695 NBS TECHNICAL NOTE NATIONAL BUREAU OF STANDARDS The National Bureau of Standards’ was established by an act of Congress March 3, 1901. The Bureau’s overall goal is to strengthen and advance the Nation’s science and technology and facilitate their effective application for p.ublic benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation’s physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to pro- mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the lnstitute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the Office of Experimental Technology lncentives Program. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consist- ent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essen- tial services leading to accurate and uniform physical measurements throughout the Nation’s scientific community, industry, and commerce. The Institute consists of the Office of Measurement Services, and the following center and divisions: - Applied Mathematics - Electricity - Mechanics - Heat - Optical Physics - Center for Radiation Research Lab- oratory Astrophysics’ - Cryogenics’ - Electromagnetics‘ - Time and Frequencyg. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measure- ment, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational insti- tutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials, the Office of Air and Water Measurement, and the following divisions: Analytical Chemistry - Polymers - Metallurgy - Inorganic Materials - Reactor Radiation - Physical Chemistry. THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services developing and promoting the use of avail- able technology; cooperates with public and private organizations in developing technological standards, codes, and test meth- ods; and provides technical advice services, and information to Government agencies and the public. The Institute consists of the following divisions and centers: Standards Application and Analysis - Electronic Technology - Center for Consumer Product Technology: Product Systems Analysis; Product Engineering - Center for Building Technology: Structures, Materials, and Safety; Building Environment; Technical Evaluation and Application - Center for Fire Research: Fire Science; Fire Safety Engineering. THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment: and serves as the principal focus wthin the exec- utive branch for the development of Federal standards for automatic datl processing equipment, techniques, and computer languages. The Institute consist of the following divisions: Computer Services - Systems and Software - Computer Systems Engineering - Information Technology. THE OFFICE OF EXPERIMENTAL TECHNOLOGY INCENTIVES PROGRAM seeks to affect public policy and process to facilitate technological change in the private sector by examining and experimenting with Government policies and prac- tices in order to identify and remove Government-related barriers and to correct inherent market imperfections that lmgede the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific informa- tion generated within NBS: promotes the development of the National Standard Reference Data System and a system Of in- formation analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists Of the following organizational units: Office of Standard Reference Data - Office of Information Activities - Office of Technical Publications - Library - Office of International Standards - Office of International Relations. Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washineton, D.C. 20234. * Located at Boulder, Colorado 80302. Time and Frequency Users' Manual Edited by George Kamas Time and Frequency Division Institute for Basic Standards National Bureau of Standards Boulder, Colorado 80302 ~~ ~ U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary Sidney Harman, Under Secretary Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director Issued May 1977 NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 695 Nat. Bur. Stand. (U.S.), Tech Note 695, 217 pages (May 1977) CODEN: NBTNAE U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1977 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 CONTENTS PAGE CHAPTER 1. INTRODUCTION : WHAT THIS BOOK IS ABOUT . 1.1 THE DEFINITIONS OF TI.M E AND FREQUENCY - 1 1.2 WHAT IS A STANDARD? . 2 . 1.2.1 Can Time Really Be a Standard? . - 3 1.2.2 The NBS Standards of Time and Frequency - 3 1.3 HOW TIME AND FREQUENCY STANDARDS ARE DISTRIBUTED . . 5 . 1.4 THE NBS ROLE IN INSURING ACCURATE TIME AND FREQUENCY . 5 1.5 WHEN DOES A MEASUREMENT BECOME A CALIBRATION? . 6 1.6 TERMS USED . 7 1.6.1 Mega, Milli, Parts Per... and Percents - 7 . 1.6.2 Frequency . 8 . 1.7 DISTRIBUTING TIME AND FREQUENCY SIGNALS VIA CABLES AND TELEPHONE LINKS . 10 1.8 TIME CODES 12 CHAPTER 2. THE EVOLUTION OF TIMEKEEPING . . 2.1 TIME SCALES 15 . . 2.1.1 Solar Time . . 15 2.1.2 Atomic Time 16 . A. Coordinated Univers.a l Time * 17 B. The New UTC System * 17 . . 2.1.3 Time Zones 18 . . 2.2 USES OF TIME SCALES 18 . 2.2.1 Systems Synchronization . . 18 2.2.2 Navigation and Astronomy 20 . . 2.3 INTERNATIONAL COORDINATION OF TIME AND FREQUENCY ACTIVITIES 21 . . 2.3.1 The Role of the International Time Bureau (BIH) . . 21 2.3.2 The Role of the National Bureau of Standards (NBS) 22 . 2.3.3 The Role of the U. 5. Naval Observatory (USNO) and the DOD PTTI Program 23 CHAPTER 3. THE ALGEBRA AND CONVENTIONS FOR TIME AND FREQUENCY MEASUREMENTS . 3.1 EXPRESSING FREQUENCY DIFFERENCE (IN HERTZ, kHz, MHz, ETC.) ,. 26 3.2 RELATIVE (FRACTIONAL) FREQUENCY, F (DIMENSIONLESS) 26 3.3 RELATIVE (FRACTIONAL) FREQUENCY DIFFERENCE, S (DIMENSIONLESS) * 27 3.3.1 Example of Algebra and Sign Conventions in Time and Frequency Measurements 27 . A. Television Frequency Transfer Measurements . .* 27 B. Television Line-10 Time Transfer Measurements 28 . 3.4 USING TIME TO GET FREQUENCY *. 29 3.5 A MATHEMATICAL D. ERIVATION 30 . 3.6 DEFINITIONS 31 iii PAGE CHAPTER 4. USING TIME AND FREQUENCY IN THE LABORATORY . 4.1 ELECTRONIC COUNTERS 33 . 4.1.1 Frequency Measurements 35 . A. Direct Measurem. ent 35 B. Prescaling . 36 C. Heterodyne Converters . 36 D. Transfer Oscillators 37 . 4.1.2 Period Measurements 40 . 4.1.3 Time Interval Measurements 42 . 4.1.4 Phase Measurements . 43 4.1.5 Pulse-Width Determination Measurements 44 . 4.1.6 Counter Accuracy 45 . A. Time Base Erro.r 45 B. Gate Error 46 . C. Trigger Errors 47 . 4.1.7 Printout and Recording 50 4.2 OSCILLOSCOPE METHODS 50 . k2.1 Calibrating the Oscilloscope Time .E ase 51 4.2.2 Direct Measurement of Fr.e quency 52 4.2.3 Frequency Comparisons 52 . A. Lissajous Patterns . . 53 B. Sweep Frequency Calibration: An Alternative Method 58 . 4.2.4 Time interval Measurements 59 4.3 WAVEMETERS 59 4.4 HETERODYNE FREQ~ENCY'METE~S 60 . 4.5 DIRECT-READING ANALOG FREQUENCY METERS 62 . 4.5.1 Electronic Audio Frequency Meters 62 . . 4.5.2 Radio Frequency Meters 63 . . 4.6 FREQUENCY COMPARATORS. 63 4.7 AUXILIARY EQUIPMENT 64 . 4.7.1 Frequency Synthesizers 64 . 4.7.2 Phase Error Multip.li ers 65 4.7.3 Phase Detectors . 65 4.7.4 Frequency Dividers 67 . A. Analog or Regenerat.iv e Dividers 67 6. Digital Dividers 68 . 4.7.5 Adjustable Rate Dividers 69 . 4.7.6 Signal Averagers 69 . 4.8 PHASE LOCK .T ECHNIQUES 70 4.9 SUMMARY 71 CHAPTER 5. THE USE OF HIGH FREQUENCY BROADCASTS FOR TIME AND FREQUENCY CALIBRATIONS . 5.1 BROADCAST FORMATS 74 . 5.1.1 WWV/WWVH 74 5.1.2 CHU 75 IV -PAGE . . 5.2 RECEIVER SELECTION 76 5.3 CHOICE OF ANTENNAS AND SIGNAL CHARACTERISTICS 77 . 