[time-nuts] Testing frequency using NTP Bruce GPS ps
Tom Van Baak
tvb at LeapSecond.com
Sat Oct 4 03:28:49 EDT 2008
> 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?
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
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
> 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:
> 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
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:
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.
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