From: "Harry Dellamano"
Subject: Re: Who thinks this?
Date: Thu, 31 Oct 2002 16:42:52 -0800
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Christopher R. Carlen" wrote in message
> 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.)
> Good day!
> Christopher R. Carlen
> Principal Laser/Optical Technologist
> Sandia National Laboratories CA USA
I say the line looks capacitive if the driving rise time is more than 4x
the line's one way prop time. As the rise time increases then the lines
inductance comes into play.
Just a WAG