alan smith wrote:
> I was asked this the other day and my first gut reaction was..no, shouldnt be.  For a BNC coupler, is there a 50 and 75 ohm version?  Or is it just a coupler so the impedance should be zero or close to it, since its coupling an impedance matched connector system.
>   
Impedance in this context reffers to characteristic impedance 
(characteristic impedance is basically the impedance you would see if 
you measured an infinitely long line) . For echo free transmission you 
want at least one (preferablly both) of your transmitter and receiver 
impedance matched to the lines characteristic impedance and you want to 
keep the characteristic impedance consistant  througout your 
transmission line. In a multidrop transmission line things are a little 
more complex, typically you would terminate the ends of the lines then 
make the receivers high impedance and the transmitters low impedance 
when transmitting and high impedance otherwise. Connectors will always 
cause some distortion but by using high quality impedance matched 
connectors you can keep it to a minimum.

*A coupler like a connector will become part of the transmission line 
and hence needs the right characteristic impedance for optimal 
performance of the line.*

Does it matter? that depends on the application. In thin ethernet for 
example the T peice arrangement going into the back of a computer is 
likely to cause far worse distortion than a badly matched coupler. But 
since thin ethernet only runs at up to 20mhz (10 megabit per second with 
manchester encoding) it copes with said distortion.


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