On Sun, Jan 20, 2019 at 5:10 PM Magnus Danielson <[email protected]> wrote:
> Hi Mark, > > I have not seen anyone comment on this post yet. > > It's an interesting approach and in general, this is one of the ways we > can expect that frequency/stability measurements to be done these days, > by sampling the RF and analyze it. > > It would be interesting to figure out why you have a "bump" there, so I > wonder what part does that. > The bump in the later plot, with the Bodnar measured against the Trueposition is almost certainly due to a PLL. There is the Bodnar and also the Wenzel and I don't know exactly what each is doing and what all the time constants are. I have no clue why there is a bump on the first wraparound plots at about Tau=1000s. > > For one thing, if the frequency estimation algorithm you depend on does > averaging/least-square style of algorithms, the lower taus will be > significantly lower than expected. TvB has some pretty good plots from > experiments illustrating this. > > In my various experiments I could see when it was averaging and the lower Taus did decrease. I chose such that this did not happen. I reviewed TvB's paper at http://www.leapsecond.com/pages/adev-avg/ many times and saw most of the effects described there, resolution, noise and averaging. > Another thing, one reason to get a "bump" is due to high Q in a PLL > circuit. > > However, I'd be careful to judge it to be any of these without more > careful look on the data. > > It can be useful to alternate views to figure things out. For instance, > swapping between MDEV and TDEV could give you some hints. Similarly > swapping between frequency and phase does the same in raw-data view. > > For phase/frequency plots, it may be worthwhile to average the data > using the +/- keys in TimeLab, as the filtering away high-frequency > noise may make it easier to see lower-frequency variations. > I've done this ad nauseam, eventually finding some anomaly that happened at various times of the day. > > In general, I recommend you to have a third reference to play around > with and measure. As one fools around with different combinations one > learns to see which artefacts follows which device. > I'm pretty sure the Bodnar is the least stable device I have, as it is based on a TCXO and it does have some issues that I know Leo has improved, but I can't spare the time to send it back to him right now. I don't have another device to use that will give me the 80 MHz I need for the SDR clock that also gives me good phase noise. Both the Trueposition and Bodnar/Wenzel are pretty good, at least better than what I am measuring. > > Do keep up the investigations. Try different approaches and learn. > > At this time, I am going to use what I have. I think it is the best approach I could come up with, using the available equipment. Regards, Mark _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
