[time-nuts] Hanging Bridges

Tom Clark, K3IO K3IO at verizon.net
Tue Dec 19 23:26:30 EST 2006


   [1]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
   oscillators.
   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
   [2]ftp://ftp.cnssys.com/pub/PTTI/PTTI_2006.ppt or
   [3]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 [4]http://gpstime.com to be
   useful.
   73, Tom

References

   1. ftp://ftp.cnssys.com/pub/PTTI/PTTI_2006.pdf
   2. ftp://ftp.cnssys.com/pub/PTTI/PTTI_2006.ppt
   3. ftp://ftp.cnssys.com/pub/PTTI/PTTI_2006.pdf
   4. http://gpstime.com/



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