Vince, The value I see in mixed mode S parameters is that in cases where common and differential modes are mostly uncoupled, it allows easier conceptualization of the excitations on the network through analogy with a simple 2-port network. For example, with experience one can scan through a 2-port S parameter file and tell how well matched it is, or the gain, or the reverse isolation. Same thing with a mixed mode file, if you can separate out (with your eyes), the appropriate terms. As far as modeling goes, I agree with you that a mixed mode S parameter file for input to a simulation is an unnecessary complication. Regards, Chuck Hill, consultant -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of vince_cavanna@xxxxxxxxxxx Sent: Wednesday, September 15, 2004 2:59 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] why do I need mixed mode S parameters? I have some philosophical questions about mixed mode S parameters that I = have been struggling to understand as I re-enter the field of signal = integrity and attempt to catch-up on some of the new = measurement/analysis techniques. I would appreciate any insight you can = offer. I understand mixed mode S parameters and can compute them from standard = (single-ended) S parameters or from a model - or the other way around. = I can appreciate their usefulness in understanding how an n-port, that = may have been designed to operate mainly under differential stimulus, = responds to (reflects and scatters the incident power) differential and = common-mode stimulus. What I am trying to understand is why I would ever want to use mixed = mode S parameters in a time-domain or frequency domain simulation, and = how to use them. I am also interested to learn what simulators support = mixed mode S parameters directly, as using them in a simulator such as = Hspice seems cumbersome. My approach today is to simply use standard S = parameters directly. The "why" I really don't understand at all. With regards to the "how", I = know of one approach but it is cumbersome and does not seem worthwhile. = I would be interested to know if there are circuit simulators that = handle mixed mode S parameters directly but most important I need to = understand why I need them. One way to use mixed mode S parameters, that has been suggested on this = mailing list, is to use the S element in Hspice, but represented with = the mixed mode S parameters instead of the standard mode S parameters, = and recognizing that the ports are conceptual (differential and common = mode) as explained in [ref1]. In order to interface the conceptual = n-port to my circuit (which expects real ports) I then have to wrap the = device with a circuit that converts the actual port waves of my circuit = into the differential and common mode waves that need to be applied to = the conceptual n-port. This approach should work but seems cumbersome = and, more important to me, I don't understand what I gain from it.=20 The approach I described seems like a round-about way to attempt to use = the mixed mode S parameters directly when they can easily be converted, = with no loss of information, into standard mode S parameters and used = directly with the S element of Hspice. Even better I would prefer to get = standard S parameters for my components so I don't need to do any = conversions at all. In my simulations I prefer to see the physical ports = rather than the conceptual differential port and common mode port = described in [ref1], and so the most appropriate model for me seems to = be the standard s parameters. I can easily compute the various = differential or common quantities from the circuit if that is what = interests me. I also don't understand why I would need mixed mode S parameters of a = device from a vendor when I can compute them from the single-ended S = parameters. I do understand that there may be benefit in mixed mode S = parameters that have been extracted using a true mixed-mode (pure mode?) = VNA, but my understanding is that most VNAs available today actually = apply single-ended stimulus and measure the standard S parameters, and = then *compute* the mixed mode S parameters. That means I derive no real = benefit from the mixed mode s parameters other than the convenience of = not having to do any computations. I don't consider this benefit = significant since the calculations are quite straightforward and do not = suffer from numerical instabilities. I may be missing some fundamental aspect about the mixed mode S = parameters that would explain their popularity and if so I would love to = understand that.=20 Vince [ref1] Combined Differential and Common-Mode Scattering Parameters: Theory and = Simulation David Bockelman and William Eisenstadt IEEE Transactions on Microwave Theory and Techniques, vol 43, no. 7, = july 1995 =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 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