Hi In fact, even without any “weird” stuff in the data, you can see short tau ADEV drop as an OCXO runs for days / weeks / months. You can test this by taking your 15 to 30 minute test run and breaking it into three or four sub runs.
Bob > On Oct 25, 2014, at 7:04 AM, WarrenS via time-nuts <[email protected]> wrote: > > > Some of the reasons that ADEV values change over time may be > caused by one of these two things that I have seen cause poor plots. > Either of which can cause changes in the ADEV values across > a wide range of taus, and the effect can change over long run ins. > Hopefully Magnus will comment if ADEV is even an appropriate > tool to use if either of these noise types are present in the raw data. > > The first thing that can cause trouble is if a bad data point > occurs every so often, aka an outlier. > The other thing is popcorn noise, a sudden frequency shift that > tends to hop between a few different values and happens at an > unpredictable time but at a somewhat repeatable rate. > I've seen Popcorn noise change over long time periods after days > or weeks of run in, say from a typical 1e-10 freq hop a few times a minute > to maybe 2e-11 hops a couple times every 5 minutes. > Even when this happened on some of my poor oscillators the basic > short tau ADEV values between hops stayed pretty much constant. > . > If either of these somewhat repeatable but random events are included in the > raw data, ADEV plots can become pretty misleading or downright useless. > For that reason Plotter allows outliers to be removed automatically. > As far as I know outliers have to be manually removed when using TimeLab. > > ws > > PS, and Yes the fact that I've posted this > shows that I have to be at least somewhat crazy, > aren't all time nuts in one way or another?. > > ------------ >> >> From: Bob Camp >> Subject: Re: [time-nuts] Changing ADEV, (was Phase, One edge or two?) >> >> Hi >> >> Grab an OCXO that has been powered off for a long time. >> >> Turn it on and start plotting ADEV. Do it from about 10 minutes after turn >> on. Run 15 to 30 minute tests every so often for the first few hours. >> >> Come back the next day and run the same series for a few hours. Repeat a >> week later, and a month later. >> >> Curve fit out the drift and run the ADEV numbers out to < 100 seconds tau. >> That’s true even if you compare the best of each batch. It really is >> getting better. >> >> Do that on enough oscillators and you will indeed find many that do get >> better (like 2X better for some, 10 or 20% for others) on ADEV after they >> have been on a while. >> >> —————————— >> >> Run an OCXO and watch the ADEV on the Time Pod. Look at enough of them and >> you will find some that drop ADEV for a while (say 10 minutes or so) and >> then climb by a bit (say 1.5:1). Hmmm, what’s going on? Look at the phase >> plot and there’s an abrupt shift in phase over some period ( which depends >> on the cause, there’s more than one possibility). Let’s say it’s 10 >> seconds. The whole ADEV plot climbs, not just the part for > 10 second >> tau. Why - there’s energy there both at short and long tau. >> >> —————————————— >> >> Look at a GPSDO / disciplined oscillator / temperature compensated Rb. Let >> it run for a good long time. If it’s got a loop that steps out to *long* >> time constants, it may only bump the frequency once every 15 minutes or >> longer. Plot the ADEV over the time segment when it steps and compare it >> to the time period it does not step. Short tau ADEV is worse at the step. >> >> ———————————————— >> >> Look at a very normal OCXO on your TimePod. After 100 seconds, the 1 >> second ADEV *should* be pretty well determined. After 1000 seconds it >> should be *very* well determined. Flip on the error bars if you want an >> idea of how good it should be. >> >> Watch for a while, Does it move outside the error bars? Hmmmm ….. It’s not >> the error bars that are the problem. The math is correct. The statistics >> are what is the issue. The ADEV hast changed for the worse as the run has >> gone on. It’s a very common thing. >> >> ——————————— >> >> Those are just the first few off the top of my head. >> >> Bob >> >> ************************** >> >>>>> ADEV most certainly does change with time, even for short tau's. >>>> >>>> Can you elaborate? > > _______________________________________________ > 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.
