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

Uwe Klein uwe at klein-habertwedt.de
Thu Feb 23 08:58:41 UTC 2012

unruh wrote:
> 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. 
Is the 110Ohm twisted pair the only connection to your pps sink ?

If you connect one conductor to GND on both sides you just introduce
some funny groundloops.

i.e. the assymetric source driven transmission line works as a baloon
and produces a symmetric signal at the other end. If you short circuit
that to gnd on one conductor...


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