[time-nuts] Some results of PRS10 and Trimble Resolution
SAIDJACK at aol.com
SAIDJACK at aol.com
Tue Jun 27 21:38:02 EDT 2006
We approached the sawtooth correction on the software side: we sample the
1PPS with 6.66ns resolution in our new Fury GPSDO and apply software adjusted
sawtooth correction (post-capture), and this yields an easily visible, very
significant reduction in the 1PPS capture noise with the Motorola M12+
receivers. You can really see the difference second-to-second when the correction is
turned on/off. This yields an average sampling quantization noise of +-3.33ns.
The physical sawtooth on the 1PPS signal is actually helpfull in this setup
as you mention since it dithers the LSB quantization noise, and thus actually
improves the quantization resolution over time (with simple averaging low
pass filtering of the captured data). This trick is called dithering in audio
Using delay lines may be tricky and expensive, they usually are temperature
sensitive, and only yield good results if the capture of the 1PPS is done in
fast enough (<10ns capture resolution). But if your capture is fast enough
anyways, there is no need to use a delay line since the correction can be done
Delay lines also have another disadvantage: the 1PPS correction from the GPS
can be positive or negative in time, so in theory you would need a negative
time delay (Einstein would be happy :). So for the delay line to work
correctly, you have to set the 0ns delay tap equivalent to the most negative pulse,
all other pulses will incur more than 0ns delay. You are now effectively
delaying the average 1PPS by 1/2 the spread of the pulses. Thus the output of the
delay line is always late (by about 30ns for commercial receivers) on
average for standard GPS receivers. This error will have to be subtracted later.
One trick when using Motorola GPS's is to set the cable delay to an additional
30ns to make the GPS receiver itself compensate for this delay by issuing the
1PPS output 30ns "too fast".
An interesting effect of sampling at +-3.33ns is that the GPS errors such as
multipath, atmospheric, and GPS crystal temperature related issues become
clearly visible with this kind of resolution...
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