In article <[EMAIL PROTECTED]>, Unruh <[EMAIL PROTECTED]> wrote:
> [EMAIL PROTECTED] (Moe Trin) writes: > > >On Tue, 15 Jan 2008, in the Usenet newsgroup comp.protocols.time.ntp, in > >article > ><[EMAIL PROTECTED]>, Unruh wrote: > > >>"David L. Mills" <[EMAIL PROTECTED]> 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 _______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
