[time-nuts] NTP, PPS and < 10 us offsets

Chris Albertson albertson.chris at gmail.com
Tue May 14 20:59:32 EDT 2013


Sorry forgot to include this, read section  5.1.3.3 it descibes how to
min/max poll are used
http://www.ntp.org/ntpfaq/NTP-s-algo.htm#Q-ALGO-CLK-UPD


On Tue, May 14, 2013 at 5:55 PM, Chris Albertson
<albertson.chris at gmail.com>wrote:

> Long intervals are a good thing.
>
> What NTP does is discipline the rate of the clock.   So lets say you
> wanted to adjust the rate of some mechanical clock.  You first set it
> as good as you can then wait an hour and see if it gained or lost
> time. then you move the fast/slow lever on the back of the clock.
> After a few hours the clock is pletty good and you have to wait a full
> day to see an error.  Then as you zero in on the exact rate setting it
> takes a week to see any error.
>
> This is kind of what NTP is doing if you want to correct out tiny
> errors in the clock rate you have to wait a long time
>
> That said I think there is a "maxpoll" parameter you can add o the
> /etc/conf file.
>
> But why?
>
> Maybe not you, but I think some people think "Oh my, NTP is not
> checking the time very often.  It must not be accurate."   That is
> thinking backwards, you need the long poll interval to see small
> errors in rate.   But this assumes a stable local clock.  NTP balances
> this.
>
>
>
> On Tue, May 14, 2013 at 5:30 PM, Miguel Barbosa Gonçalves <m at mbg.pt>
> wrote:
> > Hi!
> >
> > I am trying to figure out why NTP takes so long to react to oscillator
> > changes... I want it to track the PPS from a Trimble Acutime as closely
> > as possible.
> >
> > When checking ntptime on startup I see that the NTP daemon is using
> > PLL and starts the adjustment interval at 8 s. It then increases to
> > 16, 32, 64, 128 and stops at 256.
> >
> > I read in the documentation that this limit should be 128 but is now 256.
> >
> > --- start from http://www.ntp.org/ntpfaq/NTP-s-config-adv.htm ---
> > intervals 57 says that there were 57 calibration intervals. When PPS
> > pulses are arriving, this number should increase. Each frequency
> > adjustment requires a good calibration interval. The length of the
> > current calibration interval can be found as interval 128 s (128
> > seconds is the default maximum length). Remaining numbers count
> > abnormal conditions as explained below.
> > --- end ---
> >
> > How can I make it stop earlier? At 64 s for example?
> >
> > Any ideas?
> >
> > TIA,
> > Miguel
> > _______________________________________________
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>
>
>
> --
>
> Chris Albertson
> Redondo Beach, California
>



-- 

Chris Albertson
Redondo Beach, California


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