High stability 40 Kelvin cryo-cooled sapphire oscillator

We present initial test results for a new short-term frequency standard, the 40 K compensated sapphire oscillator (40 K CSO). Included are measurements of resonator quality factor, operational frequency, turn-over temperature, acceleration (g) sensitivity, frequency drift, and preliminary frequency stability tests. Long-term operation is facilitated use of a small cryo-cooler (1 KW wall power) with no moving cryogenic parts, and thus no preset service interval for the cold head. Initial tests with this single-stage pulse-tube cooler show a stability of 3×10-14 (1 second ≤τ≤10 seconds) and a flicker floor of 2×10-14