[time-nuts] pc clock crystal long term variations

Attila Kinali attila at kinali.ch
Sun Aug 8 21:01:43 UTC 2010


I just had a little bit too much time at hand and had a look at
the ntp loopstats of one of my machine. Specifically at the ppm value.
After filtering out all items, where the ntp loop isnt running with
a poll intervall of 2^10, i got a more or less nice curve (see ntp.png).

The noise seen is quite expected, takeing into account that the
references used are behind an ADSL connection. The jumps are
exactly at the dates when the machine was rebooted.

What surprises me though is the stability of the crystal.
In the months between reboots, there is less than +-1ppm variation,
rather in the range of +-0.2ppm (i havent made any stasticial
sound analysis, so take the values with a grain of salt).
Especially when comparing to the +-40ppm jumps on reboots.

The machine is a 16y old DEC PC, which has been used as a desktop
before it came into my hands, and has been permanently running for
the last ~5 years.

So, the question is: are the ntp loopstats a valid way to judge
a crystal or am i looking at a GIGO-System? And why doesnt the
crystal relax back into it's old ppm value, but stays where it
is after a reboot/power cylce?

				Attila Kinali
Why does it take years to find the answers to
the questions one should have asked long ago?
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