Here is some ASCII art with "colored" waves. The LLL and RRR represent portions of a T-line charged by a wavefront. <,> represent wavefronts traveling in indicated direction. Assume 50 ohm lines, 20mA current changes resulting in 1V wavefronts. Incident waves of equal magnitude: LLLLLLLLLLLL> <RRRRRRRRRRR Reflection model : <LLLLLLLLLLLRRRRRRRRRR> LLLLLLLLLLLLRRRRRRRRRRR Wave propagation model: <LLLLLLLLLLLRRRRRRRRRR> RRRRRRRRRRRLLLLLLLLLLLL Unequal incident waves, left is 2V, 40mA, right is 20mA, 1V: LLLLLLLLLLLL> LLLLLLLLLLLL> <RRRRRRRRRRR Reflection model: <LLLLLLLLLLLLLLLLLLLLLL> LLLLLLLLLLLLRRRRRRRRRR> LLLLLLLLLLLLRRRRRRRRRRR For the unequal wave reflection model above, at the point where the wavefronts meet, no charge can flow (I=0) from the right wavefront to the left T-line because the lines are charged to the same potential. I=0 means the right wavefront sees an open circuit and reflects. When this point reaches 2V, the 40mA left incident wavefront can charge both the T-lines with 20mA each, sending a 1V wavefront into the right T-line, and a 1V reflected wavefront going back on the left T-line. So the left incident wavefront can be described as having encountered a high impedance (>50 ohm and <open circuit) whereby a 2V incident wave produced a 1V reflected wave. Wave propagation model: <RRRRRRRRRRLLLLLLLLLLL> LLLLLLLLLLLLLLLLLLLLLLL> LLLLLLLLLLLLRRRRRRRRRRR Thanks, Vinu steve weir wrote: > Ihsan, I've presented two methods that both correctly predict the > results: One based on modeling the intersection as an open to the > even mode, while short to the odd mode, and the other on what I think > is far simpler: continuous propagation of each of the original wave > fronts. Use whichever model makes your day simpler, but for my money > I'll stick with the latter. I prefer the view that discontinuities > and resulting reflections in quasi uniform, infinite length, ie > terminated transmissions are the result of physical variations in the > channel, not patterns of energy I happen to launch into them. > > Consider for example +1.0V step from the left, and a +0.5V step from > the right. After they meet, the voltage moving rightward continues to > rise by +1.0V from its previous value, and the voltage moving leftward > continues to rise by +0.5V from its previous value. The waves just > linearly superimpose. > > Regards, > > > Steve. > > > Ihsan Erdin wrote: >> Steve, >> >> The wave propagation is simply the transfer of the energy in space. >> For the special case a line symmetrically driven at both ends, one can >> use the model of an unterminated transmission line driven from one >> side only and no one can tell the difference. This is based on the >> fundamental electromagnetic principle: image theory. >> >> For the uneven drivers of your example, I can rightfully argue that >> the equal frequency components "bounced" and cancelled out while the >> residual part kept on propagating. The idea of waves passing through >> each other is simply a matter of perception; not a rocksolid physical >> reality which ridicules the idea of waves bouncing in the middle. Both >> cases have equal footing and at the end it all boils down to the >> choice of modeling. >> >> The billiard ball example was an interesting attempt but not quite >> equivalent. At the collision the balls will have to come to a >> momentary full stop before accelerating in the reverse direction. This >> is not symmetrical to the case where they (might) pass through each >> other at constant speed. >> >> Best regards, >> >> Ihsan >> >> On 7/30/07, steve weir <weirsi@xxxxxxxxxx> wrote: >> >>> Vinu but for the discussion at hand: >>> >>> First: The driver is back terminated in the example so both wavefronts >>> are completely absorbed and the characteristic impedance is the >>> effective impedance of the line everywhere. Energy propagating forward >>> or backwards in the line does not change the impedance. >>> >>> Second: At the point in time where the apparent reflection occurs, no >>> wavefront has reached an impedance discontinuity. And in fact as >>> stated >>> above, if the source matches perfectly, never will. There are no >>> reflections in this system at all. Each wavefront launches, goes its >>> merry way around the path and gets identically absorbed back at the >>> driver. >>> >>> To an observer monitoring the line two equal and opposite wave fronts >>> will indeed appear to bounce like a perfectly elastic mechanical >>> collision. So let's ask ourselves which is the illusion: the apparent >>> 100% reflection, or the continuous propagation of each front. Several >>> useful experiments have been offered to resolve the issue. In each we >>> send two wavefronts which are not identical and monitor the behavior. >>> What do we find? We find that rather than each waveform reflecting >>> identically as predicted by the reflection model, the difference >>> continues to propagate forward. IE, the observation EXACTLY matches >>> the >>> wave propagation model, while it does not match an unmodified >>> reflection >>> model. In order to fix the reflection model we have to artificially >>> create a short to the odd mode at the same point where we have an open >>> to the even mode INCIDENT waveforms. >>> >>> Best Regards, >>> >>> >>> Steve. >>> Vinu Arumugham wrote: >>> >>>> "There is only one impedance at any given point on the line, and for >>>> constant line parameters, the impedance is constant throughout." >>>> Yes, that's the characteristic impedance of the line. >>>> >>>> The input impedance of an unterminated line can vary from zero to >>>> infinity depending on the frequency of the driving signal. In other >>>> words, the line driver "sees" a high or low impedance that is a >>>> function of the magnitude and phase of the reflected wavefront. The >>>> same thing happens when wavefronts meet in a loop. The effective >>>> impedance seen by each wavefront is a function of the magnitude and >>>> phase of the other wavefront. So, why is this interpretation >>>> "nonsensical"? >>>> >>>> Thanks, >>>> Vinu >>>> >>>> olaney@xxxxxxxx wrote: >>>> >>>>> 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 >>>>> <mailto: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 >>>>>> >>>>>> >>>>>> >>>>>> >>>>> >>>>> >>> -- >>> 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