5.4 USE OF HF BROADCASTS FOR TIME CALIBRATIONS 81 . 5.4.1 Time-of-Day Announcements 81 5.4.2 Using the Seconds Ticks 81 . A. Receiver Time Delay Measurements 82 . . B. Time Delay Over the Radio Path . 83 . C. Using an Adjustable Clock to Trigger the Oscilloscope 83 D. Delay Triggering: An Alt.e rnate Method that Doesn't Change the Clock Output . 86 E. Using Oscilloscope Photography for Greater Measurement Accuracy 88 . . 5.4.3 Using the WWV/WWVH Time Code 89 . A. Code Format 89 B. Recovering the Code 91 . 5.5 USE OF HF BROADCASTS FOR FREQUENCY CALIBRATIONS 92 . 4.5.1 Beat Frequency Method 93 . 5.5.2 Oscilloscope Lissajous Pattern Method 95 5.5.3 Oscilloscope Pattern Drift Method . . 97 5.5.4 Frequency Calibrations by Time Comparison of Clocks 98 . 5.6 FINDING THE PROPAGATION PATH DELAY 99 . 5.6.1 Great Circle Distance Calculations 99 . 5.6.2 Propagation Delays 100 . . 5.7 THE NBS TELEPHONE TIME-OF-DAY SERVICE 104 . . 5.8 SUMMARY 104 CHAPTER 6. CALIBRATIONS USING LF AND VLF RADIO TRANSMISSIONS . . 6.1 ANTENNAS FOR USE AT VLF-LF 105 6.2 SIGNAL FORMATS . . .. 106 6.3 PROPAGATION CHARACTERISTICS AND OTHE.R PHASE CHANGES . 106 6.4 FIELD STRENGTHS OF VLF-LF STATIONS 110 . . 6.5 INTERFERENCE 112 . . 6.6 USING WWVB FOR FREQUENCY CALIBRATIONS 112 6.6.1 Phase-Shift Identification . . 113 . . 6.6.2 Methods of Frequency Comparison 114 . . 6.7 USING OTHER LF AND VLF SERVICES FOR FREQUENCY CALIBRATIONS I15 . . 6.7.1 The Omega Navigation System 115 . . 6.7.2 NLK, NAA, and MSF 116 . . 6.8 MONITORING DATA AVAILABILI.T Y . 116 6.9 USING THE WWVB TIME CODE I16 . . 6.9.1 Time Code Format . 117 . 6.9.2 Time Transfer Using the Transmitted Envelope 118 . . 6.10 SUMMARY 120 V - PAGE CHAPTER 7. FREQUENCY CALIBRATIONS USING TELEVISION SIGNALS . . 7.1 HOW THIS FITS INTO OTHER NBS SERVICES . . 121 7.2 BASIC PRINCIPLES OF THE TV FREQUENCY CALIBRATION SERVICE 122 . . 7.2.1 Phase Instabilities of the TV Signals . . 123 7.2.2 Typical Values for the U. S. Networks 124 . . 7.3 HOW RELATIVE FREQUENCY IS MEASURED 126 . . 7.3.1 Color Bar Comparator . . 127 7.3.2 The NBS System 358 Frequency Measurement Computer 129 . . A. Block Diagram Overview 130 . 8. Use of FMC with Unstable Crystal Oscillators 133 . . 7.4 GETTING GOOD TV CALIBRATIONS 133 CHAPTER 8. FREQUENCY AND TIME CALIBRATIONS USING TV LINE-1 0 . . 8.1 HOW THE SERVICE WORKS 135 . . 8.1.1 Using Line-IO on a Local Basis . . 136 8.1.2 Using Line-IO for NBS Traceability 136 . . 8.2 EQUIPMENT NEEDED : I38 . 8.3 TV LINE-10 DATA, WHAT DO H;E Ni.M BERS MEAN? . 139 8.4 WHAT DO YOU DO WITH THE DATA? 140 . . 8.5 RESOLUTION OF THE SYSTEM . 140 8.6 GETTING TIME OF DAY FROM LINE-iO 142 . . 8.7 MEASUREMENTS COMPARED TO THE USNO . 142 . 8.8 LINE-IO EQUIPMENT AVAILABILITY 142 CHAPTER 9. LORAN-C TIME AND FREQUENCY METHODS . . 9.1 BASIC PRINCIPLES OF THE LORAN-C NAVI'GATION SYSTEM I45 . . 9.1.1 What is the Extent of Loran-C Coverage? 147 . . A. Groundwave Signal Range . . 147 8. Skywave Signal Range 147 . . 9.2 WHAT DO WE GET FROM LORAN-C? 148 . . 9.2.1 Signal Charac.t eristics . 148 9.2.2 Time Setting . . I50 9.2.3 Frequency Calibrations Using Loran-C 152 . 9.3 HOW GOOD IS LORAN-C? 152 . . 9.3.1 Groundwave Accuracy. . 153 9.3.2 Skywave Accuracy 154 . . 9.4 ARE THE DATA VALID? 154 CHAPTER 10. AN INTRODUCTION TO FREQUENCY SOURCES . . 10.1 FREQUENCY SOURCES AND CLOCKS . . 156 10.2 THE PERFORMANCE OF FREQUENCY SOURCE. S . 158 10.3 USING FREQUENC.Y STABILITY DATA . I60 10.4 RESONATORS . . 161 10.5 PRIMARY AND SECONDARY STANDARDS 163 vi -PAGE . . 10.6 QUARTZ CRYSTAL OSCILLATORS 164 . . 10.6.1 Temperature and Aging of Crystals 166 . . 10.6.2 Quartz Crystal Oscillator Performance I67 . 10.7 ATOMIC RESONANCE DEVICES 168 . . 10.7.1 State Selection . . I68 10.7.2 How to Detect Resonanc.e . 