[time-nuts] Trimble Resolution T - PPS offset and stability
SAIDJACK at aol.com
SAIDJACK at aol.com
Wed Sep 4 16:02:09 EDT 2013
Hi Bob,
it's not +/-100ns on all receivers.
Our Fury GPSDO that uses Motorola designed M12M receivers allow +/-1ns
antenna delay phase adjustment resolution. No effect on timing stability.
Almost all of our other products using uBlox GPS also allow +/-1ns antenna
delay phase adjustment resolution.
It may take many minutes to track the new phase since typical GPSDO time
constants are set for 10's of minutes typically, but once settled, the
stability will not be affected by this phase offset.
Bye,
Said
In a message dated 9/2/2013 15:24:01 Pacific Daylight Time, lists at rtty.us
writes:
Hi
On all the GPS's I have tried it on, shifting the PPS has no real impact
on stability. A few things to consider:
Normally the shift is a few hundred ns either way
The shift process is always in steps of the main clock (100 ns for 10 MHz)
GPS by it's self bounces around a bit.
If you are talking about a shift of a big fraction of a second (and it
sounds like you are) then the stability of the GPS's local clock could come
into play. On something like a TBolt that's not going to matter. On a TCXO
based gizmo that is only corrected to 1.0x10^-7 you could get an extra 50 ns
of error at a half second offset. Weather you see that on this or that GPS
depends a lot on who wrote the firmware and what they worried about when
they did.
The better alternative is to use a counter with a reasonable time base to
look at the difference between pps signals. If the counter has an OCXO time
base and it's properly calibrated you are about 10 to 100X better off than
the 50 ns in the example above.
Bob
On Sep 2, 2013, at 6:05 PM, Lachlan Gunn <lachlan at twopif.net> wrote:
> Hello.
>
>
>
> Has anyone here tried varying the PPS offset on a ResT (or UT+ or any
other
> GPS receiver for that matter) and measuring the resulting stability?
>
>
>
> I ask because my Rb has only a 1PPS output, and while I have been able to
> get at one of its internal HF signals, would like to see what I can do
with
> just 1PPS. The obvious problem with doing this is that I will need to
align
> the PPS outputs together to get reasonable accuracy, but I worry that a
> large offset in the GPS receiver will degrade stability as the pulse
moves
> away from the relevant packet in the GPS signal.
>
>
>
> Am I being over-cautious?
>
>
>
> Thanks,
>
> Lachlan
>
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