# [SI-LIST] Re: Transmission lines and why there are reflections

• From: "Leonard Alexman" <lalexman@xxxxxxxxxxx>
• To: <si-list@xxxxxxxxxxxxx>
• Date: Sun, 16 Oct 2005 17:51:58 -0700

```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

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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