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 accept.
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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