On 2015-12-21 18:24, [email protected] 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?
--
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada
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