[time-nuts] Pre-averaging Phase Data

[Simon Lewis]

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