[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)
Danny Mayer
mayer at ntp.isc.org
Sun Jan 27 16:33:55 UTC 2008
David L. Mills wrote:
> It's easy to make your own Allan characteristic. Just let the computer
> clock free-run for a couple of weeks and record the offset relative to a
> known and stable standard, preferable at the smallest poll interval you
> can. The PPS from a GPS receiver is an ideal source, but you have to
> jerry-rig a means to capture each transition.
>
> Compute the RMS frequency differences, decimate and repeat. Don't take
> the following seriously, I lifted it without considering context, but
> that's the general idea. Be very careful about missing data, etc., as
> that creates spectral lines that mess up the plot.
>
> p = w; r = diff(x); q = y; i = 1; d = 1;
> while (length(q) >= 10)
> u = diff(p) / d;
> x2(i) = sqrt(mean(u .* u) / 2);
> u = diff(r) / d;
> x1(i) = sqrt(mean(u .* u) / 2);
> u = diff(q);
> y1(i) = sqrt(mean(u .* u) / 2);
> p = p(1:2:length(p));
> r = r(1:2:length(r));
> q = q(1:2:length(q));
> m1(i) = d; i = i + 1; d = d * 2;
> end
> loglog(m1, x2 * 1e6, m1, x1 * 1e6, m1, y1 * 1e6, m1, (x1 + y1) * 1e6)
> axis([1 1e5 1e-4 100]);
> xlabel('Time Interval (s)');
> ylabel('Allan Deviation (PPM)');
> print -dtiff allan
>
> Dave
And for those of you who didn't recognize it, that's MatLab code.
Danny
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