Low-noise, high resolution microwave synthesis for atomic frequency standards

221-227The performance of newer generations of ultra high stability atomic frequency standards based on laser cooled atoms and ions appears to be limited by the local oscillator and the microwave synthesis chain. To improve this performance, the design considerations of microwave synthesizers for Cs hyperfine clock transition at 9.192 GHz that emphasize high resolution, low phase modulation (PM) and amplitude modulation (AM) noise, low spurs near the carrier, and very high phase stability with respect to environmental effects have been studied. Typical performance achieved are: internal fractional frequency stability of 1.5 × 10-15 at 10 s and 1 × 10-18 at 1 day; fractional frequency step of 2 × 10-17 and temperature coefficient less than 0.1-0.5 ps/K. Our design is quite general and enables generation of microwave outputs corresponding to several different atomic transitions. The phase locking of the synthesizer to a high stability external frequency reference based on a cavity stabilized oscillator has been studied