In the past I've mentioned the Navsync CW-12 GPS board. Oncore M12
drop-in replacement, 1 PPS ( measured Standard Deviation < 5 ns., range
~ 30 ns. from min to max for 1000 measurements), and a 10 MHz output
that's 'steered by the GPS receiver'.
The recent discussions about a cheap, simple GPSDO motivated me to try
an idea I've had in the back of my mind for a while. I wanted to use a
double-balanced mixer as a phase detector to phase-lock a 10 MHz OCXO
to the 10 MHz output of the CW-12 at 10 MHz - no dividers. I threw
together a proof-of-concept circuit and it worked as expected. But then
I compared the OCXO output to the 10 MHz output of both a Z3801A and a
Tbolt and discovered that the CW-12's 10 MHz output is about 1.5e-11
(i.e. 1.5e-4 Hz) low in frequency. I emailed Navsync and they replied:
"The CW12 Motorola Binary and NMEA versions both do not phase align the
frequency output so the long term drift that Ed is seeing is expected."
Incredible. It's 'steered by the GPS receiver' - that's a direct quote
from the data sheet and the user manual - but it's not quite on
frequency and that's fine with them.
I checked the 1 PPS and it looks okay. Compared to the 1 PPS from the
Tbolt, over 20K seconds it wanders back and forth over a range of about
90 ns. Lady Heather reported about 50 ns. of wander for the Tbolt over
the same period.
I know that some of you have CW-12s and I just wanted to let you know
about this. I'd also be interested if you can run similar tests and
confirm my results.
Ed
P.S. I have to laugh at myself. Here I am whining about something
that's off by 15 parts per trillion. I'm a time-nut all right! :-)
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