Frequency stability and phase noise of a pair of X-band cryogenic sapphire oscillators

Oscillators based on cryogenic sapphire resonators can supply the levels of microwave phase noise and frequency stability required for advanced time-and-frequency applications. NPL has realized two such oscillators of identical design, which operate at 9.204 GHz and incorporate both Pound-loop and power stabilization. Running both simultaneously, and comparing their outputs both to each other and to H-maser-referenced frequency sources, the phase noise and absolute frequency stability of these oscillators have been measured for the first time, based on both FFT-spectrum analyser and frequency-counter data. We report a double-sided phase noise of -87.5 dBc at 1 Hz, scaling as 1/f(3), and a modified Allan deviation of less than 5 x 10(-15) at 1 s; the fractional frequency drifts of the two oscillators with respect to the H-maser were -2.9 x 10(-11) and -6.0 x 10(-12) per day, respectively. Scope for further improvement is assessed.