I got some of the cheap Pletronics OHM40480526 26MHz OCXO parts for
experiments (couldn't resist $2 each :-). For this purpose I modified the
Richard H McCorkle/Tom Van Baak code for generating a 1 pps output from a
PIC, to work with 26 MHz input, and also target the PIC 16F1823 since
that's the part I had handy. But, being new to the PIC16F family it took
some time to get all the configuration bits correct. So in case anyone else
is in that position, here's the code I ended up with:
http://bealecorner.org/best/measure/time/source/PIC16F1823-26MHz-1pps_asm.txt
My M12T+ GPS board is coming on a slow boat from China, so I don't yet have
any real standard to compare this with.
But I set up two OCXOs driving two PICs, and compared the two 1pps outputs
on a 2-ch scope, after tweaking my 10k 10-turn trim pot feeding the ADJ pin
on one part to be pretty close in frequency to the other. I see a slow
steady increase in phase lag, accumulating 167 ns in 7 minutes, so I have a
frequency difference between my two oscillators of 1.67E-7/420 = 4E-10 or
0.4 ppm. For better measurements I guess I want a PIC-TIC or equivalent.
-John Beale
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