Hi The problem with absolute frequency is the one they ran into in the 60’s (and before):
There is no really good way to measure it. You certainly can take data. The data can have lots of resolution. That part has always been fairly easy. The problem is that the more carefully you look, the larger a number you get. The number can grow 2, 3, 4, 5X as you look in more depth. There is no nice simple way to limit the measurement so that does not happen. This makes it a very hard thing to characterize. Many frequency measurement systems have built in limits. They have a bandwidth (often narrow). They have an integration time (sometimes FIR, sometimes IIR). They have internal noise processes. All of that “colors” the result. This is in addition to the basics of observation time and number of samples. Running two differently designed devices in parallel can result in two very different “frequency readings”. We do “hand waving” conversions of ADEV (or some other variance). That gives a number to some number of decimal places. The gotcha is still that the underlying noise processes may or may not be “nice enough” for the conversion to have any meaning. That issue has caught a lot of people over the years. On things like GPSDO’s, the noise processes rarely are “nice”. You have humps and bumps from control loops and multiple noise sources. It’s a messy problem. None of this really addresses the question of “how do I characterize absolute frequency”. It simply goes back to the 1960’s and the whole reason we *have* ADEV. It is a quantity that you can measure and the measure converges as you take more data. Absolute frequency diverges as you take more data. Yes, the papers explain it a lot more clearly with a lot more math. Bob > On Sep 1, 2016, at 5:23 PM, Bert Kehren via time-nuts <[email protected]> > wrote: > > maybe some one smarter than us can working with the parameters that Tbolt > makes available better performance can be achieved but it is a fact that > the frequency is being changed to compensate for time and Tom's frequency > data matches our's and we do not care about ADEV, we care about the actual > frequency at that moment it goes in to the measuring device and there is room > > for improvement if you are a frequency nut and are looking for ways to > improve. Bob made a good point. > > > > In a message dated 9/1/2016 4:27:02 P.M. Eastern Daylight Time, > [email protected] writes: > > Tom wrote: > >> No, again it sounds like you have a bad TBolt. Or something is wrong > (antenna? reception? time constant? environment? China resoldered parts?). I > appreciate that Juerg did lots of testing -- do you happen to have his ADEV > plot? >> >> Your claim of 1e-10 is order(s) of magnitude worse than the TBolts that > I see. Something is wrong. > > I second that (not that any further evidence is necessary following > Tom's comprehensive response). Additionally, as far as I know, the > units Tom was testing would have had the default tuning parameters (Tom, > please comment). Most Tbolts I've seen can be tuned for much better > performance than this at tau > 100 seconds, if they are equipped with a > 37265 OCXO. > > Note that the Tbolt has tuning parameters that limit how far the > frequency is allowed to wander to adjust the PPS phase -- if Bert's > unit(s) have these parameters set to allow very fast PPS recovery, that > could well cause the behavior he describes. > > You should also check all of the other tuning parameters to see if there > are errors in the settings. > > However, rather than mucking about in the myriad tuning settings > starting where they are now, I recommend doing a full factory reset to > get all parameters back to the original settings. Then (after several > weeks of undisturbed running), compare ADEV performance with Tom's graphs. > > Best regards, > > Charles > > > _______________________________________________ > 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. _______________________________________________ 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.
