[time-nuts] GPSDO time constant

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Jan 8 08:57:26 UTC 2009


In message <B6F04190894A41EEAAE0305A898A548E at pc52>, "Tom Van Baak" writes:
>Here are ADEV plots and interesting results from a recent
>experiment on varying the time-constant of a GPSDO:
>
>http://www.leapsecond.com/pages/tbolt-tc/

Tom, part of the reason for the sub-optimally short default time
constant is to be able to cope with worst-case specs of the
oscillator.

Even though they are pretty good, shifts in voltage, temperature
and orientation does affect them.

A very good example of this effect is the NTPD PLL, which
uncritically belives otherwise unplausible good news, and
lengthens the poll-period to 20 minutes and then refuses
to accept that it did it wrong, until i thas seen three
or four (= one hour) samples saying so, after which it
breaks lock and starts over.

In a lab setting, it makes sense to increase over the default
time-constant, just remember that it impacts your loops ability
to cope with for instance a 2g turnover.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.




More information about the Time-nuts_lists.febo.com mailing list