[time-nuts] Strange 100ns jumps on Motorola M12+T

Joseph Gwinn joegwinn at comcast.net
Tue Nov 19 10:09:51 EST 2013


On Tue, 19 Nov 2013 08:51:32 -0500, time-nuts-request at febo.com wrote:
(time-nuts Digest, Vol 112, Issue 59)
> Message: 1
> Date: Tue, 19 Nov 2013 13:17:26 +0200
> From: Stephan Sandenbergh <ssandenbergh at gmail.com>
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: [time-nuts] Strange 100ns jumps on Motorola M12+T
> Message-ID:
> 	<CADbj3vZgFbnbZL-vKHJmfZn4mKOPGNi9fGXAHLFXBqOdbO3exQ at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
> 
> Hi,
> 
> I've recently measured the 1PPS outputs of three Motorola M12+T GPS
> receivers using two HP53131A TICs. Antennas are located next to one another.
> 
> Now I notice the one M12+T has changed its time offset by 100ns three times
> over the period of 48hrs. The jitter remains the same, only the offset that
> changes.
> 
> I'm currently having a re-run of the measurement. But, in meanwhile, has
> anyone seen this kind of behaviour? A firmware issue perhaps?

I've seen this in old Symmetricom GPS receivers.  The 100ns jumps occur 
when lock is lost or gained, which is typically caused by the presence 
or absence of at least one good-enough satellite in view.

A bad connection in the antenna path is one cause.  Interference can 
also provoke the effect.

Joe Gwinn



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