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

Tom Harris celephicus at gmail.com
Thu Dec 1 19:41:06 EST 2011

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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