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

David J Taylor david-taylor at blueyonder.co.uk.invalid
Thu Feb 23 12:24:03 UTC 2012

"Uwe Klein" <uwe at klein-habertwedt.de> wrote in message 
news:1nbh19-jvp.ln1 at klein-habertwedt.de...
> 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

What you say is true, but only when the cable is a significant fraction of 
a wavelength.  For the relatively short lengths and ~200 ns (or slower) 
edges being discussed here it's unlikely to matter.  For the ~1 us 
accuracy of the PPS edge likely it would be better to simply slow the edge 
down to avoid any ringing effects (and would the ringing matter in any 
case when the receiver has an RS-232 specification, likely including 

Different if you're chasing a 50 ns difference, of course.  You try to do 
the job properly then.


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