NNTP-Posting-Date: Thu, 31 Oct 2002 21:38:30 -0600
Date: Thu, 31 Oct 2002 21:34:10 -0600
From: Roy McCammon
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Subject: Re: Who thinks this?
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Christopher R. Carlen wrote:
> 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.
All close enough to true if the line is unterminated
and the driver resistance not too low. Actually a
series of stair steps, but looks like an RC on a slow
scope. In fact, the same C as the capacitance per
inch times the total inches.
You got to chase the echoes.
> 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?
I think you forgot to add "terminated in its
characteristic impedance" to that list.
> 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?
chase the echoes.