[ntp:questions] Oddities in termination of cable from gps18.

unruh unruh at invalid.ca
Wed Feb 22 22:18:22 UTC 2012


I have a garmin gps 18 connected to my computer with a long (maybe 15m)
cat 5e cable, with the PPS carried on one of the twisted pair. I figured
it would be a good idea to terminate the cable with a 100 ohm resistor. 
One testing this the other day, I notices that the signal level was down
at 1V, with a staggered risetime of the pulse. -ie it would
exponentially rise, to a little plateau, then rise a bit more. Thinking
that I had underestimated the cable impedance ( it is after all a single
sided pulse, not a balanced signal) I upped it to 200 ohm. Now the pulse
rose to 2V but with a very similar shape to the rise. I finally removed
the resistor entirely, and now got a 4.5 V pulse, but the shape of the
rising edge remained much the same. I would have expected a much sharper
rise, with ringing , but no ringing in evidence. I do not understand
this. Clearly the 100 ohm was overdriving the output of the gps, but the
cable should have looked like 100 ohm to the pulse anyway (at least at
first). The open termination of the line should surely have resulted in
much more structure to the pulse. (The scope's input impedance should
not have altered things much since that is more like a Mohm.)
>From this it seems that trying to terminate the line is a mistake, and I
do not understand why. 
 



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