Hi The “risk” with any fitting process is that it can act as a filter. Fitting a single sine wave “edge” to find a zero is not going to be much of a filter. It will not impact 1 second ADEV much at all. Fitting every “edge” for the entire second *will* act as a lowpass filter with a fairly low cutoff frequency. That *will* impact the ADEV.
Obviously there is a compromise that gets made in a practical measurement. As the number of samples goes up, your fit gets better. At 80us you appear to have a pretty good dataset. Working out just what the “filtering” impact is at shorter tau is not a simple task. Indeed this conversation has been going on for as long as anybody has been presenting ADEV papers. I first ran into it in the early 1970’s. It is at the heart of recent work recommending a specific filtering process be used. Bob > On Nov 22, 2017, at 10:58 AM, Ralph Devoe <[email protected]> wrote: > > Hi time nuts, > I've been working on a simple, low-cost, direct-digital method for > measuring the Allan variance of frequency standards. It's based on a > Digilent oscilloscope (Analog Discovery, <$300) and uses a short Python > routine to get a resolution of 3 x 10(-13) in one second. This corresponds > to a noise level of 300 fs, one or two orders of magnitude better than a > typical counter. The details are in a paper submitted to the Review of > Scientific Instruments and posted at arXiv:1711.07917 . > The method uses least-squares fitting of a sine wave to determine the > relative phase of the signal and reference. There is no zero-crossing > detector. It only works for sine waves and doesn't compute the phase noise > spectral density. I've enclosed a screen-shot of the Python output, > recording the frequency difference of two FTS-1050a standards at 1 second > intervals. The second column gives the difference in milliHertz and one can > see that all the measurements are within about +/- 20 microHertz, or 2 x > 10(-12) of each other, with a sigma much less than this. > It would interesting to compare this approach to other direct-digital > devices. > > Ralph DeVoe > KM6IYN > <Capture.JPG>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
