[time-nuts] PPS sync

Mark Sims holrum at hotmail.com
Mon Jun 5 16:06:13 EDT 2017

Yes, that is the sawtooth correction parameter.  If a receiver reports the sawtooth correction value, but not a GPSDO EFC dac setting,  Heather plots the sawtooth value as the GD plot and shows it where the DAC value is normally shown.

Sawtooth values are seldom simple ramps.   They represent the difference between the GPS PPS signal and (usually) the GPS CPU clock.   They have all sorts of interesting structure to them (like the infamous "hanging bridges") The magnitude of the sawtooth range depends upon the GPS CPU.  Older devices may have a +/- 30 ns range and newer ones like the Venus timing receivers are in the +/- 6 ns range.

A GPSDO usually derives its PPS output by dividing the 10 MHz OCXO output down.   The GPSDO control loop removes the sawtooth error.   A good GPSDO takes advantage of the sawtooth correction message from the receiver to minimize the effects of the sawtooth error on  the control loop.   The Thunderbolt locks the GPS receiver clocks to the 10 MHz OCXO and does not have any sawtooth error to compensate for.


> Mark Sims, can you comment on the SawT parameter, I assumed being reported by the M12 GPS, displayed on Lady Heather?

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