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*Post by Poul-Henning Kamp*--------

*Post by jimlux*What about doing some sort of fit to the measurement data before

calculating the ADEV? Similar to removing a linear ramp.

basically you'd solve for the three sine parameters (f, phase,

amp/deviation), then remove that from the data, then run the ADEV

calculation.

That is incredibly shaky ground from a scientific point of view.

If you know the *source* of the bias you're trying to remove (i.e. you

know it's a sinusoidal frequency modulation), I don't know that it's any

different than removing long term drift.

As a practical example of where something like this is done, consider

measuring the Allan Deviation of a coherent deep space transponder that

is orbiting a moon or planet (i.e. there's a Doppler on both uplink and

downlink).

Coherent transponders produce an output that is precisely a rational

fraction of the input frequency (e.g. for X-band 880/749 for a 7.15 GHz

input, 8.45 GHz out) and the frequency/phase of the received signal is

compared with the frequency/phase of the transmitted signal to measure

range and range rate to the spacecraft (as well as the propagation

properties of the path).

We'll test them with fixed frequency (put a spectrally pure tone in and

measure the ADEV of the , and back in the day when the "turnaround" was

done with all analog circuitry, you could be reasonably sure that the

whole thing would have comparable performance in a Doppler shifted

environment.

These days, the turnaround is implemented in digital logic, and in

particular, the receive carrier tracking loop is done digitally, and one

wants to make sure that it tracks "adequately" in the face of Doppler.

So one builds a test set that produces a well characterized frequency

modulated input (representative of the Doppler profile) and you make

your measurements, remove the Doppler profile (with the appropriate

phase ratio) and make your ADEV measurement.

[This, of course, means that you have to generate a waveform with good

ADEV for testing, a technical challenge in its own right]

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