Direct Stability Measurement of Cryogenic Sapphire Oscillators with Tracking DDSs and Two-Sample Covariance

We demonstrate the first stability measurement of 10 GHz cryogenic sapphire oscillators (CSOs) at the 100 MHz output by direct comparison of three units. Thanks to the low background noise of the Tracking DDS, and to the use of two-sample covariances, it is no longer necessary to beat the oscillators down to the HF region. The stability of our oscillators is of 2×10
^-15
at 1 s, limited by the CSO internal synthesizer, while the flicker floor of one unit is of 3×10
^-16
.