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

unruh unruh at invalid.ca
Thu Feb 23 00:30:53 UTC 2012

On 2012-02-22, David Lord <snews at lordynet.org> wrote:
> 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. 
> PPS from my Garmin is at TTL level.

Yes, that is what I get if I leave the cable unterminated.

> There are scope traces in "The ART of Electronics" in
> section on RS422 where the traces are bumpy ramps with
> several plateaus due to the reflections.

That is what it looks like, no matter what the termination is. 
As the termination resistance goes up, the reflected pulse should be
negative, giving ringing. But no ringing is seen.

> For RS232 the conditioning is by limiting the rise time by
> a capacitor across the output and from MAX232 datasheet
> about 330pF would seem ok for limiting the slew rate.

Slowing down the risetime is not what I want. That pulse edge of the
PPS  gives the time. 

> David

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