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

dan at irtelemetrics.com dan at irtelemetrics.com
Mon Dec 21 20:24:37 EST 2015

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. 

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