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

Bob Camp lists at rtty.us
Thu Dec 1 19:59:08 EST 2011


Is your GPS module a GPSDO or is it something more basic? If it's a GPSDO, then it's accuracy is not an issue. If it's a basic navigation board, then it could easily be off by enough to impact the results.


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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