[time-nuts] PPS offset between GPS receivers

Azelio Boriani azelio.boriani at screen.it
Sat Dec 8 22:43:27 UTC 2012


OK, I use the two GPSDOs method because I can set the PPS position but the
cable is useful and only one GPSDO (or any other stable PPS source) is
enough.

On Sat, Dec 8, 2012 at 6:53 PM, Don Latham <djl at montana.com> wrote:

> I use a tee connection and terminated 100 foot (33 m )piece of RG58 coax
> for a known delay to the stop pulse.
> Don L
>
> Azelio Boriani
> > Yes, this is a good test: to evaluate how your preferred uP can perform
> > as
> > a time interval counter, you can hook two GPDSOs' PPS to it and see the
> > result. The best would be to have at hand also a real TIC (HP53132A,
> > PM6681, SR620 or similar) and compare.
> >
> > On Sat, Dec 8, 2012 at 4:28 AM, Bob Camp <lists at rtty.us> wrote:
> >
> >> Hi
> >>
> >> A lot depends on exactly what the interrupt structure is. It may also
> >> depend on the phase of the cpu clock relative to the pps signal.
> >> What's
> >> reasonably sure is that there is indeed some offset between the two
> >> where
> >> the answer is indeed "ft's" random. Another thing to check - how wide
> >> is
> >> the random region?
> >>
> >> Bob
> >>
> >> On Dec 7, 2012, at 9:19 PM, Chris Albertson
> >> <albertson.chris at gmail.com>
> >> wrote:
> >>
> >> > One more test to try.  Connect one PPS signal to both GPIO ports and
> >> see
> >> > how close to zero offset you get.   It would likely be random which
> >> gets
> >> > read first.
> >> >
> >> >
> >> > On Wed, Dec 5, 2012 at 5:37 AM, Gabs Ricalde <gsricalde at gmail.com>
> >> wrote:
> >> >
> >> >> Hi everyone,
> >> >>
> >> >> As Tom suggested, I redid the test with less than 1 ft. of wire
> >> from the
> >> >> PPS output to the GPIO without any logic gates or line receivers.
> >> Same
> >> >> result,
> >> >> the SKG25A1 was 2 microseconds ahead of the 58534A. Without any
> >> other
> >> way
> >> >> of
> >> >> testing, I would probably trust the output of the timing receiver
> >> more
> >> >> than the SkyNav module. Anyway the SkyNav board is an inexpensive
> >> unit
> >> and
> >> >> I wouldn't mind setting an offset in ntpd.
> >> >>
> >> >> I don't have a scope yet, and a low jitter PPS GPIO is the closest
> >> thing I
> >> >> have
> >> >> to a TIC.
> >> >>
> >> >> _______________________________________________
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> >> >>
> >> >
> >> >
> >> >
> >> > --
> >> >
> >> > Chris Albertson
> >> > Redondo Beach, California
> >> > _______________________________________________
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> >>
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>
> --
> "Neither the voice of authority nor the weight of reason and argument
> are as significant as experiment, for thence comes quiet to the mind."
> De Erroribus Medicorum, R. Bacon, 13th century.
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>
>
> Dr. Don Latham AJ7LL
> Six Mile Systems LLP
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