On Wed, 13 Mar 2013 23:07:01 -0400, Michael Tharp <g...@partiallystapled.com> wrote:
>On 03/13/2013 09:05 PM, David wrote: >> This brings up something that I have wondered about for a while. >> >> The Garmin GPS18x (and many other receivers) specify the PPS output as >> within 1uS but does that mean it wanders around over say 12 or 24 >> hours within 1uS of GPS Time or does it mean something else? I can >> easily see the granularity of the PPS output but that is obviously not >> what the specification refers to. > >If it's anything like the ancient Motorola receivers I purchased off >fleabay on the mistaken assumption that they were UT+, it might have a >1uS phase skip every 20s or so. In other words, after a random number of >seconds the PPS (and all subsequent PPS) will be exactly 1uS early. The >idea being that it loosely tracks UTC/GPS, but jumps whenever it gets >too far away. Totally useless for timing but at least they were cheap. I have watched the output pretty carefully on my repaired Racal-Dana 1992 reliably to the nanosecond and I have never seen a jump like that and I doubt I would have missed it. At least for the Garmin 18x, the output pulse is definitely synchronous to the basic 16 MHz internal clock which is expected and I can see the sawtooth error change frequency as the GPS unit's temperature changes. It actually makes a pretty good temperature sensor. My 1992 lacks GPIB so I can not use it for logging unfortunately and I lack a better timebase for comparison purposes anyway. Those are on my list of things to take care of. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.