Latest results of the U.W.A. cryogenic sapphire oscillator

Two nominally identical separate 12 GHz oscillators based on 5 cm diameter cryogenic sapphire resonators were compared using a double heterodyne method. Locking of the oscillator frequency to the resonator is provided by active Pound stabilization and a second servo which removes the deleterious effects of amplitude modulation generated by the phase modulator. The resonator temperature and incident microwave power are also kept under servo control. The diode detectors for the power and Pound frequency stabilization servos are adjacent to the resonator in the cryogenic environment, which eliminates their room temperature sensitivity. At short times, from 0.3 to 30 s, the Allan standard deviation is about 2.5×10^-15 τ^-1/2 and limited by the measurement system and servo system noise floors. At about 50 s the Allan deviation reaches a minimum of 8×10^-16 . This represents frequency stabilization to better than I ppm of the resonator bandwidth. Up to 100 seconds the stability degrades as approximately 1 to 2×10^-16 τ^1/2, which is consistent with the earlier measurement of 4×10^-15 for one oscillator against a H-maser. This medium term drift appears to be associated with a high oscillator acceleration sensitivity (~0.5 to 3×10^-9 g^-1) and mechanical instability of the resonator