[ntp:questions] Atheros AR9331 w/GPS + PPS

Charles Swiger cswiger at mac.com
Thu Sep 10 17:18:38 UTC 2015

On Sep 10, 2015, at 3:46 AM, Gabs Ricalde <gsricalde at gmail.com> wrote:
> TL-WR703N, polling GPIO driver, ntpd maxpoll 2
> https://imgur.com/a/ILF9Y <https://imgur.com/a/ILF9Y>
> The offset depends on these things: the stability of the local clock,
> PPS jitter, and loop time constant (controlled by the poll interval).
> A low PPS jitter means you can use a faster time constant, which can
> control the local clock faster when it drifts, which means lower
> offsets.
> https://www.eecis.udel.edu/~mills/ntp/html/discipline.html <https://www.eecis.udel.edu/~mills/ntp/html/discipline.html>
> https://www.eecis.udel.edu/~mills/database/brief/algor/algor.pdf <https://www.eecis.udel.edu/~mills/database/brief/algor/algor.pdf>
> You can also improve the stability of the local clock by avoiding rapid
> temperature changes. This is the loopstats of a TL-WR841N placed inside
> a box where the temperature doesn't change that fast.
> https://imgur.com/a/eNT0k <https://imgur.com/a/eNT0k>

Ah, excellent.  You can clearly see the frequency offset change vs time
over time in that last image due to daily temperature changes rather than
load; the first link shows freq vs t graphs with clear steps, suggesting
those systems have enough load-- and changing load-- to dominate the
frequency changes due to temperature.

Looks like your crystal sees ~1.5 pps drift from a 10C change in temp, but I'm
just guesstimating.  :-)  Do you have measurements of the ambient temperature?


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