[time-nuts] Pre-averaging Phase Data

[Bob kb8tq]

Hi

In general the way you go from 1 to 10 to 100 seconds is to decimate
( = throw away) the data to get a phase record at the tau you are after.
Any averaging that you do will filter out some noise. Since ADEV is a 
measure of noise, getting rid of it is not a real good idea. You can indeed
make your results look *very* good by doing this. What those results 
would actually represent …. who knows ….

Bob

> On May 2, 2020, at 8:24 AM, Simon Lewis <siaclewis at gmail.com> wrote:
> 
> Hi all,
> 
> I have a fairly newbie question on averaging of phase data, prior to ADEV.
> 
> In a paper by Sherman and Jordens (https://arxiv.org/pdf/1605.03505.pdf) on
> oscillator metrology using SDR (Ettus N210), for long term measurements (1
> tau up) the group estimates an average phase to reduce the data volume.
> They use both a novel lower-bound variance estimate, and what I assume was
> a rectangular average of N samples per second (original data rate was
> 1Msamples/s). Both apparently provided similar results.
> 
> From what I've read/tried to understand, pre-averaging phase isn't
> practically a good idea, considering the ADEV natively does this, and you
> end up with a lower estimate than reality. They state that time interval
> counters effectively do this for frequency measurements.
> 
> I know that one can do this with caveats, and in essence this is what MDEV
> does (I believe?), but is it not more 'real' to just downsample the phase
> data? That is, drop N-1 samples per second (not decimating which would
> filter the higher order components), and then ADEV the downsampled data.
> I'm sure I'm missing some understanding here!
> 
> Thanks,
> Simon
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