FreeLists / si-list / [SI-LIST] Re: How to simulate length mismatch of PCB traces

[steve weir <weirsi@xxxxxxxxxx>]

Yongjin,  for the situation that you describe, W will work for the 
common parallel runs.  Beyond that you can cascade on any:  LC, LRC, 
U (RCRD), or 4O RLCG sections depending on just how accurate you 
would like to be.  If the assumption is that the coupling on that 
final segment is weak, then the simplest answer would be a single LRC 
stage or the 4O model.  You can derive the L and C values from the 
telegrapher's equations.  R is going to be quite small.  Your segment 
is quite small at 1mm even compared to 100ps rise time.

When you are done, what you are going to see is a simple delay of 
1mm*eR^-.5/C.  For an eR of 4.0, that translates to 1mm/150E9mm/s or 
6.7ps.  L is going to be about 325pH, and C is going to be about 0.13pF.

If you go with a cascade of RLCGs or 4O elements, you can estimate 
the line to line coupling with a field solver.  Likely you will be in 
the 5-10% range.

Regards,


Steve.


At 07:49 AM 3/31/2006, yc2158 wrote:
>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
> > > > >
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