[time-nuts] Changing ADEV, (was Phase, One edge or two?)
warrensjmail-one at yahoo.com
Sat Oct 25 07:04:36 EDT 2014
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.
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?)
> 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.
>>>> ADEV most certainly does change with time, even for short tau's.
>>> Can you elaborate?
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