The progress in the development of a solid nitrogen cooled dual-mode frequency-temperature-compensated sapphire-resonator oscillator

A dual-mode single crystal sapphire resonator with a frequency-temperature turning point at approximately 50.6 K has been designed (patent pending). It utilizes the inherent anisotropy of the temperature coefficient of frequency of two orthogonally polarized quasi-TE/TM modes separated by approximately 3.5 GHz as the oscillator output. The Q-factors of both modes have been measured to be both around 10⁸. The dual mode oscillator was constructed as two individual loop oscillators, each connected to the cryogenic resonator via separate coaxial cables and coupling probes in transmission. The probes were oriented to maximize coupling to the design mode and minimize coupling to the other orthogonally polarized mode. The loop oscillators were operated with Pound locked servos. The beat frequency between the oscillators was measured against a HP8673 synthesizer referenced to an oscilloquartz 8600 OCXO stable reference. A fractional frequency Allan deviation of about 7×10^-13/τ¹2 / was obtained from 1 s to 26 s. Above 26 s the measurement was limited by the noise floor of the measurement system.