[ntp:questions] oscillations in ntp clock synchronization

Moe Trin ibuprofin at painkiller.example.tld
Thu Jan 17 02:56:52 UTC 2008


On Tue, 15 Jan 2008, in the Usenet newsgroup comp.protocols.time.ntp, in article
<SE6jj.4473$vp3.3451 at edtnps90>, Unruh wrote:

>"David L. Mills" <mills at udel.edu> writes:

>>Assuming the residuals are in the low microseconds and the variation
>>period is much longer than the time constant and the same in both NTP
>>and chrony, it would seem very likely that what you see is due to small
>>temperature changes. This is the same thing seen in our primary servers
>>synchronized by GPS and PPS. They hold typically within a few
>>microseconds but occasionally budge twice that. I have traced this to
>>small variations in machine room temperature as the air conditioning
>>system cycles.

Servers as in ordinary computers of some kind?  Depending, those are
usually either cheap plug-in oscillators (under two bucks in onezies)
that typically spec out at +/- 50ppm over the range 0-50C (or sometimes
+/- 100ppm over 0-70C), or just an HC-49 crystal. Good oscillators cost
way to much money for this market, though I have seen systems where
someone stuck a chunk of styrofoam over the oscillator to improve the
thermal isolation.

>Maybe I will have to put in a digital thermometer into the case. The
>chrony oscillations are 10's of milliseconds. and the oscillations in
>the rate of the RTC vs the system time are larger than that.

Are they moving in more/less the same direction at similar times?

>Of course those might also be temperature oscillations-- different on
>different parts of the motherboard (I assume that the rtc has a different
>crystal to the one that runs the cpu clock).

Most of the time - but "that depends".  Are you talking PCs?  The RTC
is usually something low frequency (originally 1.8432 MHz, but now just
as easily a 32768 Hz tuning fork). They could also be part of the
CMOS RAM (usually a "Dallas Semiconductor" device - 24 to 34 pin DIP
that also has the crystal and lithium battery in the same package).
There are temperature compensated versions, but I've never seen one in
a commodity type of computer.

        Old guy




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