator, and stating both characteristics in a quantitative 

way. 

CLOCK - A device that counts a specific number of oscillations from a frequency 

source, and then displays or records the amount of time that has elapsed. For example, a 

quartz wristwatch counts 32 768 oscillations of a quartz crystal and records that one 

second has elapsed.

COARSE ACQUISITION CODE (C/A CODE) - A 1023 bit pseudo random 

noise code that is transmitted by the GPS satellites every millisecond. 

COORDINATED UNIVERSAL TIME (UTC) - An international atomic time 

scale governed by the BIPM. UTC is computed by averaging data from the world’s major 

metrology laboratories. 

DEVICE UNDER TEST (DUT) - The device being calibrated. Sometimes 

referred to as the unit under test (UUT). 

DIURNAL PHASE SHIFT - A phase shift that occurs at about the same time each 

day. A common example is a shift in the path delay of a radio signal caused by a change 

in the height of the ionosphere at sunrise and sunset. Diurnal phase shifts are common 

with LF and HF transfer standards. 

EPHEMERIS - A table that predicts the positions of extraterrestrial objects (such as 

the Sun, Moon, planets, or artificial satellites). 

FLICKER FLOOR - The averaging period that produces the best possible stability 

value for a frequency standard. Once a standard reaches its flicker floor, averaging for a 

longer period will not improve the stability estimate. Often called the flicker noise floor, 

or simply the noise floor. 
95 

Appendix D - Glossary 

FREQUENCY COUNTER - An electronic device that counts the number of 

cycles produced by a signal over a given time interval and, by doing so, determines the 

frequency of the signal. 

FREQUENCY DIVIDER - A device that divides a frequency to a lower 

frequency while preserving its phase. 

FREQUENCY OFFSET - The difference between the nameplate frequency and 

the reference frequency. Usually expressed as a unitless value, for example, –1 × 10-9, 

but it can be expressed in units of hertz. Sometimes called frequency accuracy, 

frequency difference, frequency error, or relative frequency. 

FREQUENCY UNCERTAINTY - The limits (upper and lower) of the 

measured frequency offset. Typically, frequency uncertainty is expressed using a 2σ

test, meaning that there is a 95.4 % probability that the frequency offset of the device 

under test will remain within the stated limits during the calibration. 

GLOBAL POSITIONING SYSTEM (GPS) - A radionavigation system 

maintained by the United States Department of Defense. GPS consists of a 

constellation of earth-orbiting satellites that carry atomic oscillators, and is widely used 

as a reference for frequency calibrations. 

HYDROGEN MASER - An advanced type of atomic oscillator based on the 

resonance frequency of hydrogen. 

INTRINSIC STANDARD - A standard used to define a unit of measurement 

that is based on a fundamental constant in nature. Atomic oscillators are intrinsic 

standards. 

ISO - International Organization of Standardization. 

LORAN-C - A ground-based radionavigation system operated by the United 

States Coast Guard and often used as a frequency reference. LORAN-C signals are 

broadcast at 100 kHz. 

LOW FREQUENCY (LF) - The portion of the radio spectrum from 30 to 300 

kHz. 
96 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

MEASURAND - The quantity being measured during a calibration. 

MICROSECOND - One millionth of a second (10-6 s). 

MILLISECOND - One thousandth of a second (10-3 s). 

MODIFIED JULIAN DATE (MJD) - A way of telling what day it is with the 

least possible ambiguity. The Julian date number is obtained by counting days with a 

starting point of noon on January 1, 4713 B.C., which was defined as Julian day zero. The 

Modified Julian date is equal to the Julian date shifted so that its origin occurs at midnight 

on November 17, 1858. The MJD differs from the Julian date by exactly 2 400 000.5 

days. 

NAMEPLATE FREQUENCY - The frequency labeled on an oscillator’s output, 

or the frequency that an oscillator is supposed to produce. The nameplate frequency is an 

ideal frequency with zero uncertainty. Often called the nominal frequency. 

NANOSECOND - One billionth of a second (10-9 s). 

