[time-nuts] Hanging Bridges
Tom Clark, K3IO
K3IO at verizon.net
Tue Dec 19 23:26:30 EST 2006
ftp://ftp.cnssys.com/pub/PTTI/PTTI_2006.pdfHal Murray said:
> Noise like the oncore sawtooth isn't always a bad thing.
I was going to comment on that area... Thanks for the reminder.
The problem is that the sawtooth isn't noise in the normal Gaussian sense.
If you happen to hit a long/wide hanging bridge, the resulting offset may get
past your PLL filter.
It might be possible to avoid hanging bridges by dithering the sawtooth. I'm
thinking of something like a heater under the xtal for the GPS unit that gets
driven by a medium frequency - slow relative to the normal sawtooth but fast
relative to the PLL time constant. This somehow feels not-good, but I can't
see the obvious screwup.
Lots of experience shows that the hanging bridges are a killer. I have
tried very hard to come up with PLL algorithms so that they don't kill
performance. Unfortunately, the characteristic time between them falls
right at the loop time constants you want to use to correct typical
Rick Hambly and I gave a paper at the PTTI meeting 2 weeks ago on how
to remove them in hardware -- see "Improving the Performance of Low
Cost GPS Timing Receivers" at
ftp://ftp.cnssys.com/pub/PTTI/PTTI_2006.pdf (the PPT is a huge
33MB in size, the PDF is a much smaller at ~2.5MB). You might find
some of the other papers archived on http://gpstime.com to be
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