If you suppose that the waves meet and rebound like billiard balls, that would be incorrect. Each passes through the other as if it was the only wave on the transmission line. Only a real open circuit (or other impedance discontinuity) can cause reflection. Though identical wavefronts might create the illusion of a "virtual open circuit" to the viewer, that is not the physical reality. The simultaneous "high impedance / low impedance" interpretation is nonsensical. There is only one impedance at any given point on the line, and for constant line parameters, the impedance is constant throughout. Especially note that the impedance of a linear xmsn line has nothing to do with the shape or direction of the waves that happen to be traveling on it. To suppose otherwise wrenches the laws of physics. Sorry if I have to be blunt. Wavefronts passing through each other is the bedrock reality, all else is armwaving. Orin Laney, PE, NCE On Mon, 30 Jul 2007 10:47:33 -0700 Vinu Arumugham <vinu@xxxxxxxxx> writes: When identical wavefronts are sent through the two branches of the loop and meet at the far end, each wavefront can be described as being reflected by the virtual open circuit. When one wavefront is "marked", the wavefronts do not encounter a virtual open circuit. One wavefront encounters a high impedance and the other a low impedance compared to the line impedance. The subsequent reflections of opposite polarity can be described as producing the illusion of the wavefronts flowing through rather than being reflected at that point. In other words, it seems to me that both the reflection and reinforcement descriptions are perfectly valid and each is as real or illusory as the other. Thanks, Vinu olaney@xxxxxxxx wrote: There is a difference, Ron, and my experiment illustrates it. It is that rather than bouncing back as a relection on the same trace, the loop return signals are the result of a round trip without reflection. Two open ended lines in parallel will show an impedance profile similar to that of the loop *only* if the trace lengths are matched. The fact that this special case is indistinguishable from a loop at the driving point is interesting, but does not make it equivalent in terms of the origin of each return signal. If you have a means to mark the driving signals so that they can be distinguished from each other, the difference between double open ended traces and with the ends shorted together can be observed. As you say, try it with a couple of pieces of coax and a TDR if you disagree. It'll work best if you use a separate series termination for each trace rather than a single backmatch resistor for both so that you can see the return signals separately. I mentioned ferrite but a high frequency LC trap on one leg to notch out a specific frequency might be more convincing. With two traces, the marked signal returns on the same trace. Create a loop by shorting the ends (making sure that the short maintains the correct path impedance), and the marked signal returns on the other trace. With identical traces (or coax) and identical driving signals, as you propose, the difference is there but you can't see it. That does not mean that the cases are equivalent, just that your experimental setup cannot distinguish between them. Hence, the need to mark the signals. Steve explained it well. This would make a good question for the electrical engineering professional licensing exam. Orin On Sat, 28 Jul 2007 23:29:35 -0700 steve weir <weirsi@xxxxxxxxxx> writes: Ron, yes if the signals exactly match then Ron's description of the apparent open end matches the illusion. It is an illusion just the same. This is where Orin's proposed experiment can provide insight. Any difference between the two wavefronts is not accounted for by the open end model. That odd mode if you will encounters the illusion of a dead short at the same juncture where the even mode Ron and you describe encounters the illusion of an open. Account for both the even and odd signal modes and you will get the right answer from the illusion just as you will if you follow the formal, exact, and I think simpler view: that the two wavefronts continue to propagate until they are absorbed. Steve. ron@xxxxxxxxxxx wrote: Consider for a moment a 50 ohm source driving two equal length 100 ohm lines unterminated(open circuit) TDR will show the open circuit at the end of the lines just as if there were one 50 ohm open ended line. Next consider what will happen if you connect the open ended lines together. No change. It will still reflect back as an open. Ponder that for a little and try it with a couple pieces of coax and a TDR if you disagree. -- Steve Weir Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 California office (408) 884-3985 Business (707) 780-1951 Fax Main office (401) 284-1827 Business (401) 284-1840 Fax Oregon office (503) 430-1065 Business (503) 430-1285 Fax http://www.teraspeed.com This e-mail contains proprietary and confidential intellectual property of Teraspeed Consulting Group LLC ------------------------------------------------------------------------- ----------------------------- Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC ------------------------------------------------------------------ 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.net 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.net 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