[SI-LIST] Re: Buried Microstrips
- From: "Alexander Ippich" <Alexander.Ippich@xxxxxxxxxxxxx>
- To: "Ty Beeman" <ty.beeman@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 16 Jul 2009 16:58:56 +0200
Hi Ty, Logan,
as Steve already pointed out, there is a reason, why people use 2D simulation
programs to predict impedance as "accurately" as possible instead of closed
loop equations.
The equations are typically ok for a very limited range of the independent
variables (like dielectric thickness, Dk,...). So if you try to use them, make
sure to check for what ranges they are valid.
In general, I would not use the closed loop equations for anything else as a
first indication or to find out how sensitive impedance is to a given change in
an independent variable.
Instead, we use a 2D field solver to extract impedance out of the geometrical
sizes and the Dk and Df values for the dielectrics.
However, since these field solvers can be expensive, let me point you to two
free solvers, that are quite ok regarding accuracy.
The first (http://mdtlc.sf.net) is pretty simple. Can handle trapezoid shaped
lines two dielectrics with different Dk. However, dissipation factor is not
used.
The second one (http://mmtl.sourceforge.net/) is really nice. You can have
basically arbitrary stackups with many dielectrics. Also gives much more
parameters, like C, L and R-matrix. Unfortunately, it disregards loss tangent
values (though you can type them in) and it's also not looking at frequency.
It's also not calculating total loss.But beside that, it looks really good.
Best luck,
alex
__________________________________________________________________
Alexander Ippich
Sr. Signal Integrity Engineer
Multek Germany
Multilayer Technology GmbH & Co. KG
Herrenberger Strasse 110
71034 Boeblingen - Germany
Phone +49 7031 211 3375
Fax +49 7031 211 5309
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Email alexander.ippich@xxxxxxxxxxxxx
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Ty Beeman
Sent: Thursday, July 16, 2009 4:26 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Buried Microstrips
Hello,
As part of a class project, we have been given the assignment of using closed
loop equations to "accurately" predict the characteristic impedance, etc. of
microstrips.
To solve for surface microstrips, we have used the equations from: E.
Hammerstad and O. Jensen, "Accurate models for microstrip computer-aided
design." These equations give us results that are very close to field solvers.
However we're kind of stuck in getting accurate results for a buried
microstrip. We are using the equations found in: Brian C. Wadell, "Transmission
Line Design Handbook." When we use the surface microstrip equations above to
solve for Z-buried we can get close, end up with some errors as much as 4% and
off by as much as 3 ohms or more.. If we use the Wadell equation for surface
microstrip to solve for Z-buried, our error is around 5% when comparing to
field solvers.
Looking at it a little closer, we find that our calculated effective er is
different than that given by the field solvers.
All we can seem to find in the reference materials is the same (or similar)
calculation that is in Wadell's book or old IPC equations.
So our questions are:
1. Are there any other equations for buried microstrips that others have found
to be more accurate, especially for effective er and impedance?
2. Can anyone recommend any other reference material on buried microstrips?
Are there any good IEEE papers on the subject?
We know we may have to settle for what we have now, given the reference
material, but we're hoping there's something a little more accurate available.
Thank you,
Ty Beeman
Logan Talada
Students
Tompkins-Cortland Community College
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