Power stabilized cryogenic sapphire oscillator

Microwave oscillators of exceptional short-term stability have been realized from cryogenic sapphire resonators with loaded Q factors in excess of 10/sup 9/ at 11.9 GHz and 6 K. This has been achieved by a power stabilized loop oscillator with active Pound frequency stabilization. These oscillators have exhibited a fractional frequency stability of 3-4/spl times/10/sup -15/ for integration times from 0.3 to 100 s. The relative drift of these two oscillators over one day is a few times 10/sup -13/. To reduce the long-term drift, which is principally due to excessive room temperature sensitivity, we have added cryogenic sensors for the power and frequency stabilization servos to one of these oscillators. We have also implemented a servo to reduce the room temperature sensitivity of our phase modulators. Testing of this oscillator against a Shanghai Observatory H-maser has shown an Allan deviation of 4/spl times/10/sup -15/ from 600 to 2000 s.>