Speaker
Description
The cryogenic sapphire oscillator (CSO) is a highly specialized oscillator delivering a microwave signal exhibiting extremely low instability. The Allan deviation $σ_y(τ)$ is of parts in $10^{-15}$ at 1 s, with a flicker floor of parts in $10^{-16}$, and some drift beyond a few hours.
After the results shown at the 8th Frequency Standard and Metrology Symposium, we spent a significant effort in understanding and engineering the oscillator. We gathered data about resonators of different manufacturers, including the spread of Q and temperature turning point, related to the frequency stability; and about the long-term operation, faults, interruptions, etc. Unlike optical FP etalons, no lock fault has been detected in the CSO.
The current version of the CSO can run continuously, requiring only one in-field maintenance (1 H manpower of a trained engineer) every 2nd year. Drift, in most cases $<10^{-14}$/day, proved to be extremely regular, and easy to model and remove. The CSO is now a semicommercial product available to qualified users from Franche Comté Innov, a non-profit Company owned by the University of Franche Comté, in turn a French Gov institution.
Finally, the CSO outperforms the optical FP in terms of reliability and drift, and exhibits the most desirable characteristics for use as the flywheel of atomic frequency standards. It deserves consideration for a maser-free time scale.
