[time-nuts] Measuring short term stability minus linear drift

Jim Lux jimlux at earthlink.net
Sat Oct 8 11:16:20 EDT 2011


On 10/8/11 7:56 AM, Magnus Danielson wrote:
> On 08/10/11 16:45, Jim Lux wrote:
>> On 10/7/11 9:32 PM, Rick Karlquist wrote:
>>> I want to measure the short term stability of a source
>>> with substantial linear drift. I would like some measure
>>> of stability along the lines of Allan deviation, but I
>>> only want to measure the "noise" and ignore the "drift".
>>> AFAIK, ADEV treats linear drift like a form of noise.
>>> Has this problem been solved before?
>>> Any ideas?
>>>
>>
>> what if you (least squares?) fit a straight line to the frequency
>> measurement data, remove that, then look at ADEV? We do something
>> similar with testing deep space transponders which will be handling a
>> signal with varying Doppler so our test signal is varying in frequency.
>
> This is what a simple fit does or HDEV does. The benefit of higher
> degrees fit is that it would cause better fits and high tau ADEV values
> will be less poluted by the weaker terms.
>
> For first degree effect, swap between ADEV and HDEV.
> For second degree effect, use quadratic fit and ADEV that.
>
> You still want to know the systematic behaviour and how those systematic
> effects behave, but it would be fairly ridicolous to teach you Rick on
> the merits of that.
>

Yeah, but it's always nice to know how other people do it and if someone 
has published something somewhere with more analysis.

I find that at JPL (and I assume others have found this too) that we'll 
go off and reinvent the wheel (maybe because we're working in parallel 
ignorance) for something.  And a lot of times, especially if it's in 
service of a "get the hardware tested and delivered" the analytical 
backup for whatever we did may not be as rigorous as one might like.

There's also the classic gap between the groups doing theoretical work 
in one building and groups building and testing hardware in another 
building 1000 meters away, and the two groups never have time to meet, 
and in some cases, may not even be aware of the other's existence. This 
is especially true when you're talking about early career hires (aka 
fresh-outs).  These days, with tight budgets, you may not be able to put 
two people on a job (one senior, one junior) which would provides some 
of that knowledge transfer.    (to be honest, I don't know that it's 
much worse than it ever was.. tight budgets are a perennial complaint 
since they were building pyramids in Egypt)




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