RE: [SI-LIST] : Proposal: Rs correlation/collaboration for W-Elem

Mellitz, Richard (
Tue, 3 Aug 1999 09:57:09 -0700


Measurements would be great! Any volunteers? Sounds like a good paper.

I'm not really disputing the accuracy of the W element model. I'm just
trying to figure out how to use them. Especially where I have thin lines and
in frequency ranges where dielectric losses may not be predominate.

I agree that the field solver is not the way to solve the accuracy issues.
However it is the starting point for the W element algorithm. So I really
want too very clearly nail down what I do with the field solver results. I'm
just starting with baby steps.

... Richard Mellitz,

-----Original Message-----
From: Dr. Edward P. Sayre []
Sent: Tuesday, August 03, 1999 10:15 AM
Subject: Re: [SI-LIST] : Proposal: Rs
correlation/collaboration for W-Elements

Dear colleagues:

If we are going to check the accuracy of the .W model, I
wish to make the
suggestion that we use measurements as well for
verification. Most field
solvers do not account for losses in copper or dielectric to
as correct an
extent that I would deem acceptable. Does anybody have a set
of test board
that they would volunteer?

Dr. Dmitri Kuznetsov's comments in a recent si-list
communication see
(7/29/99) are very relevant. The behavior of the skin
effect and
dielectric formulas in the frequency domain, when
transformed back into the
time domain involve functions that have very special

The losses which are proportional to frequency, namely the
losses can be shown to be related to the unit capacitance
and the loss
tangent of the material, Equation (8) for small losses.
See "OC-48/2.5 Gbps Interconnect Design Rules", Sayre, Chen
and Baxter;
DesignCon99 Proceedings. (Available on NESA's web site).
Use of equation
(8) together with the .W model has been found to be very
satisfactory when
the simulation results are compared to measurements.

Lastly, just as recently mentioned by Scott McMorrow, we too
have also seen
small differences with respect to the same problem solved by
two recent
versions of HSPICE. I do not think the way to resolve this
is through the
use of field solver predictions.


ed sayre


At 07:51 AM 8/2/99 -0700, you wrote:
>Apparently the W element model uses a pseudo-propagation
function with the
>following form.
>P(f)= exp{-sqrt[
(G0+f*Gd+j*2*pi*f*C)*(R0+sqrt(f)(1+j)Rs+j*2*pi*f*L) ]*len }
>(From HSPICE application note "Boosting Accuracy of W
>for Transmission Lines with Nonzero Rs or Gd Values")
>Let's assume that this is valid for some conditions. It
would be nice to
>know what the assumptions are.(geometry, frequency, etc.)
We can talk about
>the validity of the above in another thread.
>I would like to make a proposal. I would like to know what
various field
>solvers report in regards to the above propagation
function. Let's start
>with a microstrip first (and only look at skin effect). The
> Height over ground: 0.004"
> Width of conductor: 0.006"
> Thickness of conductor: 0.001"
> Conductivity: 0.58E8 mho/meter
>Let's all use the same units for Rs. Say:
> Ohms/(sqrt(Hz)*meter)
>Now, A colleague of mine has supplied a formula that is
used in microwave
>design. I have attached a PDF file with details. (Too tough
for text, TTFT
>:-)), I remember foobar)
>The answer, using the closed form formula for Rs is:
> 1.806E-03 ohms/(sqrt(Hz)*meter)
>If this is the magnitude of complex Rs, then Re(Rs) would
> 1.277E-03 ohms/(sqrt(Hz)*meter)
>I have received sidebar results from some of you folks, but
I don't want to
>post other people answers. However I will compile a table
of posted
>results. There are issues of complex number involved.
Remember I'm looking
>for the Rs for the above propagation formula.
>Step 2 will be to do same for a strip line geometry where:
> Height over ground: 0.005"
> Width of conductor: 0.0025"
> Thickness of conductor: 0.0005"
> Distance between ground planes: 0.0105
>It would be appreciated if we could find out what "tricks"
people are using
>to get Rs from their field solvers.
>Richard Mellitz
> <<Mathcad - ms_loss_eq.pdf>>
>Attachment Converted: "C:\TEMP\attachments\Mathcad -

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