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

Miguel Gonçalves miguel.barbosa.goncalves at gmail.com
Wed May 29 20:35:42 UTC 2013

On Tue, May 28, 2013 at 8:55 AM, David Woolley
<david at ex.djwhome.demon.invalid> wrote:
> 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.

There isn't because I did not post it. Calibration interval is not
polling interval.

# ntptime
ntp_gettime() returns code 0 (OK)
  time d550e438.3dc51d94  Wed, May 29 2013 20:31:52.241, (.241289477),
  maximum error 1739 us, estimated error 1 us, TAI offset 0
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset 0.226 us, frequency 147.458 ppm, interval 4 s,
  maximum error 1739 us, estimated error 1 us,
  time constant 4, precision 0.001 us, tolerance 496 ppm,
  pps frequency 147.458 ppm, stability 0.535 ppm, jitter 2.156 us,
  intervals 67322, jitter exceeded 7483, stability exceeded 0, errors 4.

>> 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 will look into this.

>> 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 noticed that after sending the message.

>> 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.

That's because kern_ntptime.c is in the FreeBSD kernel sources and not
in NTP 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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