[time-nuts] Some results of PRS10 and Trimble Resolution

[SAIDJACK at aol.com]

Hello Tom,
 
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  
etc.
 
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 
in software.
 
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...
 
Bye,
Said