The formula that I know of is: SDD21 =3D 1/2 (S21 - S41 - S23 + S43) For the reciprocal, symmetric system, S21 =3D S43, and S41 =3D S23. My algebra says that, for this system, SDD21 =3D S21 - S41. I didn't check your Maxim reference; please double-check it and let me know if it still disagrees with the formula. =20 Also, a quick Google search of "Pozar book" would've found the information about that book - my thanks to Jeff of Agilent for providing that info. Not to beat you up, but please invest a few ergs before responding (especially to the entire list) - I invest a bit of time in my responses (to keep from asking the obvious, mis-stating information, and/or embarrassing my employer), and I appreciate others doing the same. Aside from that, I welcome folks letting me know if I've goofed and/or asking for clarification. Jeff Loyer -----Original Message----- From: Paul Levin [mailto:levinpa@xxxxxxxxxxxxx]=20 Sent: Thursday, September 23, 2004 1:03 PM To: Loyer, Jeff Cc: vince_cavanna@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: why do I need mixed mode S parameters? Dear Jeff, Even after correcting S41 to S43, your formula, highlighted below, for SDD11 isn't quite correct. If you look at Maxim Application Note HFAN-5.1.0 and follow their equations, I think you will find that S11 and S22 also figure into the mix. Also, what book by Pozar are you referring to? Thanks. Regards, Paul ____________________ Loyer, Jeff wrote: >My take, some of which was articulated in others' responses. > >I have been forced to become familiar with mixed-mode S-parameters >because they indicate what I need to know about the behavior of a >differential bus, which is what many high-speed busses are now. The >single-ended S-parameters for a PCI Express Transmission Line (a coupled >differential pair), for instance, won't give you much indication of the >losses vs. frequency for that T-line. In fact, they may give you a very >erroneous representation. You need mixed-mode S-parameters to make >sense of your return and insertion losses for differential busses. = =3D20 > >For example, for the following circuit: >p1 ------- p2 >p3 ------- p4 > >p1 & p3 represent the 2 input halves of a differential pair (coupled), >routed as microstrip, p2 & p4 the output. >Take single-ended measurements of 1/2 of this coupled, microstrip >differential pair (S11 & S21, with ports 3 & 4 terminated to 50ohms). >If the trace is long enough and your VNA goes to a high enough >frequency, you'll find dramatic resonances (S21 drops significantly at >certain frequencies). Much of your energy appears to be "lost" at key >frequencies. What you've inadvertently created is a "coupled line >coupler", described in detail in Pozar's book. If you're unfamiliar >with mixed-mode parameters, you might conclude that a terrible thing is >happening at those resonant frequencies (and you will be in excellent >company, in my opinion). However, if you then measure S41, and from >that measurement calculate SDD21 (for reciprocal, symmetric systems, mag > =20 > > <>SDD21 is mag(S21 - S41)), you'll find that, for the differential case, > there is no resonance. P.S. - nothing too exotic here, this can be > <>duplicated in Hspice. Single-ended S-parameters gave an erroneous > indication of a resonance that doesn't occur when the system is excited > differentially; mixed-mode S-parameters were needed to judge the actual > quality of the system. > > Similar things might occur if you only measure single-ended S-parameters > for a differential pair going over a slot in a reference plane - > single-ended S-paramaters show horrible return loss (reflection), while > mixed-mode S-parameters indicate little reflection. > > As another example, take the circuit below: > > SE_p1 ------ SE_p2 > Diff_p1 Diff_p2 > SE_p3 ------ SE_p4 > > > SE_p5 ------ SE_p6 > Diff_p3 Diff_p4 > SE_p7 ------ SE_p8 > > Here, SE_p1 and SE_p3 represent the 2 input halves of one differential > pair (Diff_p1), while SE_p5 and SE_p7 represent the inputs of another > differential pair. If these are signals going through a connector, > you're probably most interested in SDD11 (differential return loss), > SDD21 (differential insertion loss), SDD31 (differential NEXT), and > SDD41 (differential FEXT). S21, S63, S33, etc. won't tell you much > about the behavior of the differential signals going through the > connector. Also, you may not be interested in things like SCD31 > (near-end crosstalk that ends up as common mode). > > Again, mixed mode S-params are necessary to give you info. about what > you care about - single-ended S-params won't. > > To my knowledge, no VNA equipment exists to measure mixed-mode S-params > directly (for the GHz frequencies I care about). You must measure the > single-ended S-params, and mathematically derive the mixed mode S-params > from those. > > A very good explanation of mixed-mode S-params is the "RF Balanced > Device Characterization" webcast by Greg Amorese and David Ballo of > Agilent. > > I think many future models will have to be 12-port representations (2 > differential aggressors, 1 differential victim), possibly as 12-port > S-params (single-ended). Results from simulations that use those models > may then be converted to mixed-mode S-params