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

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Thu Feb 23 19:59:57 UTC 2012


On 2/22/2012 2:18 PM, 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.

A low current output driver, perhaps 5ma?
 {Another reason to use a RS232 line driver?}

e.g. SiRF III chips (Garmin sometimes uses),
e.g. GSC3e/LPx and GSC3f/LPx, Output 5.25v-0.2v @100ua, 2ma max

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