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

Bob Camp lists at rtty.us
Thu Dec 1 19:53:20 EST 2011


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. 

Simple answer - yes you can do it. Longer answer - spend the $30 or $50 on an auction site counter and just do it the right way. You will spend way more than that working out all the details even if you only make minimum wage. 


On Dec 1, 2011, at 7:41 PM, Tom Harris wrote:

> I finally have a need to ask a question about time measurement!
> I want to measure the accuracy of a 1Hz signal from a real time clock
> (RTC) chip with an integrated TXCO allegedly good for +/- 2ppm
> accuracy. My employer is too tight to buy a good frequency counter,
> but I do have a GPS module outputting 1PPS. If I use a little micro
> running at 16MHz from a jellybean crystal to measure the period of the
> 1PPS signal from the GPS concurrently with measuring the period of my
> test signal, will I be able to get accuracy of sub ppm? My resolution
> for a single period is 62.5 ppb, but this is a lot different to
> accuracy. I could easily count multiple periods, as I do not need fast
> results. My thought is that the 16MHz will have jitter and a
> pronounced drift, but that as the measurements of my reference and
> signal are done nearly concurrently, the errors will be very small.
> The need for this is that the RTC chip for a product has the engaging
> property of shifting it's frequency by several ppm after being
> soldered to the board, and I need to characterise this to get accurate
> timing for the product.
> -- 
> Tom Harris <celephicus at gmail.com>
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