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

Chris Albertson albertson.chris at gmail.com
Wed Feb 22 23:53:43 UTC 2012

I could not get good result using cat-5 either.  My "fix" was to move
the GPS and computer closer so I would not need a long cable.

I sort of understand coax but cat-5 is a bundle of pairs.  If you use
the cat-5 for both RS232 and PPS then the twisted pairs are
un-balanced because the return current path is shared by the RX, TX
and PPS signals.   I think that might have been my problem

I was thinking about use balance line driver and receiver chips but
then figured moving the equipment would be better

On Wed, Feb 22, 2012 at 2:18 PM, unruh <unruh at invalid.ca> 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.
> _______________________________________________
> questions mailing list
> questions at lists.ntp.org
> http://lists.ntp.org/listinfo/questions


Chris Albertson
Redondo Beach, California

More information about the questions mailing list