Closed loop tests of the NASA sapphire phase stabilizer

A cryogenic sapphire phase stabilizer (SPS) has been developed to meet microwave oscillator phase noise requirements. The SPS uses a high-Q, X-band sapphire dielectric whispering-gallery mode resonator as a discriminator to stabilize a quartz crystal oscillator. At an untuned frequency of 7.9449 GHz with a loaded Q of 6 million, an "open loop" discriminator noise floor (referred to 100 MHz) of approximately S_phi(f) = -110 dB/f³(/Hz) was previously reported for offset frequencies from 1 Hz to 1 kHz. Precise tuning of the sapphire resonant frequency now allows implementation of simplified control loops together with suppressed-carrier phase sensing circuitry. These improvements make possible an ultralow-noise demonstration of closed-loop SPS performance. From 1 Hz to 1 kHz a comparison of the SPS with a quartz crystal reference oscillator of the highest quality showed only the noise of the reference oscillator. These results appear to represent the lowest phase noise in a closed-loop or active sapphire oscillator to date at temperatures achievable with liquid nitrogen (77 K or higher)