From: email@example.com (Bob Wilson)
Subject: Re: Who thinks this?
Date: Fri, 01 Nov 2002 05:42:25 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article <3DC1D056.firstname.lastname@example.org>, email@example.com says...
>I find many people have the following conception:
>A transmission line is like a capacitor. If you have a long line, and
>feed it a digital logic transition, the output signal at the other end
>will look like a RC exponential response. The slowness of that response
>will be related, for the most part proportionally, to the length of the
>cable and the output impedance of the driver.
>Furthermore, that putting a resistor in series with the output of a
>driver, and the input of the cable, will somehow slow the response even
>more, which seems logical if you think the line is capacitive.
>Of course all of these notions are very incorrect, assuming one is
>talking about an almost ideal line with a purely real complex
>propagation constant, and thus a purely real characteristic impedance,
>and a non-dispersing, non-distorting line, such as typical controlled
>impedance cables that we use every day.
>What do you think?
>Fun questions are then derived from these considerations like:
>You have a driver generating a very stiff 5V output step, and you
>connect it to a 50R line with a 50R series resistor.
>Why does the edge at the unterminated output end of the line snap to 5V
>after the propagation delay, with the same risetime as if the driver
>were driving a simple 50R resistor, no matter how long the line (again
>assuming that the line length is such that the line is very close to
>ideally non-distorting)? And why then is the input voltage to the line
>only 2.5V when the output voltage steps to 5V?
>Or doesn't it?
>(I'd say it does.)
A transmission line is NOT just a capacitor. As one can see from any basic
text on this subject, an ideal transmission line can be modeled as a long
network of L-Cs, not just caps alone.