Ultra-high stability cryocooled sapphire microwave oscillators

Two nominally identical microwave cryocooled sapphire oscillators (CSO) have been implemented at the University of Adelaide. The sapphire resonators have a turning point in their frequency-temperature dependence at approximately 6 K, which delivers first-order insensitivity to temperature fluctuations when operated at this point. Using a closed system ultra-low vibration pulse-tube cryocooler with a specially design cryostat [1], it is possible to control the rms temperature fluctuations at the resonator to the 10 μK level, while maintaining a low vibration environment. Combined with a loaded Q-factor of about 10⁹, similar oscillators have shown an Allan deviation of fractional frequency fluctuations of σ_y = 5.8×10^-16 at 1 s [2].