Hi Vince, The short answer to your question: 'Why do I need mixed mode S parameters?' is =20 YOU DON'T. Mixed mode S-parameters are nothing more than a linear transformation of = the single ended S-parameters. Therefore it is the transformation or the inverse transformation will let you bounce back and forth between mixed = mode and single ended. If you have N-ports and N is an even number, you can convert your single ended S-parameters to mixed mode, grouping your ports however you wish. = You are correct that it is just as easy to simulate whatever network with = single ended (natural) S-parameters, and use MatLab (or some other math = program) to do the transformation to mixed mode. Personally I use just the Sdd quadrant of a 4-port usually and typically ignore the rest. Well, maybe take a quick glance, but unless I see something horrific I usually do not care. I make a leap of faith that = my common mode rejection is good, and the mode conversion is small. What I work on is differential outputs and inputs, so the differential response is what I am most interested in. Which leads in to why one = would get mixed mode S-parameters from a device vendor. If you know you are = using a differential device, then from a datasheet/S-parameter quick glance = you can understand the differential response. In addition, if you make the = same assumption I use about ignoring the rest of the mixed mode quadrants = (Sdc Scd and Scc) then you can simulate only a two port instead of a 4 port. (Not that simulating extra ports is a big deal, but two ports simpler in your own MatLab script or Excel spreadsheet) Thank you, Bill ++++++++++++++++++++++++++++++++++++++++++++++++++++ Bill Beale CAE, HiPHY Group Synopsys phone: 503-547-6523 2025 NW Cornelius Pass Rd. fax: 503-547-6017 Hillsboro OR, 97124 email: beale@xxxxxxxxxxxx ++++++++++++++++++++++++++++++++++++++++++++++++++++ >=20 > I have some philosophical questions about mixed mode S=20 > parameters that I =3D > have been struggling to understand as I re-enter the field of signal = =3D > integrity and attempt to catch-up on some of the new =3D > measurement/analysis techniques. I would appreciate any=20 > insight you can =3D > offer. >=20 > I understand mixed mode S parameters and can compute them=20 > from standard =3D > (single-ended) S parameters or from a model - or the other=20 > way around. =3D > I can appreciate their usefulness in understanding how an=20 > n-port, that =3D > may have been designed to operate mainly under differential=20 > stimulus, =3D > responds to (reflects and scatters the incident power)=20 > differential and =3D > common-mode stimulus. >=20 > What I am trying to understand is why I would ever want to use mixed = =3D > mode S parameters in a time-domain or frequency domain=20 > simulation, and =3D > how to use them. I am also interested to learn what=20 > simulators support =3D > mixed mode S parameters directly, as using them in a=20 > simulator such as =3D > Hspice seems cumbersome. My approach today is to simply use=20 > standard S =3D > parameters directly. >=20 > The "why" I really don't understand at all. With regards to=20 > the "how", I =3D > know of one approach but it is cumbersome and does not seem=20 > worthwhile. =3D > I would be interested to know if there are circuit simulators that =3D > handle mixed mode S parameters directly but most important I need to = =3D > understand why I need them. >=20 > One way to use mixed mode S parameters, that has been=20 > suggested on this =3D > mailing list, is to use the S element in Hspice, but=20 > represented with =3D > the mixed mode S parameters instead of the standard mode S=20 > parameters, =3D > and recognizing that the ports are conceptual (differential=20 > and common =3D > mode) as explained in [ref1]. In order to interface the conceptual =3D > n-port to my circuit (which expects real ports) I then have=20 > to wrap the =3D > device with a circuit that converts the actual port waves of=20 > my circuit =3D > into the differential and common mode waves that need to be=20 > applied to =3D > the conceptual n-port. This approach should work but seems=20 > cumbersome =3D > and, more important to me, I don't understand what I gain from = it.=3D20 >=20 > The approach I described seems like a round-about way to=20 > attempt to use =3D > the mixed mode S parameters directly when they can easily be=20 > converted, =3D > with no loss of information, into standard mode S parameters=20 > and used =3D > directly with the S element of Hspice. Even better I would=20 > prefer to get =3D > standard S parameters for my components so I don't need to do any =3D > conversions at all. In my simulations I prefer to see the=20 > physical ports =3D > rather than the conceptual differential port and common mode port =3D > described in [ref1], and so the most appropriate model for me=20 > seems to =3D > be the standard s parameters. I can easily compute the various =3D > differential or common quantities from the circuit if that is what =3D > interests me. >=20 > I also don't understand why I would need mixed mode S=20 > parameters of a =3D > device from a vendor when I can compute them from the single-ended S = =3D > parameters. I do understand that there may be benefit in=20 > mixed mode S =3D > parameters that have been extracted using a true mixed-mode=20 > (pure mode?) =3D > VNA, but my understanding is that most VNAs available today actually = =3D > apply single-ended stimulus and measure the standard S=20 > parameters, and =3D > then *compute* the mixed mode S parameters. That means I=20 > derive no real =3D > benefit from the mixed mode s parameters other than the=20 > convenience of =3D > not having to do any computations. I don't consider this benefit =3D > significant since the calculations are quite straightforward=20 > and do not =3D > suffer from numerical instabilities. >=20 > I may be missing some fundamental aspect about the mixed mode S =3D > parameters that would explain their popularity and if so I=20 > would love to =3D > understand that.=3D20 >=20 > Vince ------------------------------------------------------------------ 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