[time-nuts] Deriving PPS from hockey-puck G-Mouse GPS receiver to lock an oscillator

[Steve Rooke]

Mark,

2008/12/30 Mark Sims <holrum at hotmail.com>:
>
> A long time ago, I did something very similar with an early release Magellan 5000 board  (and a couple of other models of GPS cards) that did not implement the 1PPS signal.  I think the best time sync that I ever got was around 1 mS (most boards were around 10 mS).

I'm not actually looking at time syncing, only making a GPSDO.

> True 1PPS signals are implemented and synchronized in hardware.  The NMEA messages are generated in software.  Due to the vagaries of things like software interrupt latencies,  serial port bit synchronization, etc I doubt that you could ever get better than 1 serial bit time of accuracy out of any board and 10-100 bit times is more likely.  At 4800 bits/sec the jitter/offset of even 1 bit time would be huge at best.  Perfectly OK for things like clock displays,  but totally inadequate for things like oscillator locking.

Ok, so I can see what you mean about jitter but would that not be
taken care of by a low pass filter with a reasonable time constant
following phase-frequency detection between the PPS and a divided down
ocxo? Agreed the short term stability (ADEV for small tao?) would be
poor but would it not improve significantly in the longer term (ADEV
for large tao?)? As for offset against an absolute PPS synced to GPS
time, well this is not important to me.

73, Steve
-- 
Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet