[ntp:questions] oscillations in ntp clock synchronization

isw isw at witzend.com
Thu Jan 17 17:55:15 UTC 2008


In article <kbBjj.34002$fj2.20407 at edtnps82>,
 Unruh <unruh-spam at physics.ubc.ca> wrote:

> ibuprofin at painkiller.example.tld (Moe Trin) writes:
> 
> >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
> 
> Yup ordinary computers. They are between 0-100PPM out-- across about 8 of
> them. But the cyclic variation is strange. 
> 
> >+/- 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.
> 
> What is strange is the about 1.5 hour timescale. There is nothing I know of
> in any of the rooms that has that time scale.

Could be a line voltage variation, caused by some distant load changing. 
Switching power supplies of the sort used in PCs don't have wonderful 
line regulation.
> 
> Is there any way of getting the temperature from one of those CPU
> thermometers (onboard)  to better than 1C?

The absolute accuracy won't be that good, but the resolution should be, 
assuming you can read the raw signal.

Isaac




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