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(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(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 x 10(-12) for measuring times 1 s less than or equal to tau less than or equal to 100 s with unstabilized resonator housing temperature and a mode Q of 2 x 10(6). We project a frequency stability 10(-14) for this resonator with stabilized housing temperature and with a mode Q of 10(7).