Improvements in Atomic Cesium Beam Frequency Standards at the National Bureau of Standards

Abstract

The National Bureau of Standards Frequency Standard, NBS-III, a cesium beam with a 3.66-meter interaction region, has been in operation since 1963. The last published (1966) accuracy capability for NBS-III was 1.1 × 10-12(lσ). Recently, several new solid-state broad-band frequency-multiplier chains have been constructed. Reduction of the random phase noise by more than 20 dB compared to the previous state of the art has been obtained consistently. In addition, a solid-state servo system has been installed to control the frequency of the 5-MHz slave oscillator. Comparisons were made between NBS-III and one of the commercial cesium standards in the NBS clock ensemble. The relative fractional frequency stability σ(N = 2, T = 7 days, σ = 1 day) = 1 × 10-13 was observed for nine weekly comparisons. The very-long-term frequency stability for this recently improved NBS-III system has not been evaluated fully. Due to the improvements both in electronic systems and evaluative techniques, however, an accuracy of 5 × 10-13(1σ) for a single evaluative experiment is reported. Substantial effort is being expended toward improvement of the accuracy and figure of merit (presently 10) of the NBS cesium standard. The modified system, to be called NBS-5, is expected to be in operation in the latter half of 1970 and to exhibit a figure of merit in excess of 500.