[time-nuts] Trimble Resolution T - PPS offset and stability

[Said Jackson]

Bob,

Forgot to mention that this phase offset feature is built into the GPS receivers themselves.

Bye,
Said

Sent From iPhone

On Sep 4, 2013, at 13:02, SAIDJACK at aol.com wrote:

> 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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