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. Isolation of RF fields by the special nature of the electromagnetic mode allows the very low loss of the sapphire itself to be realized. Several mode families have been identified with fairly 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. Based on the measurements and on the performance of commercially available phase detectors, the performance for a cooled resonator operating at 77 K with a Q of 3×10⁷ is projected 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