NCSL - National Conference of Standards Laboratories. 

NIST - National Institute of Standards and Technology. 

NVLAP - National Voluntary Laboratory Accreditation Program, a division of NIST. 

OSCILLATOR - An electronic device that produces signals of a specified frequency. 

OVEN - A temperature-controlled chamber used to enclose a quartz crystal. By 

providing a stable environment for the crystal, an oven improves the oscillator’s 

performance. 

PATH DELAY - The amount of time that elapses as a time or frequency signal travels 

from the transmitter to the receiver. 

PHASE COMPARISON - A measurement technique used to measure the 

difference between two frequencies. 

PHASE STEP - A sudden change in phase. 
97 

Appendix D - Glossary 

PICOSECOND - One trillionth of a second (10-12 s). 

PIEZOELECTRIC EFFECT - The oscillation of a quartz crystal that takes 

place when a voltage is applied. 

PSEUDO RANDOM NOISE CODE (PRN) - A spread spectrum waveform 

that delivers encoded information but appears to be noise. 

PULSE PER SECOND (PPS) - An electrical pulse that is generated once per 

second. If the pulse is synchronized to UTC, it is said to be on-time. 

QUARTZ OSCILLATOR - A common type of oscillator that produces a 

frequency derived from the mechanical resonance of a quartz crystal. 

REFERENCE - A device with a known uncertainty relative to national or 

international standards that is used to calibrate another device. Also called a standard. 

RESOLUTION - The degree to which a measurement can be determined. 

RESONANCE FREQUENCY - The fundamental frequency of an oscillator. 

Atomic oscillators derive their resonance frequency from the the physical properties of 

an atom. Quartz oscillators derive their resonance frequency from the mechanical 

properties of a quartz crystal. 

RUBIDIUM OSCILLATOR - An atomic oscillator which derives its 

frequency from the resonance frequency of the rubidium atom. 

SECOND - The basic unit of time interval, defined as the duration of 9 192 631 770 

energy transitions of a cesium atom. All units of frequency and time interval are 

defined based on the second. 

STABILITY - The statistical estimate of the frequency fluctuations of a signal 

over a given time interval. 

STANDARD - A device with a known level of uncertainty that is used to calibrate 

another device. Also called a reference. 
98 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

TEST UNCERTAINTY RATIO (TUR) - The performance ratio between the 

reference and the device under test during a calibration. A TUR of 10:1 is preferred, but a 

lower ratio can be used if more averaging is done. 

TIME BASE - The component inside an instrument (such as a counter) that provides 

the reference for all time and frequency related functions. 

TIME CODE - A digital data stream that contains date and time-of-day information. 

TIME INTERVAL COUNTER - A device that measures the amount of time that 

elapses between the arrival of two electrical signals. 

TIME INTERVAL METHOD - A phase comparison technique that uses a time 

interval counter. 

TIME SCALE - An unambiguous ordering of events, or a system for keeping time. 

TRACEABILITY - The property of the result of a measurement or the value of a 

standard whereby it can be related to stated references, usually national or international 

standards, through an unbroken chain of comparisons, all having stated uncertainties. 

TRANSFER STANDARD - A radio receiver that receives, processes, and 

distributes a signal that serves as a traceable reference for calibrations. Transfer standards 

used for frequency typically receive GPS, LORAN-C, or WWVB signals. 

UNCERTAINTY - Parameter, associated with the result of a measurement, that 

characterizes the dispersion of values that could reasonably be attributed to the measurand. 

USNO - The United States Naval Observatory, located in Washington, DC. 

WWV - A high frequency (HF) radio station operated by NIST in Fort Collins, 

Colorado. WWV broadcasts standard time and frequency information on 2.5, 5, 10, 15, 

and 20 MHz. NIST operates a similar station named WWVH from Hawaii. 

