With many GPS modules with 1pps the pulses keep coming even if the fix is lost, and so your precious GPSDO attempts to track these erroneous pulses, and you lose your nice holdover performance. The pulses can be wildly off.
If you know when to hold, the micro driving the D-A and OCXO EFC input can easily hold the last good value, or (as in the commercial GPSDOs) use a Kalman filter arrangement to continue moving the D-A in an open loop manner. In one of the units I built, I had a HOLD input to the GPSDO micro, and used another micro to monitor the GPS module (Motorola Binary Protocol). If the fix went below 3D, the number of satellites in the fix below 5, or the DOP was poor, this micro would simply raise a HOLD output, and not lower it until proper conditions had been restored for at least a minute. It worked well. Of course the error on 24-hour holdover was larger than desired, for the much more frequent dropouts a few minutes long, the performance was all you could desire. I achieved about 1us for one hour. The trouble is, this isn't really practical unless you write all your own code. Murray ZL1BPU _______________________________________________ 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.
