Reliability and Reproducibility of the Cryogenic Sapphire Oscillator Technology

The cryogenic sapphire oscillator (CSO) is a highly specialized machine, which delivers a microwave reference signal exhibiting the lowest frequency fluctuations [Allan deviation (ADEV)
$\sigma _{y}(\tau)$
] for the integration time
$\tau $
between 1 and
$10^{4}$
s, indeed four decades. In such interval, good units feature
$\sigma _{y}(\tau)< 10^{-15}$
, with
$< 10^{-14}$
drift in one day. The oscillator is based on a sapphire monocrystal resonating at 10 GHz in a whispering-gallery mode, cooled at
$\approx 6$
K for zero thermal coefficient and optimal quality factor
$Q$
. We report on the progress accomplished in implementing 11 CSOs in about ten years since the first sample was delivered to the Malargüe station of the European Space Agency (ESA) in Argentina. Short-term stability is improved by a factor of 3–10, depending on
$\tau $
, and the refrigerator’s electric power is reduced to a 3-kW single-phase line. Frequency stability and overall performances are reproducible, with unattended operation between scheduled maintenance every two years. The CSO is now a semicommercial product suitable to scientific applications requiring extreme frequency stability with the reliable unattended long-term operation, like the flywheel for primary frequency standards, the ground segment of global navigation satellite system (GNSS), astrometry, very long baseline interferometry (VLBI), and radio astronomy stations.