Temperature-compensated sapphire resonator for ultra-stable oscillator capability at temperatures above 77 K

We report on the design and test of a whispering gallery sapphire resonator for which the dominant (WGH/sub n11/) microwave mode family shows frequency-stable, compensated operation for temperatures above 77 K. The resonator makes possible a new ultra-stable oscillator (USO) capability that promises performance improvements over the best available crystal quartz oscillators in a compact cryogenic package. A mechanical compensation mechanism, enabled by the difference between copper and sapphire expansion coefficients, tunes the resonator to cancel the temperature variation of sapphire's dielectric constant. In experimental tests, the WGH/sub 811/ mode showed a frequency turnover temperature of 87 K in agreement with finite element calculations. Preliminary tests of oscillator operation show an Allan Deviation of frequency variation of 1.4-6/spl times/10/sup -12/ for measuring times 1 s /spl les//spl tau//spl les/100 s with unstabilized resonator housing temperature and a mode Q of 2/spl times/10/sup 6/. We project a frequency stability 10/sup -14/ for this resonator with stabilized housing temperature and with a mode Q of 10/sup 7/.<
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