Le 02/12/2011 01:53, Bob Camp a écrit :
Hi
Ok, 62.5 ppb at 1 second would be 62.5 ns. that sounds right for a 16 MHz clock.
Your accuracy will be related to the offset between the two 1 ops events
(divided TCXO and GPS PPS) and the accuracy of your crystal.
With some luck you can get the crystal frequency to< 1 ppm by counting it
against the GPS pps output. If the TCXO pps is within 100 ms of the GPS pps, you
should get about 100 ppb accuracy. The trick is to keep the TCXO pps that close.
Can be done, but it takes some fiddling.
I don't think that the measurements have to be done concurrently.
The GPS pps signal will have a low ns jitter, possibly defined in
the data sheet, that can in this context be ignored if measurements are
take over a long period as can that for the RTC.
So for me, I would characterise the micro first, counting cpu cycles
per GPS PPS period. If that is done over enough periods, a good
indication of the measuring devices rate against a known standard will
be known. (1)
Then measure the RTC pps using the same procedure. (2)
Finally adjust the result from 2 by that of (1).
However, even that will be more costly than renting a counter.
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