*From*: olaney@xxxxxxxx*To*: si-list@xxxxxxxxxxxxx*Date*: Sat, 28 Jul 2007 10:57:39 -0700

Again, no. I described it exactly as wave theory predicts. For a circular trace driven at one point, the two waves launch in each direction, meet at the electrical halfway point, continue through each other, and finally meet again at the origin, where the termination impedance determines what happens next. At the halfway point, the linear superposition of the two waves causes the voltage to double as they pass through each other, i.e. reinforcement, and current to droop to zero, but that is only a virtual open circuit, not a physical one. Wave theory teaches that a step launched into a single ended trace will propagate to the end, and meet a virtual wave headed the other direction (i.e. reflect), because the open circuit demands that the end point current must be zero. The incident wave and its own reflection add by linear superposition, causing the instantaneous voltage to double at the trace end. Assuming a step waveform in, the doubling propagates back to the origin, yielding the classic two step waveform we see on oscilloscopes. For a CW sine wave, we see nodes and antinodes along the trace that are used to calculate VSWR. But we are talking about a looped trace, where the two branches have minimal coupling between themselves. It is almost the same except that the return path for each direction is the entire loop. Here's an experiment that you can do with a TDR and a looped trace. At the drive point, make the waves launched in each direction slightly different from each other. This can be done by placing a small piece of ferrite on one of the branches as close to the origin as possible, or some other means of slowing the step edge (or making it ring, whatever) for that one direction as an ID marker. This allows us to distinguish the waveform launched into one direction from that launched in the other direction. Now, take your oscope probe and look at the waveform at each branch. Let us call them A and B. You will see the wave launched into B arrive at A, and the one launched into A returning to B. In a sense, the halfway point could be called an open circuit per your interpretation, based on the observation that where the waves have passed through each other, the local current drops to zero. But this is due to the superposition of equal but opposite currents, not any open circuit per se. To the extent that the waveforms in each direction differ (and we arranged for that) the instantaneous sum is not zero, and the difference is observable. Take your scope probe and move it around the trace to view this effect. Let's not be fooled by viewing a looped trace as a single ended one with a slot down the middle. That viewpoint puts the round trip on the same piece of copper, caused by an open circuit at the far end. In a looped configuration, the waves go all the way around the loop, and there is no far end as such. Consider that if you cut one of the branches next to the origin, that is where the far end of the just created single ended trace will be. Cutting the trace creates the ultimate directional ID, all in one direction and none in the other, which allows you to view the single propagating wave without superposition. As before, once the signal traverses the entire loop, the (open circuit for the severed branch) terminating impedance determines what happens next. You might wish to attend the three day course (bessercourse.com course 193) I will be teaching in San Jose, starting August 13. The discussion includes TDR theory, and I can make this example part of the demo. Orin Laney On Fri, 27 Jul 2007 23:22:20 -0700 ronald miller <ron@xxxxxxxxxxx> writes: olaney The two waves will not reinforce each other. They see an apparent open.. Ron olaney@xxxxxxxx wrote: No, the waves in each direction have the same phase at the source. They would meet and *reinforce* each other at the electrical halfway point, continue around the loop in both directions, and meet again at the source. Unless the source was back terminated, they would reflect from the low source impedance, this time in inverted phase from the original sense, and repeat the exercise until losses cause the waveform to die out. This is just a wierd example of exactly what happens on a regular single ended trace with an open end, except that you are driving two traces of identical length in parallel that have their ends shorted together. Also note that to back terminate the source, its impedance would have to be half that of the transmission path, because it sees both ends of the line in parallel. Example: 50 ohm source to drive a 100 ohm looped trace. Or, you could terminate the halfway roundtrip point the same way. Or both, if you double the drive level (great for video but not good practice for logic signals). Basically, ring layouts in copper foil are strange birds, useful for narrowband microwave purposes, but not of much use for wideband, time domain waveforms for all the reasons mentioned. Orin Laney On Thu, 26 Jul 2007 14:21:57 -0700 "Salkow, Steven" <steven.salkow@xxxxxxxx> writes: Mr Townsend et All made good points. DFM software would flag this as an error. Electrically, I think the idea is that the propagating standing magnetic wave of each arc path would exactly cancel each other when the waves met and no reflections would reflect back to the source. If one side of a path was not exactly equal, the point at which the waves cancel would not be at an physical midpoint rather at an electrical midpoint which may fall on a driven node. The point made by others already is better schemes exist already such as star distribution where end point termination may be used to easily terminate a line with quite predictable results. ss -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Townsend, Fred Sent: Thursday, July 26, 2007 9:05 AM To: David.Carney@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Circle bus topology; Circular Firing Squad? David: I have to ask what would be gained from such a topology? Ron and Scott both make some very good points. In the case of the Mux bus (1553)the ring encompasses the airplane and the double ring structure gives increased reliability. Microwave can make use of some structures like directional couplers to help reduce reflections. If rings are good why aren't we still using the token ring? Again the token ring was over an area much bigger than a PCB. A ring in a PCB would have all of the problems with no apparent gain. Think about your PCB router. Rings would drive the router nuts? Fred Townsend -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of David Carney (Neenah) Sent: Thursday, July 26, 2007 6:47 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Circle bus topology Has anyone ever experimented with a circle bus topology. The basic concept would be a bus with several devices attached. They would be routed in a daisy chain topology, and then the two ends of the daisy chain would be connected together. The PCB routing would look like a circle or a loop for each net on the bus. Pointers to references such as papers or application notes would be greatly appreciated. I'm particularly interested in signal integrity and EMC implications of this topology. =20 Thanks. =20 =20 ---------- David Carney Senior Hardware Engineer Plexus Corp. Phone - 920-751-5646 =20 ------------------------------------------------------------------ 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: =20 //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 =20 ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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 -- Ronald Miller Ghz Data, Signal Integrity Consulting 7721 Sunset Ave. Newark CA 94560 tel 510-793-4744 cell 510-377-9380 fax 510-742-6686 www.ghzdata.com ------------------------------------------------------------------ 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

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