Re: [SI-LIST] : Accuracy of Hspice's W element

Scott McMorrow (scott@vasthorizons.com)
Wed, 28 Jul 1999 10:48:13 -0700

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Amen

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

Doug Yanagawa wrote:

> Hspice 99.2 documentation has an app note titled > "Boosting Accuracy of W Element for Transmission Lines with Nonzero Rs > or Gd Values" > It should be required reading for anyone using Hspice W-elements. > > -- > Doug Yanagawa > doug@stratumone.com > StratumOne Communications, Inc. > 3900 Freedom Circle, Suite 102, Santa Clara, CA 95054 > (408)496-3037 (408)988-2571 fax > > "Mellitz, Richard" wrote: > > > > Dmitri, > > > > Thank you for posting the explanation. > > > > All, > > > > The main accuracy issue we've seen with W - elements is not the W element > > model, but the value we calculate for Rs. For thin lines, at high > > frequencies, the value for Rs can have a dominant damping effect that causes > > circuit to work or not work in simulations. I don't seem to be able to get > > field solvers and closed form equations to all agree. I do have my favorite > > though. I don't know if this is the forum to talk about individual products > > though. So I'll refrain. > > > > ... Rich > > > > -----Original Message----- > > From: Dmitri Kuznetsov [mailto:vdm@iname.com] > > Sent: Tuesday, July 27, 1999 11:37 PM > > To: si-list@silab.eng.sun.com > > Subject: Re: [SI-LIST] : Accuracy of Hspice's W > > element > > > > Dear SIers, > > > > I am the developer of the transmission-line simulation > > technology used > > by several popular simulators including Hspice's W element. > > Recently, a > > number of postings in this reflector pointed out a > > discrepancy between > > the ac and transient responses of W elements with > > frequency-dependent > > loss. Although I am no longer with Avant!, I would like to > > respond in > > defense of my algorithm. > > > > The answer may surprise you... It is supposed to be this > > way. And it > > does not indicate accuracy problems with either ac or > > transient model. > > This is a quite interesting phenomenon, and I would like to > > explain it > > here. > > > > It is caused by non-physical nature of the Sqrt(f)*Rs > > skin-effect and > > f*Gd dielectric-loss equations. They are lacking imaginary > > parts, or > > the corresponding frequency dependence of L and C. Real and > > imaginary > > parts of any analytic complex function cannot be arbitrary > > but are > > uniquely related by Riemann-Cauchy equations. This is not > > obvious, as > > one may envision specifying arbitrary unrelated functions > > for the real > > and imaginary parts (and have done so in this case). But > > there are laws > > for everything. > > > > As a result, transient responses of transmission lines with > > Sqrt(f)*Rs > > and f*Gd loss are non-causal, i.e. the response starts > > before the > > excitation is applied. If you take FFT of the W-element's > > ac waveforms, > > you can observe signal traveling faster than the speed of > > light, but it > > is a mathematically accurate frequency-domain solution. > > > > To assure correctness and accuracy of the transient > > solution, I change > > the frequency response as to restore the correct > > relationship between > > the real and imaginary parts. This is why the frequency > > responses of ac > > and transient models are different. > > > > The corrective change depends on line length. This creates > > another side > > effect, a slight difference between transient responses of > > segmented and > > unsegmented lines. The difference is small, as the > > correction affects > > primarily higher frequencies at which both transmission-line > > responses > > and excitation spectrum are significantly attenuated. > > > > The transient model is just as accurate for non-zero Rs and > > Gd as it is > > for constant loss, but with respect to the corrected > > frequency > > response. In fact, I use the same frequency-dependent > > algorithm for > > both cases. The accuracy is not improved by segmenting the > > line or > > changing .option RISETIME from it's actual value. It is > > important to > > set this option, especially for longer lines with low loss. > > > > Another popular skin-effect equation, Sqrt(j*2*f)*Rs, has > > correct > > real/imaginary part relationship and does not require > > correction. > > However, its inductive component is > > L(f)=Lo+Rs/(2*Pi*Sqrt(f)), which > > produces infinite inductance at dc, and causes other > > interesting > > phenomena for large Rs. > > > > I do have the solution that eliminates above problems. But > > I believe > > that present Hspice's implementation of my algorithm is > > still good as it > > provides a robust way to achieve simulation results that are > > very close > > to measurements with minimum number of model parameters. > > > > This was verified by many users, I would recommend > > downloading IMAPS > > presentation by Jim Foppiano. It was discussed recently in > > this > > reflector and contains comprehensive comparison of > > time-domain > > measurements and W-element simulations with non-zero Rs and > > Gd. > > > > I hope you found these comments useful. I have been working > > hard