[ntp:questions] Frequency Offset

David J Taylor david-taylor at blueyonder.co.uk.invalid
Fri Feb 24 18:17:43 UTC 2012

"unruh" <unruh at invalid.ca> wrote in message 
news:20Q1r.6801$Ai4.3345 at newsfe18.iad...
> Do you mean >500PPM?
> If you were running linux, you could use the adjtimex program and the -t
> or --tick adjustment to change the tick value of your system clock.
> each value of 1 adjustment speeds up or slows down the clock by about 
> 100PPM.
> You can use that to get the clock within the +- 500PPM range that ntpd
> can adjust. chrony uses it automatically.

.. and a similar facility exists in Windows by setting the 
NTPD_TICKADJ_PPM environment variable


> Temperature sensitivity is usually in the "less than 1PPM per degree C"
> so you would have had to be expriencing quite a heat wave (500 degrees
> C) to have temperture be a factor. I suspect other things might have
> been more urgent worries then.

Frequency variation from mean of three PCs in the same room is remarkable 
different here:

- Dell 4400
    typically +/- 0.7 ppm, but -1.5ppm one day

- home built Pentium 4 HT system with ASUS P4P800 motherboard
    typically -0.2 to + 0.1 ppm

- recent PC with quad-core Intel and ASUSTeK P7P55 LX motherboard
    -0.015 to + 0.009 ppm

>> One of my desktop systems, p4-2667, has just taken two
>> days to get to an offset of under 2 ms after a kernel
> ntpd is slow, but not that slow. Since for greater than 128ms offset it
> does a step, and since it fixes things by about 1/2 per hour, half a day
> is more like it to get it down to microsecond, not millisecond ranges.

Indeed, the NTPD was restarted on most of my PCs today (update from 
4.2.7p258 to 4.2.7p259) and you can see the transients here:


A fraction of an hour, not days.  I don't have a plot coming up from cold, 
though.  That may take a few hours for ultimate stability.


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