[time-nuts] GPS ADEV?

Tom Van Baak tvb at LeapSecond.com
Tue Apr 10 10:52:57 EDT 2007

> Hi Tom:
> Why is the Datum2000 so much better than the other receivers?


I think mostly because the three Motorola GPS receivers
in that plot are 1PPS OEM boards and the Datum 2000 is
a fancy GPSDO (Rb). So it's not a "fair" comparison; I just
wanted to show a baseline in the plot and the Datum 2000
was a handy well-behaved GPSDO to use.

Note, though, that for taus greater than a couple of hours
the non-disciplined plain CNS-II (based on an M12+ with
hardware sawtooth correction) meets and then follows the

The point this demonstrates to me is that for any sort of
long-term time/frequency comparison (e.g., to see how
good your Cs is, or to measure the drift in your Rb) all
you need is a plain GPS OEM 1PPS receiver; a GPSDO
doesn't give you any additional performance. This may
be obvious by now, but it was cool to see it come out of
the graph.


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