From: "Harry Dellamano"
Subject: Re: Who thinks this?
Date: Thu, 31 Oct 2002 16:51:33 -0800
Organization: Posted via Supernews, http://www.supernews.com
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"Harry Dellamano" wrote in message
> "Christopher R. Carlen" wrote in message
> > Hi:
> > 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
> > firstname.lastname@example.org
> 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
ooPs.. increases => decreases
It's difficult for me to type and breath at the same time!