[SI-LIST] Re: Transmission line match
- From: "Prakash Chauhan" <pchauhan@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 12 Nov 2003 20:34:50 -0500
Doug,
I find your observation about time constants interesting in a couple of
aspects:
1. If you see the time constant of one segment of the lumped network, it
can be seen to be (LC)^1/2, which is the inverse of the wave propagation
speed.
2. It also helps me visualize the return current ( that we keep talking
about in this forum) and how discontinuities in the transmission or return
path would impact signal quality.
Since circuit designers work more with circuit elements and do not
indulge
frequently in solving electromagnetic wave problems, this technique might
help bring a better qualitative understanding.
--Prakash.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Knighten, Jim L
Sent: Wednesday, November 12, 2003 7:39 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
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.
____________________________________________________________________________
__
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http:/www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http:/www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http:/www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
