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

David Mills mills at udel.edu
Fri May 1 16:11:07 UTC 2009


Kat,

I anticipated this in the original 1993 kernel PPS design. If you look 
closely at the kernel PPS loop you will find it is actually two separate 
loops, one for time and theother for frequency. The OCXO can be used to 
discipline the kernel frequency and the daemon used to compensate for 
residual time and frequency offset. It works best if the time 
constant/poll interval  is reasonably long, like 512 or 1024 s. This 
still leaves something to be desired, as the sawtooth error is still 
there, but if the intriinsic OCXO eror is 0.23 PPM as you report, the 
error should be insignificatnt.

Dave

Kat wrote:

>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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