Hi The sawtooth process “picks” the closest clock edge and spits out the PPS based on it.
If the internal TCXO is off of a point that divides to 1Hz, the edge guess changes fairly often and you can average it out. A drifting TCXO will (effectively) never be at a modulo 1 Hz frequency long enough to matter. If the TCXO happens to hit a modulo 1 Hz point *and* remain fairly stable, the pick process will always pick the same edge of the clock. Now the averaging process stops working. The edge guess is no longer bouncing back and forth. If you graph what happens, it is a parabolic looking structure in the data between two triangle waves. The normal term for this is a hanging bridge. The gotcha is that depending on the TCXO, the temperature, and the guess process, the bridge can “hang in there” for quite a while. If it hangs on one edge of the clock for a few hundred seconds, the offset will likely chug right through your filter. ====== On a receiver with sawtooth correction, you have a manufacturer specific message that gives you information on the state of the receiver. It is defined as either applying to the next pps or to the pps that just came out. There is a field that may give you picosecond resolution (as opposed to accuracy) data on the proper location of the PPS edge. Depending on how you evaluate the correction, it can get the jitter down below 1 ns (again, jitter as opposed to accuracy). A device that uses the sawtooth data shoves it into the control loop along with the measured early / late information on the PPS. Bob > On Jul 18, 2016, at 6:31 PM, Nick Sayer via time-nuts <[email protected]> > wrote: > > I've read Tom's page about sawtooth PPS jitter and I believe I understand > where it comes from. My current GPSDOs ignore the phenomenon. Certainly at > the moment, I'm satisfied with that. The systems gravitate towards PLL time > constants that average it all away. > > What I'd like to understand is how sawtooth compensation works with receivers > that support it. Is it that I expect an NMEA sentence with a nanosecond > offset value that I add to any phase difference observation that I get? > > Sent from my iPhone > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