169 10.7.3 Atomic Oscillators . . 172 10.7.4 Atomic Resonator Frequency Stability and Accuracy I72 lo. 8 AVA I LABLE ATOM IC FREQUENCY DEV I CES . 173 . . 10.8.1 Cesium Beam Frequency Oscillators . . 173 10.8.2 Rubidium Gas Cell Frequency Oscillators 175 . . 10.8.3 Atomic Hydrogen Masers 176 . . 10.9 TRENDS 177 CHAPTER 11. SUMMARY OF AVAILABLE SERVICES . . 179 . GLOSSARY 183 INDEX 191 TABLES Table 1.1 PREFIX CONVERSION CHART . Table 1.2 CONVERSIONS FROM PARTS PER... TO PERCENTS . Table 1.3 CONVERSIONS TO HERTZ . Table 1.4 RADIO FREQUENCY BANDS Table 4.1 READINGS OBTAINED WITH VARIOU.S SETTINGS OF TIME BASE AND PERIOD MULTIPLIER CONTROLS 41 . Table 5.1 HF TIME AND FREQUENCY BROADCAST STATIONS 73 . Table 5.2 IMAGE FREQUENCIES FOR WWV/WWVH 77 . . Table 6.1 LF-VLF BROADCAST STATIONS 105 . . Table 9.1 U. S. EAST COAST LORAN-C CHAIN 145 . . Table 9.2 U. S. WEST COAST LORAN-C CHAIN 146 Table 9.3 GROUP REPETITION INTERVALS . . 148 Table 10.1 COMPARISON OF FREQUENCY SOURCES . . 178 . Table 11.1 CHARACTERISTICS OF THE MAJOR T&F DISSEMINATION SYSTEMS 179 . . Table 11.2 HOW FREQUENCY IS CALIBRATED 180 . . Table 11.3 HOW TIME IS CALIBRATED 180 vi i FIGURES -PAG-E . Figure 1 .I Organization of the NBS Frequency and Time Standard 4 . Figure 1.2 IRlG Time Code, Format H 12 . Figure 1.3 IRlG Time Code Formats 13 Figure 2.1 Universal Time Family Relationships . 16 . Figure 2.2 First Atomic Clock. 16 . Figure 2.3 Classes of Time Scales & Accuracies 17 Figure 2.4 Dating of Events in the Vicinity of a Leap Second . 17 . Figure 2.5 Standard Time Zones of the World Referenced to UTC 19 Figure 2.6 NBS, USNO, and BIH Interactions 21 . Figure 4.1 Electronic Counters 33 Figure 4.2 Simplified Block Diagram of an Electronic Counter . 34 Figure 4.3 Diagram of a Counter in the Frequency Measurement Mode . 35 Figure 4.4 Prescaler-Counter Combination . 36 . Figure 4.5 Typical Manually-Tuned Heterodyne Converter 37 . Figure 4.6 Typical Manually-Tuned Transfer Oscillator 38 Figure 4.7 Automatic Transfer Oscillators . 39 . Figure 4.8 Diagram of Counter in the Period-Measurement Mode 40 . Figure 4.9 Diagram of Counter in the Multiple-Period Mode 41 Figure 4.10 Effect of Noise on Trigger Point in Period Measurements . 42 . Figure 4.11 Diagram of Counter in the Time Interval Mode 43 Figure 4.12 Optimum Trigger Points for Start-Stop Pulses . * 43 Figure 4.13 Use of Time-Interval Unit for Phase Measurements . 44 Figure 4.14 Time-Interval Counter with Start-Stop Channels Connected to . Common Source for Pulse-Width or Period Measurements 45 Figure 4.15 Typical Time Base Stability Curve . 46 Figure 4.16 Constant Gating Interval with Ambiguity of 1 Count . 46 Figure 4.17 Accuracy Chart for Period and Frequency Measurements . 47 . Figure 4.18 Undifferentiated Schmitt-Trigger Waveforms 48 Figure 4.19 Time Error Produced by Improper Calibration of Trigger Level Control . 49 Figure 4.20 Sine Wave Method of Checking Trigger Level Calibration . 49 Figure 4.21 Digital-to-Analog Arrangement for Chart Recording . 50 . Figure 4.22 Simplified Block Diagram of a Cathode-Ray Oscilloscope 50 Figure 4.23 Waveform of Horizontal Deflection Voltage From Time-Base Generator 51 Figure 4.24 Time Base Calibration Hookup Using an External Frequency Source . 51 Figure 4.25 Sine Wave Display as Viewed on Oscilloscope . 52 . Figure 4.26 Equipment Hookup for a 1:l Lissajous Pattern 53 Figure 4.27 Elliptical Lissajous Patterns for Two Identical Frequencies of . Different Phase 53 . Figure 4.28 Development of a 3:l Lissajous Pattern 54 . Figure 4.29 Lissajous Patterns 55 Figure 4.30 Arrangement for Measuring Small Frequency Differences with . Oscilloscope and Frequency Multipliers 56 Figure 4.31 Double-Balanced Mixer 56 viii