,>'.)*• r>rJfl HKra mitt* >\ « i If I * I IT I . Ip I * .aference nbs Publi- cations / % ^m \ NBS TECHNICAL NOTE 695 U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards Time and Frequency Users' Manual 100 .U5753 No. 695 1977 NATIONAL BUREAU OF STANDARDS 1 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 public benefit To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, a technical (3) 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 Institute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the Office of Experimental Technology Incentives 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- 2 oratory Astrophysics — Cryogenics 2 — Electromagnetics 2 — Time 3 and Frequency . 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 ENSTITUTE 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 data 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 impede 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. 1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234. a Located at Boulder, Colorado 80302. I . 1 I C C 1 ! 1977 6 CC- Time and Frequency Users' Manual r |9 H Edited by George Kamas Time and Frequency Division Institute for Basic Standards National Bureau of Standards Boulder, Colorado 80302 / W \ V * •"•cAU o* U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary Sidney Harman, Under Secretary Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology 0$. NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director u i 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, DC. 20402 CONTENTS PAGE CHAPTER 1. INTRODUCTION: WHAT THIS BOOK IS ABOUT 1.1 TFTE DEFINITIONS OF TIME 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, Mill i , 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 Universal 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. S. 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 (DIMENS I0NLESS) 26 3.3 RELATIVE (FRACTIONAL) FREQUENCY DIFFERENCE, S (DIMENS I0NLESS) 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 DERIVATION 30 3.6 DEFINITIONS 31 PAGE CHAPTER 4. USING TIME AND FREQUENCY IN THE LABORATORY 4.1 ELECTRONIC COUNTERS 33 4.1.1 Frequency Measurements . 35 A. Direct Measurement . 35 B. Prescal ing 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 Error 45 B. Gate Error .... 46 C. Trigger Errors 47 4.1.7 Printout and Recording . 50 4.2 OSCILLOSCOPE METHODS .... 50 4.2.1 Calibrating the Oscilloscope Time Base 51 4.2.2 Direct Measurement of Frequency 52 4.2.3 Frequency Comparisons 52 A. Lissajous Patterns . 53 B. Sweep Frequency Calibration: An A ternative Method 58 4.2.4 Time Interval Measurements 59 4.3 WAVEMETERS 59 4.4 HETERODYNE FREQUENCY METERS 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 *.7 AUXILIARY EQUIPMENT . 64 4.7.1 Frequency Synthesizers . 64 4.7.2 Phase Error Multipliers 65 4.7.3 Phase Detectors 65 4.7.4 Frequency Dividers 67 A. Analog or Regenerative D viders 67 B. Digital Dividers 68 4.7.5 Adjustable Rate Dividers 69 4.7.6 Signal Averagers 69 4.8 PHASE LOCK TECHNIQUES 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 1 v 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.^.1 Time-of-Day Announcements 81 5.4.2 Using the Seconds Ticks 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 Alternate Method that Doesn't Change the Clock Output ........ 86 E. Using Oscilloscope Photography for Greater Measurement Accuracy 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 5.5.1 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 ...