Experimental results on a 9.2-GHz superconducting cavity stabilized oscillator

Experimental results of the frequency stabilization of a Gunn diode oscillator by a 9.2-GHz TM₀₁₀ niobium superconducting cavity (unloaded Q=4×10⁸) are described. The cavity is used as the frequency-determining element in a frequency discriminator circuit. The technique of frequency stabilization is reviewed. The frequency shift of the superconducting cavity stabilized oscillator (SCSO) induced by the mismatch of a square-law detector in the discriminator is studied. By improving this problem, the fractional frequency fluctuation due to room-temperature changes is reduced to less than 1×10^-13/°C. The frequency stability of the SCSO is measured by comparing it with the auto-tuned hydrogen maser. The frequency stability of σ_y(τ) reached 1.4×10^-14 at τ=500 s, after subtraction of the linear frequency drift. The measured SNR of the discriminator shows that even with the comparatively low Q cavity, short-term frequency stability of 1.4×10^-14/√τ is achieved in the SCSO. The frequency fluctuation due to mechanical deformation of the cavity caused by the tilt and vibration of the dewar, however, seems to be serious. The measured acceleration sensitivity of the SCSO was 6.5×10^-8/g for a vibration frequency of less than 80 Hz.