for 6 > > years developing my simulation technology, and can say with > > confidence > > that it is by far the most accurate and general > > transmission-line > > simulation method. > > > > Regards, > > Dr. Dmitri Kuznetsov > > > > Scott McMorrow wrote: > > > > > > One might be careful about comparing simulator accuracy to > > > Hspice, especially the w-element transmission line model. > > > It seems there are some issues with simulating frequency > > > dependent loss effect. With the latest release 99.2 they > > have > > > published an app note titled: > > > > > > "Boosting Accuracy of W Element for Transmission Lines > > > with Nonzero Rs or Gd Values" > > > > > > It is a very interesting paper. > > > > > > Scott McMorrow > > > SiQual > > > > > > > > Mike Degerstrom wrote: > > > > > > John, > > > > > > Are you refering to the W-element model instead of the > > T-element model? > > > If so then, WRT the W-element inaccuracies: did you try to > > adjust the > > > .options RISETIME parameter? We've looked at the > > w-element > > > model in some detail and we totally agree that the AC > > models > > > give a very different response than that predicted with > > the > > > transient w-element model. By adjusting the RISETIME > > parameter > > > you can get the transient loss correct at one or two > > frequencies. > > > In general, I think the w-element model can be used with > > > good results for modeling of wide-band digital signals. > > The > > > trick is knowing whether you are getting good results or > > not! > > > > > > However, this is getting way off the subject of the > > original > > > posting. I suspect the original post was not concerned > > with losses. > > > If that is the case then the w-element does a great job > > with > > > multiple coupled conductors. We often run at least 3 > > coupled > > > conductors together and run many randomly switching bits > > in > > > the simulation. Then we view the resulting eye diagram to > > > see the effects of the crosstalk, i.e., coupling modes. > > It took > > > us some time to develop these capabilities. You can, of > > course, > > > use the approach suggested by Dima which is to use convert > > > the coupled lines to decoupled lines. In fact, it is > > probably > > > best to start with Dima's approach so you better > > understand > > > the theory instead of just trusting that some fully > > coupled > > > EM result is correctly translated to a fully coupled > > transmission > > > line model. > > > > > > Mike > > > > > > On Jul 16, 8:57am, John Williamson wrote: > > > > Subject: RE: [SI-LIST] : even-odd mode influence > > > > Folks, > > > > > > > > A comment on HSPICE lossy coupled T-line accuracy. Be > > careful when using the > > > > W-element. > > > > We've found that the time and frequency domains do not > > agree. For example, a > > > > set of extracted > > > > RLGC parameters resulted in a frequency domain > > attenuation of -25dB at > > > > 10GHz. However when > > > > the same model was used in the time domain driven by a > > 10GHz sine wave, the > > > > attenuation was > > > > only -8dB. We've found that this can be corrected by > > dividing the model up > > > > into several shorter > > > > ones. It's not computationally efficient, but it works > > fairly well. > > > > > > > > Regards, > > > > > > > > jw > > > > > > > > John M. Williamson, 0V30, Electromagnetic & interconnect > > technology > > > > Nortel Networks > > > > (613) 763 3198, ESN 393-3198 E-mail: > > williamj@nortel.ca > > > > > > > > > > > > Jim Foppiano wrote: > > > > > > Scott, > > > > > > I presented a workshop paper on simulating Fibre Channel > > Loss at the > > > IMAPS High Speed seminar in January. The simulations were > > done using the > > > > > > W element model in HSPICE. I correlated modeled and > > measured > > > results. I would be happy to send you a copy if you wish. > > > It is a 10MB Power Point presentation that I have divided > > > up into 5 files. > > > Regards, > > > > > > Jim > > < > > > Jim Foppiano > > > Senior Electrical Engineer II > > > Microelectronics Technology Center > > > Digital Signal Processing Systems > > > <jim-foppiano@raytheon.com> > > > P.O. Box 660246 M/S 261 > > > Dallas > > > Texas > > > 75266 > > > Pager: 972-597-2053 > > > Fax: 972-344-3287 > > > Work: 972-344-288 > > > > Tarek Ali - WGS Board Design Technology wrote: > > > > > > Hi, > > > > > > If anyone needs to download the imaps presentation > > please access > > > my freedrive account at http://www.freedrive.com. > > > > > > login: tarek > > > password: tarek > > > > > > And you can download the ppt file from the imaps > > folder. > > > Most email systems will not permit files above 5 > > Meg, and this > > > file is 10 Meg. > > > > > > Thanks, > > > > > > Tarek Ali > > > > > > > **** To unsubscribe from si-list: send e-mail to > > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE > > si-list, for more help, put HELP. si-list archives are accessible at > > http://www.qsl.net/wb6tpu/si-list **** > > > > **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. 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--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

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