Microwave oscillators of exceptional short term stability have been realised from cryogenic sapphire resonators with loaded Q-factors of 109 at X-band and 6 K by employing a synthesis of loop oscillator operation with active Pound frequency stabilisation. These oscillators have exhibited a fractional frequency stability of 3 – 4 × 10-15 for integration times from 0.3 to 100 degrading to 10-14 at 1000 s, and long term drifts of a few parts in 1013 per day.
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Luiten et al. (1994) studied this question.
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