ntpviz - Don't plot a line during data absence

[Achim Gratz]

Gary E. Miller writes:
> We already discussed this.  The load on a RasPi distorts the timing
> so badly that it shows up on the ntpizv plots.  The Heisenberg Uncertainty
> Principle in action: Mmasuring distorts the thing being measured.

The load itself actually doesn't distort the timing.  It warms up the
processor and RAM and finally the XO, however, and _that_ changes the
timing.  Also, the Heisenberg uncertainty principle is about the
attainable precision of measurement of complementary pairs of variables,
not about the disturbance of the system being measured by the act of
measurement.

For the rasPi 2B that I have here, I've logged the PPS arrival times
together with the CPU temperature for two weeks now.  After I've closed
the box it's in, which made things much more stable and about 4K warmer
overall, the loop values can be predicted by a kernel density filtered
CPU temp measurement quite nicely and I end up with almost exactly
linear 200ppb/K response over the 4K temperature range of the last week.

Now, kernel density is a non-causal filter and can't be used in a feed
forward loop and I haven't yet looked into a causal filter that gets rid
of the noise and short spikes and can still predict correctly where the
loop should be.  But it seems possible to do that with only a small
error, which would allow the main NTP FLL/PLL to just deal with the
oscillator drift and the residual noise from the environment, which
should become about an order of magnitude smaller, maybe a bit better
even.

> Look at the spikes in the red line on the first graph here:
>
>     https://pi4.rellim.com/day/
>
> Each one of those is ntpviz running.  When ntpvis runs faster (finishes
> in less time) the spikes get smaller.  The spikes used to me much, much,
> bigger.

Sorry, but you're doing that wrong.  Pull the data from the machine with
rsync (using -rsh=ssh) and process it offline.


Regards,
Achim.
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