State-of-the-art cryocooled sapphire oscillators

Two nominally identical ultra-low vibration cryocooled microwave sapphire oscillators are compared. The phase noise of a single oscillator was measured to be -105 dBc/Hz at 1 Hz offset on the 11.2 GHz carrier and is characterized by white frequency noise. The Allan deviation at 1s integration time, with drift removed, is 5.8 × 10^-16. Between 0.1 and 100 s, the frequency stability is white frequency noise limited with a minimum of 1.6 × 10^-16 around 100 s. The long-term stability of the oscillator degrades due to a sensitivity to diurnal room temperature changes. In the absence of this effect the oscillator has potential to reach a stability below 10^-16.