[ntp:questions] PPS from an external OCXO source. Correcting drift is it possible?

Kat Schwar3Kat at gmail.com
Fri May 1 11:09:04 UTC 2009

To try and maintain better accuracy on the ntp server that feeds my
small network and increase the maxtime for server polling, I thought
that I would dig into my scrap box and build a PPS with a home built
OCXO as a spare time project.  The NTP documentation stated that e a
PPS source could be used with an NTP server as the reference clock, so
I built the PPS source.

My homebrew PPS source that is vastly more stable than my servers
internal clock, but it has a slight constant drift that I can't easily
correct.  Also I can expect the drift to change slowly with time due
to crystal aging.

I hooked it up to my FreeBSD servers serial port and tweeked it for
minimum drift relative to another internal NTP server referencing
three external NTP servers. (NTP 4.2.5p5-a).  Unfortunately this setup
doesn't appear to work as I would have expected it to.

I have two problems with it.  1. It starts off with a random offset.
2. The offset drifts at about 20 milliseconds a day.

If this was the internal PC clock then  NTP would take care of the
drift with by calculating the drift constant with respect to other
external NTP servers and correcting for it whilst reducing the offset
to zero. Does the PPS clock not have a similar mechanism?

Is there any way to do this or is the NTP documentation misleading me?

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