[SI-LIST] Re: Transmission line match
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <JK100005@xxxxxxxxxxxxxxxx>, <doug@xxxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Thu, 13 Nov 2003 07:10:37 -0800
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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