Stability and phase noise tests of two cryocooled sapphire oscillators

A cryocooled compensated sapphire oscillator (CSO), developed for the Cassini Ka-band Radio Science experiment, and operating in the 7 K-10 K temperature range was previously demonstrated to show ultra-high stability of /spl sigma//sub y/=2.5/spl times/10/sup -15/ for measuring times 200 seconds /spl les//spl tau//spl les/600 seconds using a hydrogen maser as reference. CSO-1 and CSO-3 are now both operational with new low noise receivers. We have made initial phase noise and Allan deviation measurements that show more than ten times stability improvement over the hydrogen maser for measuring times 1 second /spl les//spl tau//spl les/10 seconds, and indicate performance for the individual units of /spl sigma//sub y//spl ap/3/spl times/10/sup -15/ for measuring times from 10 to 1000 seconds. Phase noise is reduced by 20 to 28 dB over the design offset frequency range from 1 Hz to 40 Hz. Receiver design is also discussed.