MEASUREMENT AND ANALYSIS OF A MICROWAVE-OSCILLATOR STABILIZED BY A SAPPHIRE DIELECTRIC RING RESONATOR FOR ULTRA-LOW NOISE

Phase-noise measurements are presented for a microwave oscillator whose frequency is stabilized by a "whispering gallery" mode sapphire ring resonator with Q of 2 × 105. The nature of the mode, which involves little metallic conduction, allows nearly full use of the very low dielectric loss in sapphire. Several mode families have been identified with good agreement with calculated frequency predictions. Waveguide coupling parameters have been characterized for the principal (lowest frequency) mode family, for n = 5 to n = 10 full waves around the perimeter. For a 5-cm wheel resonator in a 7.6-cm container, Q-values above 105 were found at room temperature for all of the modes in this sequence. Coupling Q-values for these same modes ranged from 104 (n = 5) to 105 (n = 10) for a WR112 waveguide port at the center of the cylindrical wall of the containing can. Phase noise measurements for a transistor oscillator locked to the n = 10 (7.84 GHz) mode showed a 1/f3 dependence for low offset frequencies, and a value of ℒ (ƒ) = −55 dB/Hz at an offset of 10 Hz from the carrier. We believe this value to be lower than any previously published for a non-cryogenic X-band oscillator. Based on these measurements and on the performance of commercially available phase detectors, we project performance for a cooled resonator operating at 77 K with a Q of 3 × 107 to be −85 dB/Hz at an offset of 1 Hz. This value is 30 dB below that of the best X-band source presently available, a frequency-multiplied quartz crystal oscillator. © 1990 IEEE