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

Mark C. Stephens marks at non-stop.com.au
Thu Feb 23 14:55:54 UTC 2012

To clarify, Chassis ground should only be connected one end as opposed to signal ground which should be connected both ends.

It is common practice if using shielded cable to solder only one end of the shield to the shell and link the shell to chassis ground pin.

> -----Original Message-----
> From: Uwe Klein [mailto:uwe at klein-habertwedt.de]
> Sent: Thursday, 23 February 2012 7:59 PM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] Oddities in termination of cable from gps18.
> 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...
> uwe

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