[time-nuts] Question about effect of spurious frequency modulation on Allan Deviation

[jimlux]

On 8/6/18 7:25 AM, Poul-Henning Kamp wrote:
> --------
> In message <66ac003f-ff72-b2e2-ebfd-f81ab64ef2c2 at earthlink.net>, jimlux writes:
> 
>> 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]