Re:[SI-LIST] : questions about Spicelink/Ansoft tools

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From: Nirmal Jain (
Date: Mon Jan 10 2000 - 11:32:40 PST

Dear Teddy,

When you use "Return Path" in Maxwell Quick 3D Extractor, it is actually
computing loop inductance assuming the return current is flowing through
assigned return path conductor. So if I have a signal conductor over a
gnd plane, the original matrix would look like

                      L11 L12
                      L21 L22

where L11 represents the partial inductance of the signal conductor, L22
is the partial inductance of the gnd plane and L12 (or L21) represents
the mutual
inductance between the signal conductor and the gnd plane.

When you use "Return Path" operation, the software is computing the loop
inductance using the expression

Lloop = L11 + L22 - 2 * L12

The Loop inductance would be higher than the partial inductance of the
signal conductor if

1) Ground plane is far off, hence my L12 is very small
2) or if my mutual inductance number is negative in the original matrix,
which means when I do my return path operation, my current in the gnd
plane is not
flowing in the opposite direction to the current in the signal
conductor. In this case the L12 term would added instead of being
subtracted ( negative of
negative is +ve). So if your mutual number is -ve, you could either
re-solve the model by swapping sink/source terminal or swapping the
terminals in the reduce matrix operation. I prefer the reduce matrix
step as it is much faster. After this operation, the mutual between the
gnd plane and
the signal should be +ve, and it would actually get subtracted in the
return path operation instead of being added and you would see a much
lower number!

3) the solution may not be converged

Hope it helps. Let me know if you have any further questions. If you
still have problems feel free to send me the model.

Best regards


> > -----Original Message-----
> > From: Teddy Chou
> > Sent: Friday, January 07, 2000 11:00 AM
> > To: ''
> > Subject: HFSS: questions about Spicelink/Ansoft tools
> >
> > Hi,
> > Recently, I have started to use the Spicelink / Ansoft
> > simulation tools( it resembles the HFSS/Ansoft, but only the
> > quasi-TEM waves guided by transmission lines are
> > considered ). I found some problems when using the
> > "REDUCE MATRIX" item in Maxwell 3D Extractor. I needed to
> > get some characteristics of microstrip lines. However, when I
> > assigned the ground plane to the "return path" or "ground"
> > ( part of the return path ), the inductance would increase as
> > the size of ground plane increases. Even when the ratio of
> > the width of ground plane and the width of a trace is over 100,
> > it never got a stable value. For avoiding this phenomenon, I
> > tried to decrease the area of the I_sink and I_source surface
> > in the ground plane, it still got the resembling results. Who can
> > tell me how to assign the reasonable size of the ground plane
> > for microstrip lines.
> >
> >
> > Best Wishes,
> >
> > Teddy Chou
> > Signal Integrity Engineer,
> > VIA Technologies, Inc. Taipei, Taiwan, ROC
> > TEL : 886-2-22185452 ext : 6046
> >
> > Very Innovative Architecture
> >
> >

Nirmal Jain
Senior Application Engineer
Signal Integrity Products

Ansoft Corp.
2560 Oak Rd. #322
Walnut Creek, CA 94596
(925) 938-6490 (Tel)
(925) 215-2477 (Fax)

Pittsburgh Office:

Four Station Square
Pittsburgh, PA 15219
(412) 261-3200 (Tel)
(412) 471-9427 (Fax)

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