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

Paul Nicholson vlf0308 at abelian.org
Thu Jul 1 12:06:54 EDT 2010

Thanks all for the various replies, on and off list.

John WA4WDL wrote:
 > Perhaps you should use only the leading edges for the
 > pulse-to-pulse interval measurement.

Yes, I'm just using the leading edge, turning it into a pulse
using a couple of RC networks, slowing the rise and fall enough
to give a nice shape for accurate locating of the centroid.

Antonio CT1TE wrote:
 > I can send you the yesterday record of my GPS TBolt #2 against
 > DCF77, which apparently doesn't show any periodic phase
 > variation.

Thanks for the offer.  Hold on that, I'll assume for now that
this slow cycle is not ionospheric.

Rob Kimberley wrote:
 > I'm assuming your location is fixed, and you are tracking
 > sufficient SVs,

Fixed.  Maybe I can relocate the GPS antenna for a better view of
the sky.

Peter Vince wrote:
 > I don't believe that cyclic variability is GPS - I've not
 > noticed such a thing on any of the systems I monitor.

Thanks, that's very helpful.  Knowing that this is not normal
behaviour for GPS timing means that I can set about tracking down
a system fault.

There are a few things I can think of to check.  The cycle must
correlate with something - soundcard rate, temperature, CPU clock,
etc.  It is pretty consistent, here is a bit more of MSF 60kHz
and also DCF at 77.5kHz,


Thanks again, I'll report back when I find the problem.
Paul Nicholson

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