> The single-shot is supposed to eliminate the 1PPS jitter? So it must > be triggered on the 1PPS, and the variable delay gives an average of > half the clock period?
Yes, right. Note another equivalent technique is to use two serial ports; one for the receiver (record sawtooth corrections) and one for a 53132A-style counter (record TI measurements) and then do the calculations in software. TAC32 does this. I think TBoltMon also allows it. Roughly, it's a trade-off in equipment and complexity. They give essentially the same performance result. There are still many other sources of noise, both short- and long-term in an OEM GPS receiver/antenna system. That's why even if there were zero quantization error, you would still see a couple of ns rms error in the 1PPS output. > Thanks. I had not found that page yet. Just judging by eyeball, the > pink and yellow traces don't seem to track very well. Any reason? On that plot, the various runs weren't concurrent so you can't look for common-mode effects. Separate runs, showing typical levels of jitter, all plotted on the same x-y scale. You've got a good eye. > And what's the blue trace for? I can't seem to find the parent page, > so I don't know if you have already explained it. This will help a bit more: http://www.febo.com/pipermail/time-nuts/2005-April/018114.html > Overall, I was a little disappointed to find the sawtooth correction > only gives about a factor of 3 or so improvement. The results I have That's partly because the sawtooth of the M12+ is small to begin with, at least compared to the earlier VP. So there isn't much room for sawtooth correction to have a massive gain. Also the granularity of the quantization message from the receiver is 1 ns. And there's an internal delay of one or two seconds which has a slight effect on the quality of the correction. Another way to look at it is this. If a perfect M12+ had, say, 2.5 ns of jitter, and you saw 10 ns rms without sawtooth correction and 3 ns rms with sawtooth correction you could say you achieved a 15x improvement! (10-2.5)/(3-2.5). Or if you didn't say it, the marketing department certainly would. The closer you get to a few nanoseconds of jitter anyway, the more all the other errors in a typical cheap OEM GPS receiver and antenna system come into play. So once you average beyond a couple of minutes the wander you see in the 1PPS has less and less to do with sawtooth and more to do with the sum of all the other subtle errors. Expensive geodetic or timing receivers use an assortment of techniques to reduce the effect of these remaining error sources, to the point where you'll hear of millimeters and picoseconds (and priced accordingly). > obtained with my method have given over two orders of magnitude > improvement in noise reduction. That's with no optimization. Averaging over multiple samples, of course, gives a reduction in noise. You can see this in the ADEV plots at the end of: http://www.leapsecond.com/pages/58503-cns2/ On the other hand, if you found a way to get two orders of magnitude better performance out of an M12+ without the use of averaging then I'm all ears. /tvb _______________________________________________ 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.
