[SI-LIST] Re: Transmission line match
- From: Doug Brooks <doug@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 13 Nov 2003 09:03:01 -0800
Several of you gave very helpful (and sometimes different) views of this.
Thanks for helping me see it different ways.
What a great list this can be sometimes!!
Doug
At 07:10 AM 11/13/2003 -0800, Loyer, Jeff wrote:
>seconded...
>
>Jeff Loyer
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Knighten, Jim L
>Sent: Wednesday, November 12, 2003 4:39 PM
>To: doug@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Transmission line match
>
>
>Doug,
>
>With all respect, I think you are on the wrong track. Instead of =
>thinking
>in terms of time constants, think about Kirchoff's voltage and current =
>laws
>at the boundary between the transmission line and the termination. The
>concept of transmission lines means that they are long enough so that =
>you
>have to think in terms of traveling waves. At the boundary, the voltage
>across the two conductors of the transmission line and the voltage =
>across
>the load must be the same. Also, at the boundary, the currents must be =
>the
>same. If the impedances are different, the current in the load must be
>different from the current in the transmission line. The only way this =
>can
>be is if there is third current (reflected current) to satisfy =
>Kirchoff's
>current law. This is written in a simplistic way, but I think you will =
>find
>that this is the way to think about mismatch behavior in a qualitative
>manner.
>
>Regards,
>
>Jim
>
>________________________
>James L. Knighten, Ph.D.
>Teradata, a division of NCR http://www.ncr.com
>17095 Via del Campo
>San Diego, CA 92127
>tel: 858-485-2537
>fax: 858-485-3788
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
>On
>Behalf Of Doug Brooks
>Sent: Wednesday, November 12, 2003 2:43 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=20
>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=20
>R=3DZ=3D(L/C)^.5, then RC=3DL/R. That is, when correctly terminated, the =
>time=20
>constants are matched.
>3. If the terminating resistance is high, then the RC time constant is=20
>relatively longer. If it is low, then the L/R time constant is =
>relatively=20
>longer.
>
>At this point I am having trouble getting over the "so what?" and=20
>completing the argument. What is the next step in this argument..... or =
>am=20
>I on the wrong track altogether?
>
>Thanks
>Doug Brooks
>
>
>Doug Brooks' new book, "Signal Integrity Issues and Printed Circuit =
>Board=20
>Design" has just been released by Prentice Hall. See details and =
>ordering=20
>info at www.ultracad.com. Also check out UltraCAD's new differential=20
>impedance calculator.
>_________________________________________________________________________=
>___
>__
>
>
>
>
>
>
>
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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
______________________________________________________________________________
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