Ultra-Low Vibration Pulse-Tube Cryocooler Stabilized Cryogenic Sapphire Oscillator With ${hbox{10}}^{-16}$ Fractional Frequency Stability

摘要

A low maintenance long-term operational cryogenic sapphire oscillator has been implemented at 11.2 GHz using an ultra-low-vibration cryostat and pulse-tube cryocooler. It is currently the world's most stable microwave oscillator employing a cryocooler. Its performance is explained in terms of temperature and frequency stability. The phase noise and the Allan deviation of frequency fluctuations have been evaluated by comparing it to an ultra-stable liquid-helium cooled cryogenic sapphire oscillator in the same laboratory. Assuming both contribute equally, the Allan deviation evaluated for the cryocooled oscillator is $sigma_y approx 1 times 10^{-15}tau^{-1/2}$ for integration times $1 < tau < 10$ s with a minimum $sigma_y = 3.9 times 10^{-16}$ at $tau = 20$ s. The long term frequency drift is less than ${hbox{5}} times {hbox{10}}^{-14}$/day. From the measured power spectral density of phase fluctuations, the single-sideband phase noise can be represented by ${cal L}$$_{phi}(f) = 10^{-14.0}/f^4+10^{-11.6}/f^3+10^{-10.0}/f^2+10^{-10.2}/f+ 10^{-11.0}$ rad$^2$ /Hz for Fourier frequencies $10^{-3} < f < 10^3$ Hz in the single oscillator. As a result, ${cal L}$$_{phi} approx -97.5$ dBc/Hz at 1-Hz offset from the carrier.