Steve, Thanks for pointing me out my wrong answer(don't work at midnight!!). You are totally right. I am considering if I have "a line with a little extra segment on the end that the other coupled lines don't" like what you wrote. To simplify the problem, I am not considering eR variation and don't have meander. My simulation purpose is to see the length mismatch effect as waveforms at each line end. Like Ihsan mentioned, a ladder circuit seems like one solution. But how can I decide the coupling cap/inductor values between a longer line and the other lines? Also how can I use U element in this situation? Thanks, Yongjin > >remember --- In si-list@xxxxxxxxxxxxxxx, steve weir <weirsi@...> wrote: > > Yongjin, just using a C on the end of the line will distort the line > impedance. You want to think in terms of the entire length of the > line before representing etch as a lumped C. > > I think the real problem here is you need to decide what it is that > you want/need to evaluate. Do you have a 1% effective eR variation > line to line, some distributed meander, or do you have a line with a > little extra segment on the end that the other coupled lines don't? > > Steve. > > At 08:51 PM 3/30/2006, yc2158 wrote: > >Thanks for the replies. > >After reviewing my problem again with Tr(rise time) around 100ps > >(Er=3D3D4.3), 1mm~2mm line should act as a lumped capacitor based on the > >formula Tr/(5*To)>length [eq.(6.32),pp239,Bakoglu]. For my case, > > 100ps/(5*69ps/cm) =3D3D 20/69cm > length=3D3D1mm~2mm. > >Thus I added only a lumped capacitor to one line like this. > > > > in1 --------------- out1 --capacitor- connected to gnd > > in2 --------------- out2 > > in3 --------------- out3 > > |<----10cm---->| |<--1mm-->| > > > >Should I also include an inductor together with a lumped capacitor? > >If the extra length I am considering were greater than 20/69cm, I > >think I should follow the Ihsan's advice. > >Thanks, > > > >Yongjin > > > >--- In si-list@xxxxxxxxxxxxxxx, "Ihsan Erdin" <erdinih@> wrote: > > > > > > The question implies that the mutual coupling between the lines be > >included > > > otherwise the solution is trivial. If the length difference is short > > > cascading a w-element is not a good solution. Indeed the w-element for > > > electrically short lines should be avoided at all times. A quick and > >dirty > > > way is to make a ladder circuit or use a U-element (if anybody can > >remember > > > that...) to account for the length difference. > > > Regards > > > > > > Ihsan > > > > > > > > > On 3/29/06, YongJin CHOI <yc2158@> wrote: > > > > > > > > Dear SI members, > > > > I am looking for a simple simulation method to get a > > > > rough idea if one or two of the four traces is a > > > > little bit longer like 1~2mm. The transmission line > > > > models I am having are W-model from Hspice field > > > > solver(2-D) and S-parameter model from HFSS. I can use > > > > Hspice or Spectre depending on the simplicity. > > > > Any comments would be appreciated. > > > > Thanks, > > > > > > > > Yongjin CHOI > > > > > > > > __________________________________________________ > > > > Do You Yahoo!? > > > > Tired of spam? Yahoo! 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