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

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 approximately 1 10 - 15 tau - 1 / 2 for integration times 1 < tau < 10 s with a minimum sigma y = 3.9 10 - 16 at tau = 20 s. The long term frequency drift is less than hbox 5 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 approximately - 97.5 dBc/Hz at 1-Hz offset from the carrier.