> Apart from the delayed measurement point, I have not been able > to identify any issues. > > The frequency spectrum filtered out by the averaging is waaaay to > the left of our minimum Tau. > > Phase wrap-around inside bursts can be detected and unfolded > in the processing. > > Am I overlooking anything ?
I think this is basically a valid thing to do, under specific conditions. It works well with the Wavecrest DTS boxes, which can take several thousand samples per second and distill them down to a single TI reading. I've observed good agreement between ADEV plots taken with the DTS2070 and direct-digital analyzers down to the ~2E-12 level at t=1s. The main caveat is that the burst of averaged readings needs to take up a very small portion of the tau-zero interval, as you point out, to keep the averaging effects out of the visible portion of the plot. This might not be a very big problem with MDEV but it is definitely an issue with ADEV. The second thing to watch for is even more important: while the host software (TimeLab, Stable32, etc.) can handle phase wraps that occur between counter readings, the sub-reading averaging algorithm in the counter will not. Phase wraps in the middle of a burst will corrupt the data badly, and I don't see any reliable way to detect and unfold them after the fact. So if John's firmware can handle intra-burst phase wraps before returning each averaged reading to the software, it could be a big win. Otherwise this technique should be confined to cases where two extremely stable sources are being compared with no possibility of phase wraps. It would be a good way to keep an eye on the short-term behavior of a pair of Cs or GPS clocks, for instance. -- john, KE5FX Miles Design LLC _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.