[time-nuts] Low cost GPS module for < 100ns timestamping error

Tom Van Baak (lab) tvb at leapsecond.com
Sat May 3 17:41:33 UTC 2014

Tony, Chris, Bert,

Since all you want is a 10 ns time stamp / data logger you do not need a GPSDO, or OCXO, or VCXO.

The solution is cheap and very simple.

Your GPS receiver provides a 1PPS to the microprocessor. Use a plain XO or TCXO; the frequency does not need to be accurate, just stable to about 1e-9 (many $1 xtals do this). Each second your code [re]computes the drift between the clock and GPS. You may average over 10 to 100 seconds if you wish.

Even though your clock is off-time and off-frequency your software knows what the offset is. Therefore, you can simply adjust the time stamp reading by the current clock error.

This "software GPSDO" gives equal or actually slightly better performance than a real GPDSO but it is much simpler: no DAC, no EFC, no OCXO, no VCXO, no PLL.

/tvb (i5s)

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