[time-nuts] Using GPS 1PPS for accurate period measurement

Bob Smither smither at c-c-i.com
Fri Dec 2 09:09:43 EST 2011


mike cook wrote:
> 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.

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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