[time-nuts] PPS offset between GPS receivers

Don Latham djl at montana.com
Sat Dec 8 17:53:15 UTC 2012


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.
"If you don't know what it is, don't poke it."
Ghost in the Shell


Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com





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