Low noise, microwave signal generation using cryogenic, sapphire dielectric resonators: an update

Results obtained using sapphire dielectric resonators for automatic frequency control (AFC) stabilization of an X-band surface acoustic wave resonator voltage-controlled oscillator (SAWR VCO)-based source are reported. The resonators are operated in a low order mode and employ high temperature superconducting films in the metal enclosure covers in order to obtain unloaded Q-factors of 3×10⁵ to 5×10⁵. Signal source flicker-of-frequency noise and noise floor levels are measured at X -band. The measured values for flicker-of-frequency noise are 10 dB poorer than anticipated, based on the large value of discriminator sensitivity obtained. Possible causes for current limitations in signal near-carrier spectral performance include resonator short-term frequency fluctuations resulting from environmentally induced effects and/or frequency-drive (AM-to-FM) effects. Resonator vibration sensitivities on the order of 1×10^-8 per g are measured