[time-nuts] Long period variation of GPS PPS timing?

Paul Nicholson vlf0308 at abelian.org
Thu Jul 1 14:01:25 EDT 2010

Matthew Kaufman wrote:
 > What is the soundcard sample rate? This feels like an artifact
 > of sample phase vs. pulse phase to me.

There are actually two soundcards, running at

  192003.8285 and

samples/sec, varying a little.  They are on different computers.
One takes an East/West signal, the other a North/South signal.
Each is timed and resampled to 192000.0 based on the PPS supplied
to both soundcards.  The two timestamped data streams are then
brought together for bearing and phase extraction.

The plotted phase is actually the mean (weighted by signal
strength) of the phases separately determined for each signal.

Tests with a signal generator show that the signals are being
correctly resampled, only a small and fairly constant phase
error between the two signals.

Perhaps there is some subtle aliasing between the two signals
and/or soundcards. I have altered things now to only look at the
signal from one antenna, one soundcard.  Will know in an hour
or so if this has made a difference to the slow cycle.

Joe Gwinn wrote:
 > The centroid of pulses triggered by the leading edge of the
 > 1PPS pulse will vary with the width of the pulses.

I see.  It is a passive network but unprotected from temperature
changes.  It is shut in a box with 2 PCs and a bunch of other
stuff.  Various fans on and off. Temperature varies 26-30C in
the box.   I need to look at temperature, but gut feeling tells
me the cycle is a little too regular and is some subtle software
defect, aliasing or something.   I must plot soundcard drift
rate against the slow cycle, etc.   I can tackle this with
confidence now that I know it is not a GPS limitation.  Thanks
for that.
Paul Nicholson

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