During this, the NIST Centennial Year, I look back on the first 100 years and summarize NIST work on time and frequency measurement, a topic of some prominence from the early years of the institution. Initial work at NIST (then NBS, the National Bureau of Standards) focused primarily on frequency standards needed to support the control of broadcast frequencies in the early days of radio. The Time and Frequency Division was not created until 1967, by which time a number of primary atomic frequency standards had already been developed, and it was clear that a broader program in this field was needed. This paper describes a variety of programs including the development of primary frequency standards, the international coordination of standards, statistical techniques for characterizing noise, dissemination methods, and methods for measuring optical frequency.

Time and frequency measurement at NIST: the first 100 years

The authors present a broad overview of time and frequency technology, particularly those trends relating to the generation and distribution of time and frequency signals. The authors provide a general look at these trends. They refer the reader to other papers, particularly to those in this issue, for greater detail. The topics considered are: background, including accuracy and stability, frequency standards, time transfer systems, and network synchronization; time generation, including various types of atomic clocks and quartz oscillators; and time distribution, including applications of time transfer concepts and practical synchronization limits. The characterization of components and systems is also addressed. >

Time generation and distribution

http://ieeexplore.ieee.org/iel5/7585/20668/00956152.pdf

2001 IEEE International Frequency Control Symposium and PDA Exhibition TIME AND FREQUENCY MEASUREMENT AT NIST: THE FIRST 100 YEARS

A coded time/date information dissemination system (CTD), based on telephone lines and commercial modems, is now in its experimental phase in Italy at IEN. This service, born from a cooperation with other metrological laboratories (TUG, Austria, SNT, Sweden, VSL, The Netherlands), represents an attempt towards an European standardization. Some results of an experimental analysis in which a few modems were tested, both in laboratory conditions and connected to the telephone network, in order to evaluate the timing capability of the system are given. When the system is used in a one-way mode, in many practical cases the modems delay turns out to be the main factor which limits the accuracy, even more than the telephone line delays. If the two-way mode is used, the modems asymmetry, i.e., the delay difference between transmission and reception, is almost always the most important source of uncertainty, provided the link is not including a space segment. Comparing the widely used V.22 modems to the old V.21 ones, the latters turn out to be better both in delay time (30-100 ms V.22, and 7-15 ms V.21) and asymmetry (10-50 micro-s V.22, and 10 ms V.22). Time transfer accuracies of 10 micron-s (same turn) to 100 micro-s (long distance calls) were obtained in two-way mode with commercial V.21 modems.

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Performances of a date dissemination code on telephone lines using commercial modems

The state of development of advanced timing systems at NIST is described. The work on cesium and rubidium frequency standards, stored-ion frequency standards, diode lasers used to pump such standards, time transfer, and methods for characterizing clocks, oscillators, and time distribution systems is presented. The emphasis is on NIST-developed technology rather than the general state of the art in this field.

/pdf/time-and-frequency-technology-at-nist-4dsqgmg1pk.pdf

Time and frequency technology at NIST

: Telephone time dissemination can serve a broad spectrum of users who require time accuracy in the millisecond range and who are reluctant to use radio time dissemination services. A system which uses a simple protocol for telephone loop-delay correction is presented. The protocol uses ASCII characters transmitted by the frequency shift keying techniques of standard 300 baud modems. The protocol is designed for the transfer of the correct time from a master clock to a slave clock. The slave clock places the telephone call, puts the master clock into a prompt echo mode, measures the loop delay time, and applies half the loop delay time as a correction to the time it receives from the master clock. The precision and accuracy of this protocol are discussed and measurements are presented on the precision of time transfer using Leitch CSD-5300 units as master and slave clocks. The capabilities of the full system, and the Leitch CSD 5300 units which are used at each distribution node, are discussed as possible solutions to questions that are often addressed (or misaddressed) to "Precise Time" research groups.

D. Jackson

Papers

A Telephone-Based Time Dissemination System