  This article shows the first measurement of three 100 MHz signals exhibiting
fluctuations from 2E-16 to parts in 1E-15 for integration time tau between 1 s
and 1 day. Such stable signals are provided by three Cryogenic Sapphire
Oscillators (CSOs) operating at about 10 GHz, also delivering the 100 MHz
output via a dedicated synthesizer. The measurement is made possible by a
6-channel Tracking DDS (TDDS) and the two-sample covariance tool, used to
estimate the Allan variance. The use of two TDDS channels per CSO enables high
rejection of the instrument background noise. The covariance outperforms the
Three-Cornered Hat (TCH) method in that the background converges to zero "out
of the box," with no need of the hypothesis that the instrument channels are
equally noisy, nor of more sophisticated techniques to estimate the background
noise of each channel. Thanks to correlation and averaging, the instrument
background (AVAR) rolls off with a slope 1/sqrt(m), the number of measurements,
down to 1E-18 tau = 1E4 s. For consistency check, we compare the results to the
traditional TCH method beating the 10 GHz outputs down to the MHz region. Given
the flexibility of the TDDS, our methods find immediate application to the
measurement of the 250 MHz output of the FS combs.
