mike cook wrote:
Le 02/12/2011 01:53, Bob Camp a écrit :HiOk, 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.
This may depend on the GPS unit. I have a old Ref Tek GPS clock box (used to time earthquake monitoring systems) with a 1 PPS output pulse. That pulse has about +/- 2 usec of jitter. Looking at the box, the Trimble GPS receiver output is "handled" by a 68000 series microprocessor. I suspect the micro is interrupted by the Trimble 1 PPS, and therefor the signal out of the Ref Tek has the stated jitter. Of course, for the application intended, 2 usec of jitter is not significant.
I am using this 1 PPS to monitor a precise crystal oscillator, but I have to do some serious averaging.
--
Bob Smither, PhD Circuit Concepts, Inc.
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