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

timeok at timeok.it timeok at timeok.it
Tue Dec 22 00:31:16 EST 2015

To verify if the where the delay is generated, antenna and splitter or GPS,  simply invert the antenna cable on the GPSs antenna connector and verify if the delay change as sign.


On Tue 22/12/15 02:24 , dan at irtelemetrics.com wrote:

> Hi All,
> 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. 
> Thanks,
> Dan
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