to understand those > results. > > Hope this helps without muddying the waters... > > Jeff Loyer > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > On Behalf Of vince_cavanna@xxxxxxxxxxx > Sent: Wednesday, September 15, 2004 1:59 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 > =3D3D > have been struggling to understand as I re-enter the field of signal =3D3D > integrity and attempt to catch-up on some of the new =3D3D > measurement/analysis techniques. I would appreciate any insight you can > =3D3D > offer. > > I understand mixed mode S parameters and can compute them from standard > =3D3D > (single-ended) S parameters or from a model - or the other way around. > =3D3D > I can appreciate their usefulness in understanding how an n-port, that =3D > =3D3D > may have been designed to operate mainly under differential stimulus, =3D > =3D3D > responds to (reflects and scatters the incident power) differential and > =3D3D > common-mode stimulus. > > What I am trying to understand is why I would ever want to use mixed =3D3D > mode S parameters in a time-domain or frequency domain simulation, and =3D > =3D3D > how to use them. I am also interested to learn what simulators support =3D > =3D3D > mixed mode S parameters directly, as using them in a simulator such as =3D > =3D3D > Hspice seems cumbersome. My approach today is to simply use standard S =3D > =3D3D > parameters directly. > > The "why" I really don't understand at all. With regards to the "how", I > =3D3D > know of one approach but it is cumbersome and does not seem worthwhile. > =3D3D > I would be interested to know if there are circuit simulators that = =3D3D > handle mixed mode S parameters directly but most important I need to =3D3D > understand why I need them. > > One way to use mixed mode S parameters, that has been suggested on this > =3D3D > mailing list, is to use the S element in Hspice, but represented with =3D > =3D3D > the mixed mode S parameters instead of the standard mode S parameters, =3D > =3D3D > and recognizing that the ports are conceptual (differential and common =3D > =3D3D > mode) as explained in [ref1]. In order to interface the conceptual = =3D3D > n-port to my circuit (which expects real ports) I then have to wrap the > =3D3D > device with a circuit that converts the actual port waves of my circuit > =3D3D > into the differential and common mode waves that need to be applied to =3D > =3D3D > the conceptual n-port. This approach should work but seems cumbersome =3D > =3D3D > and, more important to me, I don't understand what I gain from it.=3D3D20 > > The approach I described seems like a round-about way to attempt to use > =3D3D > the mixed mode S parameters directly when they can easily be converted, > =3D3D > with no loss of information, into standard mode S parameters and used =3D > =3D3D > directly with the S element of Hspice. Even better I would prefer to get > =3D3D > standard S parameters for my components so I don't need to do any = =3D3D > conversions at all. In my simulations I prefer to see the physical ports > =3D3D > rather than the conceptual differential port and common mode port = =3D3D > described in [ref1], and so the most appropriate model for me seems to =3D > =3D3D > be the standard s parameters. I can easily compute the various =3D3D > differential or common quantities from the circuit if that is what = =3D3D > interests me. > > I also don't understand why I would need mixed mode S parameters of a =3D > =3D3D > device from a vendor when I can compute them from the single-ended S =3D3D > parameters. I do understand that there may be benefit in mixed mode S =3D > =3D3D > parameters that have been extracted using a true mixed-mode (pure mode?) > =3D3D > VNA, but my understanding is that most VNAs available today actually =3D3D > apply single-ended stimulus and measure the standard S parameters, and =3D > =3D3D > then *compute* the mixed mode S parameters. That means I derive no real > =3D3D > benefit from the mixed mode s parameters other than the convenience of =3D > =3D3D > not having to do any computations. I don't consider this benefit =3D3D > significant since the calculations are quite straightforward and do not > =3D3D > suffer from numerical instabilities. > > I may be missing some fundamental aspect about the mixed mode S =3D3D > parameters that would explain their popularity and if so I would love to > =3D3D > understand that.=3D3D20 > > Vince > > [ref1] > Combined Differential and Common-Mode Scattering Parameters: Theory and > =3D3D > Simulation > David Bockelman and William Eisenstadt > IEEE Transactions on Microwave Theory and Techniques, vol 43, no. 7, =3D3D > july 1995 > =3D3D20 > ------------------------------------------------------------------ > 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: =3D20 > //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 > =3D20 > > ------------------------------------------------------------------ > 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 > > > > --=20 Paul A. Levin Senior Principal Engineer Xyratex, Manhattan Beach (310) 372-7352 - home & office (310) 291-8199 - cell ------------------------------------------------------------------ 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