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[SI-LIST] Re: Transmission lines and why there are reflections
- From: jan.vercammen1@xxxxxxxx
- To: lalexman@xxxxxxxxxxx
- Date: Mon, 17 Oct 2005 09:28:48 +0200
SI-list,
last year I had to give a presentation on signal integrity and I had to
explain reflections
on transmission lines to a group of colleagues.
In the IEEE trans on Education (vol 46 nr 1 February 2003) I came across
the following publication:
Animating transmission line transients with BOUNCE, by C.W. Trueman,
The BOUNCE tool from prof. Trueman is available on
http://emclab.concordia.ca/~trueman/bounce/index.htm
here you find several links to tools, one of them is BOUNCE. It is simple
and instructive and helps
to visualize transients on transmission lines, one can control various
parameters and even stubs are
allowed. The link to the article returns empty, but IEEE members can
access or buy. Maybe someone
could ask prof. Trueman to update his link.
Kind regards,
Jan Vercammen
"Leonard Alexman" <lalexman@xxxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
17/10/2005 02:51
Please respond to lalexman
To: <si-list@xxxxxxxxxxxxx>
cc: (bcc: Jan Vercammen/AMEMV/MOR/AGFA/BE/BAYER)
Subject: [SI-LIST] Re: Transmission lines and why there are
reflections
Thanks for the responses.
They all will help with my confusion.
Leonard
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Eric Bogatin
Sent: Sunday, October 16, 2005 3:09 PM
To: si-list@xxxxxxxxxxxxx
Cc: eric@xxxxxxxxxxxxxxx; susan@xxxxxxxxxxxxxxx
Subject: [SI-LIST] Transmission lines and why there are reflections
Leonard-
Why a signal reflects from an open, or any change in impedance, is a very
profound question. In a nut shell, the reflected wave is created to match
the ratio of the voltages and currents on the two sides of the interface
to
the impedances in these two regions, to keep the scalar voltage continuous
across the interface and to keep the current loop continuous across the
interface.
If you are interested, I wrote a column on this very topic in Printed
Circuit Design and Manufacture, for the Aug, 2003 issue. A pdf version of
this is can be downloaded from my web site, www.BeTheSignal.com
<http://www.bethesignal.com/> . You can also find a link to Printed Circuit
Design and Manufacture Magazine from my web site where you can sign up for
your free subscription.
I find it often screws up our intuition thinking that a signal "doubles"
upon reflection from an open. It is better to think of what happens in
terms
of the reflected signal and the incident signal. The reflection
coefficient
from an open is 1. If a 1 volt signal hits an open, a 1 volt signal
reflects
from the open. If you were a receiver with your input at the open, you
would
be sensitive to the scalar voltage between the signal and return
conductors.
What you would see is a voltage composed of the incident 1 v signal and
the
reflected 1 v signal, giving you a measured voltage of 2 volts.
If you think of it this way, then you can handle the cases when it is not
an
open but a termination of 150 ohms, for example. Knowing the impedance
change at the interface you can calculate the reflection coefficient, how
much reflects, and how much is measured across the 150 ohm load.
--eric
**************************************
Dr. Eric Bogatin
www.BeTheSignal.com
Signal Integrity on-demand training
26235 w 110th terr
Olathe, KS 66061
v: 913-393-1305
f: 913-393-0929
c:913-424-4333
e:eric@xxxxxxxxxxxxxxx
Signal Integrity-Simplified
Prentice Hall, 2004
****************************************
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Leonard Alexman
Sent: Saturday, October 15, 2005 2:18 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST]
Hi,
=20
I am trying to figure out transmission lines and reflections and trying to
understand why if the load is open or a high resistance the voltage that
arrives at the load is doubled and the signal is them reflected back to
the
source. I understand there is an impedance mismatch but in all the
articles
I have found not explains in basic terms wht the voltage doubles and
reflects back down the line. Can anyone point me to an article that might
explain this in basic terms ?
=20
TIA
=20
Leonard Alexman
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