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ard. the National Bureau o f Standards (NBSI develoDedBased on a comnercially available rubidium stand-

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. NBS4 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 X 10-l6 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 X 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 f 10) X with the value obtained for NBS-III in 1969-which value is preserved in the rate of the NBS Atomic Time Scale.

A technique is presented that allows one to measure directly the fluctuations of the natural resonant frequency of quartz crystal resonators in a passive circuit. This technique greatly aids in modeling the noise in both crystal resonators and crystal controlled oscillator circuits. Definite changes of slope in the spectral density of the frequency fluctuations,as a function of frequency offset from the natural resonant frequen of the crystals indicate that several mezanisms are contributing to the frequency instabilities in crystals. Our measurements also indicate that the electronics in the oscillators seriously degrade the frequency stability for sample times less than 100 S. The effects are especially dramatic for times less than 1 S.