[time-nuts] NTP, PPS and < 10 us offsets
Miguel Barbosa Gonçalves
m at mbg.pt
Wed May 15 15:43:27 EDT 2013
Thank you for your input.
Yes, I've set minpoll and maxpoll to 4 yet the system corrects the offset
slowly during the calibration interval that starts I believe at 8 seconds
(2^3) and stops at 256 seconds (2^8) even though I've set min and maxpoll.
Even considering this situation I am going crazy with the Trimble Acutime
Gold I installed on my roof.
I connected the port A to a RS422-to-RS232 converter. The PPS+ signal is
directly connected to the DCD pin of this RS232 port. The port B is also
connected to a RS422-to-RS232 converter and I occasionally connect a
Windows computer to check the status of the antenna.
I've reconfigured the antenna to output only NMEA sentences (and I also use
the PPS) and NTP works fine but suddenly without any explanation the NTP
process stops receiving information from the receiver. I really don't
understand what's happening here as the receiver is on the roof and sees
plenty of satellites:
56427 70907.634 127.127.20.0
56427 70923.635 127.127.20.0
56427 70939.634 127.127.20.0
56427 70955.635 127.127.20.0
56427 70971.634 127.127.20.0
Can anyone help me please?
On 15 May 2013 18:33, Chris Albertson <albertson.chris at gmail.com> wrote:
> i've seen that myself, NTP's compensation can almost be used as a way to
> measure room temperature. You can see the AC cycles. For this reason I
> keep the NTP server in a walk in closet that lacks heating duct. The room
> temp cycles slowly.
> I've also read but can't remember where about a project to replace the TTL
> can oscillator in a PC with a better quality one driven from a OCXO. That
> is a lot of work for small gain.
> One thing about that study. I notice the author had t remove the PPS
> reference clock. It looks like he was unabe to get the graph to move at
> all with PPS attached. I'd like to see the same study done with a good
> PPS reference clock let connected. Likely the results would be
> uninteresting, just a flat graph.
> On Wed, May 15, 2013 at 12:51 AM, Hal Murray <hmurray at megapathdsl.net
> > albertson.chris at gmail.com said:
> > > Maybe not you, but I think some people think "Oh my, NTP is not
> > the
> > > time very often. It must not be accurate." That is thinking
> > > you need the long poll interval to see small errors in rate. But this
> > > assumes a stable local clock. NTP balances this.
> > That "balance" is assuming a classic ADEV type V curve. That assumption
> > works better in old non-green systems that burned lots of power in an
> > loop when they didn't have any work to do. With modern systems, it's
> > to
> > change the temperature significantly by starting/finishing a big chunk of
> > work.
> > External temperature changes can also be important. Every now and then,
> > like to repost this link. This seems like a good opportunity.
> > NTP temperature compensation
> > http://www.ijs.si/time/temp-compensation/
> > --
> > These are my opinions. I hate spam.
> > _______________________________________________
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> Chris Albertson
> Redondo Beach, California
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