Microwave Local Oscillator for a Cesium Frequency Standard Synthesized from a Cryogenic Sapphire Oscillator

A synthesized microwave local oscillator for a cesium atomic frequency standard has been implemented using an ultra-stable oscillator. The oscillator was based on a high Q-factor cryogenic-sapphire-resonator cooled with liquid helium which operates on a Whispering Gallery mode. The cesium hyperfine transition frequency of 9.192 GHz was synthesized from the 10.812 GHz oscillation frequency, chosen because the corresponding mode had the best frequency stability. When compared with a hydrogen maser reference the local oscillator exhibited a fractional frequency stability of 6×10-15 for integration times of 600 to 1200 s at 9.192 GHz, limited only by the cryogenic sapphire oscillator.