[ntp:questions] Loop Frequency and Offset

Miguel Gonçalves mail at miguelgoncalves.com
Tue Sep 27 10:42:14 UTC 2011

On 27 September 2011 10:35, David Woolley <david at ex.djwhome.demon.invalid>wrote:

>  Applying a static correction to the clock frequency only helps if the
> static error is close to or greater than 500ppm.  However, if that is the
> case, the right thing to do is to replace the motherboard, with a better one
> as it is likely that the clock is unstable as well as having a high static
> error.

One had 180 ppm sand the other 46 ppm without fiddling with machdep.tsc_freq
sysctl variable. After the correction I am getting -0.041 ppm for the first
and -0.045 ppm for the second.

>  performance? The machines I have at the moment are showing worse than 1 us
>> performance perhaps because I am not using GPS timing receivers.
> Make sure the system is only used as a time server.  Keep its temperature
> very constant.  Disable any form of power management (in particular variable
> clock rates). Disable spread spectrum clocks.

Embedded machines running NanoBSD and they don't do anything else besides
running NTP. They sit in a temperature controlled room at 20 ºC. Power
management has been disabled. I checked the BIOS and did not see any
reference to spectrum clocks.

> Calibrate the systematic error by outputting a hardware pulse within the
> kernel at a precisely known system clock time and using hardware to measure
> the offset from the PPS input pulse.  You will need to also output the
> system time at which you did this, but that needn't be in real time.
> Do not try to do this on the second.

Makes sense... I'll investigate a way of doing this.

>  I know... :-) My cable connection is really bad for NTP... I can't get
>> better than 3 us. That is why I have 2 stratum 1 GPS receivers onsite.
> Reading that as 3ms.  The time you serve will have that sort of jitter in,
> it so there is little point in being accurate to better than about 100
> microseconds of jitter/wander.

You read well :-)


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