Cryogenic sapphire resonator-oscillator with exceptional stability: an update

Microwave oscillators of exceptional short term stability have been realized from cryogenic sapphire resonators with loaded Q factors in excess of 10⁹ at 11.9 GHz and 6 K. This has been achieved using a synthesis of a power stabilized loop oscillator and active Pound frequency stabilization. These oscillators have exhibited a fractional frequency stability of 3-4×10^-15 for integration times from 0.3 to 100 seconds. The relative drift of these two oscillators over one day is a few times 10^-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 on one of these oscillators. We have also implemented a servo to reduce the room temperature sensitivity of out phase modulators. Testing of this oscillator against a Shanghai Observatory H-maser has shown an Allan deviation of less than 4×10 ^-15 from 600 to 2000 s