Leo, On 1/9/19 10:10 AM, Leo Bodnar wrote: > Depends what you call "systematic"... > > I can only speak for Ublox but it is fairly representative of modern (even > though its architecture did not change for almost 10 years) GPS chipsets. > The quantisation runs off internal cheap XO or TCXO that is PLLed to produce > MCU core clock that is then phase-accumulation quantised to the current > navigation solution target - typically every 1-10Hz. > Stock Ublox has 26MHz XO/TCXO which is then PLLed by MCU core into system > 48MHz clock which runs the Cortex core - including timers - that produce 1PPS > TP. > > Somebody measured Ublox 1PPS against the stable timebase and looked at same > Ublox reported quantisation correction (ps order of magnitude) and actual > error was about 1ns order of magnitude. This would be unaccounted > discrepancy of what Ublox thinks is happening on its 1PPS I/O pin and what is > actually there. This makes analogue delay line correction only viable to > that level of accuracy. > > Whether this inaccuracy is the result of a high core PLL phase noise, sloppy > quantisation algorithm or hardware I/O drivers is an interesting academic > question but won't solve the problem.
Now, what I was talking about was frequency/time-interval counters in this case, not GPS-chips or GPSDOs. So, your comment seems out of context in that regard. However, considering that you have a free-running TCXO, the assignment of cycle in that clock which is to represent the PPS has a systematic quantization. The setup is much more complex than a normal counter setup, so it is otherwise not comparable. > Ublox core can run off many external clock frequencies from 12 to 40MHz but > they all will go through the PLL which makes it pointless to experiment with. They better PLL. External clock and steering from the time-product is a relative straightforward approach if one wants to do that, and then the PPS quantization will not be as relevant to the steering-loop. Cheers, Magnus > Leo > >> From: Magnus Danielson <[email protected]> >> >> Recall that the quantization is really a form of time-stamp value for >> the channel in it's relation to the time-base. It's a systematic pattern >> in the time-base clock and it is phase-locked to the time-base phase. >> ... > >> So, to conclude, the quantization noise that we have is very systematic >> in its nature, > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
