Determining the frequency offset

[Achim Gratz]

I have been experimenting with determining the frequency offset via the
MONOTONIC_RAW clock.  At the moment I've implemented that as a Perl
script although it should eventually go into some sort of pps-raw device
driver that timestamps the incoming PPS pulses.  Based on the Allan
deviation plots by the RADclock folks the best timescale for getting the
least amount of noise is somewhere between 300s and 3000s.  I've run the
script for 3 days on my spare PC (no PPS there, synchronized over LAN).
It measured the frequency offset each 15s and then double that interval
all the way to 256×15s.  This already looks pretty promising:

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The plot of 128×15=1920s gives the best overall fit to what the NTP loop
filter was doing, although there are some features that are on a shorter
timescale (480s plotted for reference).


Regards,
Achim.
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