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

Mellitz, Richard (richard.mellitz@intel.com)
Wed, 28 Jul 1999 07:47:19 -0700

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
>

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