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 6K in an ultra-low vibration cryostat using a pulse-tube cryocooler. The phase noise for a single oscillator is measured at −105dBc/Hz at 1Hz offset on the 11.2GHz carrier. The oscillator fractional frequency stability, after subtracting a linear frequency drift of 3 . 5 × 10 - 14 /day, is characterized by 5 . 3 × 10 - 16 τ - 1 / 2 + 9 × 10 - 17 for integration times 0 . 1 s < τ < 1000 s and is limited by a flicker frequency noise floor near 1 × 10 - 16 .