[ntp:questions] NTP and PPS calibration interval (convergence speed)

David Woolley david at ex.djwhome.demon.invalid
Tue May 28 07:55:33 UTC 2013

Miguel Gonçalves wrote:

> For me, it is not clear why calibration occurs every 256 seconds when

I don't understand what you mean by this, and I don't see any evidence 
of a 256 in your posting.

> I have a GPS receiver with a PPS output. I understand that a good
> frequency drift estimation would be good when free running but frankly
> I don't want my NTP servers to serve time when the GPS signal is not
> available. I can trust my Meinberg box to do that because it has a
> TXCO but not my regular NTP servers.

The way to do this is to set tinker dispersion on the server, to reflect 
  a realistic worst case clock frequency error, and set tos maxdist, on 
the clients, to represent how large an error margin you are prepared to 
> I was also surprised that Meinberg's NTP server calibration interval
> never goes above 4 seconds. They must have done what I did.
> I changed my FreeBSD 8.3 kernel to never go above 4 seconds and the
> results are in the graphics attached.

comp.protocols.time.ntp is not a binaries newsgroup.  I believe the 
mailing list imposes similar restrictions.

> I changed PPS_FAVGDEF in the file /sys/kern/kern_ntptime.c from the

No such file in the source distribution, at least not for 4.2.7p333.  No 
such symbol in the ntpd sources.

> default 8 (256 seconds) to 2 (4 seconds). The frequency is therefore
> adjusted every 4 seconds instead of 256.

I'm going to guess it has something to do with the averaging time for 
the frequency.  Generally a longer averaging time will give you a more 
accurate frequency, assuming that phase measurement error noise 
dominates, which should be true below the Allan intercept. 
Unfortunately, your Allan intercept may be exceedingly low and 
statistically poorly behaved.

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