WWVB - A low frequency (LF) radio station operated by NIST in Fort Collins, 

Colorado. WWVB broadcasts standard time and frequency information on a carrier 

frequency of 60 kHz. 
99 

Index 

accuracy 61, 93 

Allan deviation (or Allan variance) 41-43, 62-65, 93 

antenna, GPS 3, 6 

automatic plotting 17-19 

bar graph 15-17, 19, 21 

block diagram of system 2, 83 

calibration 55-57, 94 

changing oscillators 19 

channel names 12 

characterization 94 

color monitor 5 

commands accepted by the system 

 ALL 37 

 CalibrateTIC 49 

 CONTROL-P 17 

 CONTROL-X 20, 23 

 COPY 26-28, 34, 43, 45 

 DEMO 24, 37 

 DOS 14 

 EDIT 29, 31 

 EXIT 14 

 FILE 43 

 FILTER 31-32, 34, 53 

 GO 40 

 GPS 46-48 

 HI 31-32 

 HISTORY 44-46 

 LENGTH 40 

 LO 31-32 

 LOAD 24, 37 

 MAGNIFY 34 

Index
100 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

 MEASURE 15, 19 

 METER 44 

 NAMES 12, 19, 29 

 OFFSET 40-41 

 PICKCHANNEL 39 

 PLOT 27 

 PORTION 33-34 

 QUIT 14, 24-28, 31, 35, 37, 39, 45 

 RESET 31-32, 34 

 SELECT 38 

 SHORTTERM 39 

 STABILITY 41-43 

 STATISTICS 26-27 

 TITLE 35 

 UTILITIES 39 

 VIEW 24-26 

 XAXIS 34-35 

 YAXIS 34-35 

 YES 14 

connecting oscillators 8-9 

Coordinated Universal Time (UTC) 17, 94 

data 

 printing 26 

 retrieving 37-38, 43-44 

 viewing 24-26 

demonstration data 24 

exiting the FMAS software 14 

frequency files 45-46, 52 

frequency offset 16, 19, 26, 29, 31, 40-41, 43-46, 56, 58-61, 87, 95 

frequency uncertainty 56, 58-61, 89, 91, 95 

glossary 93-98 

Global Positioning System (GPS) 1-3, 6, 8-12, 46-48, 53, 76-78, 87-89, 91, 95 

installing the system 1-7 
101 

Index 

measurement screen 15-17 

menus 

 exiting a menu 14 

 learning to use them 13-14 

 selecting a menu item 13-14 

microsecond 18, 100 

modem 7 

Modified Julian Date (MJD) 25, 44, 96 

monitor, color 5 

nanosecond 17, 100 

NO SIGNAL message 20 

oscillators 

 changing 19-20 

 connecting to system 8-9 

 primary 8 

parts included in system 

 list of 1 

plotting a portion of the plot on screen 33-34 

power outages iii, 2 

primary oscillator 

 performance history 44-46 

 selecting 8 

 timebase, used as 8 

 traceability 9, 87 

printer 

 cable 7 

 changing types 53 

 checking status 20 

 paper 1, 9 

printing the data 26 

quick check of system 20-21 

r (correlation coefficient) 26, 29, 41 

range of data 26 

reference frequency 9, 89 
102 

NIST Frequency Measurement and Analysis System: Operator’s Manual 

removing data points 31, 32 

retrieving your data 37-38, 43-44 

setup software 

 pathname for software 52 

 pathname for phase data 52 

 pathname for demo data 52 

 pathname for frequency data 52 

 pathname for short term data 52 

 select a printer 53 

 changing the plot time 52 

 setting the file capacity 52 

short-term oscillator performance 39-44 

specifications 95 

stability 39-43, 61-65, 97 

starting the system 10-12 

supplies iii, 1 

tape backup 3-4 

telephone line iii, 7 

timebase jumper 8 

time interval counter 1, 8-9, 49, 81-84, 98 

title of plot 29 

traceability 

 definition 56, 98 

 monthly reports 9, 55-57, 87-89 

uninterruptible power supply (UPS) 1, 4-7 

viewing data 24-26 

viewing the data statistics 26-27 

xaxis 

 labels 18-19 

 position 34-35 

 tick mark interval 29 

yaxis 

 labels 18-19 

 scale 34-35