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Discussion of precise time and frequency measurement

Re: [time-nuts] Histogram of T2 arrival times

Hal Murray

Fri, Jul 29, 2016 12:26 AM

The systems collecting the data have 200-300 microsec peak-peak of
clock offset, mostly tracking daily temperature swings.

How do you know that?

Which "that"?

I don't have a formal proof, just a collection of data that all fits together.

You can get the clock offset from loopstats. That can lie, for example by using a PPS over USB without fudging to correct for the USB polling delay or getting the time from the net with asymmetric delays due to routing or last-mile link speeds.

You can confirm things by using a good NTP system to monitor your targets. That's somewhat circular since you have to ask how good is the clock on your "good" system.

You can get the temperature connection by plotting offset and temperature over 24-48 hours. You can get confirmation by plotting the drift.

Crystals are well known to have a temperature effect so the drift-temp connection shouldn't be surprising.

On Linux, you can get the temperature from
/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
or similar. The filename depends slightly on the version of the kernel you are using.

You can get confirmation of the temperature from the disk using hddtemp or smartd and/or by monitoring your room temperature. (That works better if the disk is in the same box.)

You would really like to measure the temperature at the crystal. I haven't gone that far.

--
These are my opinions. I hate spam.

gem@rellim.com said: >> The systems collecting the data have 200-300 microsec peak-peak of >> clock offset, mostly tracking daily temperature swings. > How do you know that? Which "that"? I don't have a formal proof, just a collection of data that all fits together. You can get the clock offset from loopstats. That can lie, for example by using a PPS over USB without fudging to correct for the USB polling delay or getting the time from the net with asymmetric delays due to routing or last-mile link speeds. You can confirm things by using a good NTP system to monitor your targets. That's somewhat circular since you have to ask how good is the clock on your "good" system. You can get the temperature connection by plotting offset and temperature over 24-48 hours. You can get confirmation by plotting the drift. Crystals are well known to have a temperature effect so the drift-temp connection shouldn't be surprising. On Linux, you can get the temperature from /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq or similar. The filename depends slightly on the version of the kernel you are using. You can get confirmation of the temperature from the disk using hddtemp or smartd and/or by monitoring your room temperature. (That works better if the disk is in the same box.) You would really like to measure the temperature at the crystal. I haven't gone that far. -- These are my opinions. I hate spam.

Gary E. Miller

Fri, Jul 29, 2016 12:46 AM

Yo Hal!

On Thu, 28 Jul 2016 17:26:40 -0700
Hal Murray hmurray@megapathdsl.net wrote:

gem@rellim.com said:

The systems collecting the data have 200-300 microsec peak-peak of
clock offset, mostly tracking daily temperature swings.

How do you know that?

Which "that"?

The thing just previous to "that": temp swings.

You can get the temperature connection by plotting offset and
temperature over 24-48 hours. You can get confirmation by plotting
the drift.

So, you have plotted offset versus temp? Can we see that?

Crystals are well known to have a temperature effect so the
drift-temp connection shouldn't be surprising.

I would not be surprised if temp was dominant here. But I can think
of a few other things that may be dominant instead.

If you have data, I'd love to see it, but conjecture is not very
useful, except to design a new experiment.

On Linux, you can get the temperature from
/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
or similar.

I would only consider CPU temp a very rough proxy for XTAL temp. Room
ambient would be much closer to XTAL temp.

You would really like to measure the temperature at the crystal. I
haven't gone that far.

Yup. I have some USB thermometers that are good for such things.

RGDS
GARY

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com Tel:+1 541 382 8588

Yo Hal! On Thu, 28 Jul 2016 17:26:40 -0700 Hal Murray <hmurray@megapathdsl.net> wrote: > gem@rellim.com said: > >> The systems collecting the data have 200-300 microsec peak-peak of > >> clock offset, mostly tracking daily temperature swings. > > How do you know that? > > Which "that"? The thing just previous to "that": temp swings. > You can get the temperature connection by plotting offset and > temperature over 24-48 hours. You can get confirmation by plotting > the drift. So, you have plotted offset versus temp? Can we see that? > Crystals are well known to have a temperature effect so the > drift-temp connection shouldn't be surprising. I would not be surprised if temp was dominant here. But I can think of a few other things that may be dominant instead. If you have data, I'd love to see it, but conjecture is not very useful, except to design a new experiment. > On Linux, you can get the temperature from > /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq > or similar. I would only consider CPU temp a very rough proxy for XTAL temp. Room ambient would be much closer to XTAL temp. > You would really like to measure the temperature at the crystal. I > haven't gone that far. Yup. I have some USB thermometers that are good for such things. RGDS GARY --------------------------------------------------------------------------- Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703 gem@rellim.com Tel:+1 541 382 8588