[time-nuts] Changing ADEV, (was Phase, One edge or two?)

[Bob Camp]

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 <time-nuts at febo.com> 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?
> 
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