[time-nuts] PPS phase between two GPS units.

[Mike Cook]

> Le 22 déc. 2015 à 06:16, Brian Inglis <Brian.Inglis at SystematicSw.ab.ca> a écrit :
> 
> On 2015-12-21 18:24, dan at irtelemetrics.com wrote:
>> So I've been playing with some timing hardware here, and have noticed something rather curious. I have two otherwise identical Lea-6T GPS modules, configured exactly the same. These units are tied to the same antenna, with a splitter with the same length cables running to each unit. They were given the same survey coordinates. Initially there was not any appreciable difference between the PPS outputs. The outputs were on average within about one nS of each other. However after a day or two they started to display a difference of about 21nS between the PPS outputs. This is also evident in the GPSDO output, as the phase of the 10Mhz is also skewed by 21nS or so.
>> 
>> A few days ago I started a 48 hour survey. The came up virtually identical coordinates at the end of that survey. After the survey the phase is still 21nS different.
>> 
>> At this point, my only thought is the GPS splitter. It's DC coupled to one unit, and AC coupled to the other. It is possible there are some delays on one output vs. the other due to the blocking caps and bypass inductor in the splitter. I've tried swapping the GPSDO units on the splitter, so we'll see what happens there.
>> Can anyone offer any insight as to why the two units may have a different PPS output timing? It's not really a problem, more of a curiosity and mildly academic exercise.
> 
> A sampling difference of 1 cycle with 48Mhz clocks would give you ~21ns offset if not using sawtooth correction. Can you do the sawtooth correction and see if the offset is reduced?
> 

  Good thinking Brian. But I would expect the difference to wander from 21s UNLESS the the two receivers clocks are lock stepping. Injection locking might do this if the two module are physically close enough. 

> -- 
> Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada
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