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 ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ 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