Oscilent Corporation - Technical References
Introduction to Quartz Frequency Standards

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Introduction to Quartz Frequency Standards - Specifications, Standards, Terms, and Definitions

Numerous specifications and standards exist which relate to frequency standards. The major organizations responsible for these documents are the Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC), the CCIR, and the U. S. Department of Defense, which maintains the Military Specification (MIL-SPEC) system. A listing of "Specifications and Standards Relating to Frequency Control" can be found in the final pages of the Proceedings of the IEEE Frequency Control Symposium (and its predecessor, the Proceedings of the Annual Symposium on Frequency Control). In the 1990 Proceedings, for example, 79 such documents are listed [48]. Many of the documents include terms and definitions, some of which are inconsistent. Unfortunately, no single authoritative document exists for terms and definitions relating to frequency standards. The terms and definitions in the CCIR glossary [16], in IEEE Std. 1139-1988 [19], and in MIL-0-55310's section 6 [28] are the most recent; they address different aspects of the field, and together form a fairly good set of terms and definitions for users of frequency standards.

The most comprehensive document dealing with the specification of frequency standards is MlL-0-55310 [28]. The evolution of this document over a period of many years has included periodic coordinations between the government agencies that purchase crystal oscillators and the suppliers of those oscillators. The document addresses the specifications of all the oscillator parameters discussed above, plus many others. This specification was written for crystal oscillators. Because the output frequencies of atomic frequency standards originate from crystal oscillators, and because no comparable document exists that addresses atomic standards specifically, MlL-0-55310 can also serve as a useful guide to specifying atomic standards.

MIL-STD-188-115, Interoperability and Performance Standards for Communications Timing and Synchronization Subsystems' specifies that the standard frequencies for nodal clocks shall be 1 MHz, 5 MHz, or 5 X 2N MHz, where N is an integer. This standard also specifies a 1-pulse-per-second timing signal of amplitude 10 V, pulse width of 20 mS, rise time less than 20 ns, fall time less than 1 ms; and a 24-bit binary coded decimal (BCD) time-code that provides Coordinated Universal Time (UTC) time of day in hours, minutes, and seconds, with provisions for an additional 12 bits for day of the year, and an additional four bits for describing the figure of merit (FOM) of the time signal. The FOMs range from BCD Character 1 for better than 1 ns accuracy to BCD character 9 for "greater than 10 ms of fault'' [49].