Cryo-cooled sapphire oscillator with mechanical compensation

We present test results for a short-term frequency standard, 40 K Compensated Sapphire Oscillator (CSO). Included are measured resonator Q values, cryocooler vibration data, silver spacer construction process, and copper wall sensitivity. The 40 K CSO design goals are a frequency stability of 1×10^-14 ( 1 second ≤ τ ≤ 100 seconds), a year or more continuous operation, and a compact rack-mount configuration. The 40 K CSO bridges the gap of two previous technologies: 10 K CSO and 77 K CSO. In particular, the 10 K CSO incorporated a Gifford-McMahon type of cryocooler with no cryogens, while the 77 K CSO developed thermo-mechanical compensation operating at 80 K. The 40 K CSO can serve as an independent oscillator or as a "slaved" local oscillator controlled by atomic standards like the Hg ion trap, enabling their inherent performance.