I am not sure about the Garmin GPS16, but all of the GPS receivers I use do
not rigidly define the falling edge of the 1PPS signal. Perhaps you should
use only the leading edges for the pulse-to-pulse interval measurement.
John WA4WDL
--------------------------------------------------
From: "Paul Nicholson" <[email protected]>
Sent: Thursday, July 01, 2010 6:15 AM
To: <[email protected]>
Subject: [time-nuts] Long period variation of GPS PPS timing?
I am using the PPS from a Garmin GPS16 to timestamp a VLF
signal received directly via a PC soundcard. Signal into one
channel, PPS into the other.
Using pulse centroid timing, I'm seeing about 0.5uS jitter of
the pulse-to-pulse interval, and an exponential moving average
with time constant 100 seconds is applied to smooth this out.
What I have left is a slow cyclic variation of the signal
timestamping.
It shows up when I measure the phase of an off-air signal,
eg here is MSF 60kHz from Anthorn.
http://abelian.org/vlf/live/pp100701msfb.png
The same variation appears (in unison) on other off-air timing
signals, eg DCF and HBG both show the same cycle, so I guess
this is variation of the GPS PPS itself, not the VLF signal phase.
The period is about 3000 seconds and the amplitude corresponds
to about +/- 1.4uS.
Having no prior experience of using GPS PPS timing, I was
expecting some pulse-to-pulse jitter, but this slow variation
has caught me by surprise. Question: is this typical of GPS
timing signals, or have I got a problem in my software?
--
Paul Nicholson
http://abelian.org
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