Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators

Two nominally identical ultra-stable microwave oscillators are compared. Each incorporates a sapphire resonator cooled to near 6 K in an ultra-low vibration cryostat using a pulse-tube cryocooler. The phase noise for a single oscillator is measured at -105 dBc/Hz at 1 Hz offset on the 11.2 GHz carrier. The oscillator fractional frequency stability, after subtracting a linear frequency drift of 3.5 x 10(-14)/day, is characterized by 5.3 x 10(-16) tau(-1/2) + 9 x 10(-17) for integration times 0.1 s < tau < 1000 s and is limited by a flicker frequency noise floor near 1 x 10(-16). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709479]