Long-term experience with cesium beam frequency standards

Since its introduction 7 years ago, the Hewlett-Packard cesium beam frequency standard has been installed in many national laboratories. At the present, this standard represents over 70% of the number of atomic clocks reported to the Bureau International des Poids et Mesures (BIPM). Many of these are monitored on a continuous basis. Data from 160 clocks HP currently reported by BIPM is used to estimate overall frequency accuracy. Data taken at the United States Naval Observatory (USNO) on a daily basis for a period of years for in excess of 40 clocks gives a reasonably clear picture of overall life expectancy and day-to-day performance variations. For example, instances of anomalous beam current performance, which causes a premature instrument shutdown, have been observed. Data from the National Institute of Standards and Technology (NIST) gives an unbroken string of frequency deviations for over 1500 days on two clocks, and lesser time span for two other clocks. This allows an estimate of the upper limit of a frequency stability flicker floor. The accumulated data also gives further insight into cesium beam tube end-of-life behavior. Production, test, and verification data is taken routinely on every clock in the manufacturing process. The database now contains data on well over 1000 clocks. A recent analysis of data from all sources has highlighted actual clock performance in a variety of environments. The details of the analysis and the results obtained form the core of the paper