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

Doug Yanagawa (doug@stratumone.com)
Wed, 28 Jul 1999 10:17:00 -0700

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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