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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Oct 8 10:56:55 EDT 2011

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.


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