[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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