Accuracy Evaluation and Stability of the NBS Primary Frequency Standards

The National Bureau of Standards has two primary standards for frequency and the unit of time. They are both cesium devices and are designated NBS-4 and NBS-5. The design of NBS-5 is discussed in detail, including its relationship to its predecessor NBS-III, and a brief description of NBS-4 is given. NBS-4 and NBS-5 have been used since January 1973 for a total of twelve calibrations of the NBS Atomic Time Scale. The application of pulsed microwave excitation, and the use in the accuracy evaluations of frequency shifts due to known changes in the exciting microwave power are discussed. Measurements of the atomic velocity distributions are reported. A stability of 9 × 10-15 derived from the comparison of NBS-4 and NBS-5 is reported for averaging times of 20 000 s, and data on accuracy are given. Results obtained to date give an evaluated accuracy of 1-2 × 10-13 with indications that this accuracy may be improved in the future. The bias-corrected frequencies of NBS-4 and NBS-5 agree to within (1 ± 10) × 10-13 with the value obtained for NBS-III in 1969¿which value is preserved in the rate of the NBS Atomic Time Scale.