[time-nuts] Using GPS 1PPS for accurate period measurement
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 :
>> 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
> 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
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.
The only two laws that we need (Richard Maybury):
Do all you have agreed to do.
Do not encroach on other persons or their property.
Smither at C-C-I.Com http://www.C-C-I.Com 281-331-2744(office) -4616(fax)
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 140 bytes
Desc: not available
More information about the time-nuts