[SI-LIST] Re: Transmission line match

        Doug - 
        So now, don't you go into under-damped, damped, and over-damped
discussions?
        Thanks
        Ross Amans

-----Original Message-----
From: Doug Brooks [mailto:doug@xxxxxxxxxx]
Sent: Wednesday, November 12, 2003 4:43 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Transmission line match


I am trying to develop a *qualitative* (NOT quantitative) explanation of 
what happens at the terminating end of a transmission line. I think I am on 
the track with this model and discussion:
1. The transmission line model is a lumped model of a string of Ls and Cs.
2. The termination value forms a time constant with these. That is, if 
R=Z=(L/C)^.5, then RC=L/R. That is, when correctly terminated, the time 
constants are matched.
3. If the terminating resistance is high, then the RC time constant is 
relatively longer. If it is low, then the L/R time constant is relatively 
longer.

At this point I am having trouble getting over the "so what?" and 
completing the argument. What is the next step in this argument..... or am 
I on the wrong track altogether?

Thanks
Doug Brooks


Doug Brooks' new book, "Signal Integrity Issues and Printed Circuit Board 
Design" has just been released by Prentice Hall. See details and ordering 
info at www.ultracad.com. Also check out UltraCAD's new differential 
impedance calculator.
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