COVER FEATURE
A REVOLUTION
IN THE TIME
DOMAIN
C
ST MICROWAVE STUDIO® Version 4
(CST MWS) is a major new release of
the company’s flagship 3D EM microwave modeling and design package. Over
1400 changes, enhancements and new features
have been added to make this already powerful
tool truly comprehensive and an invaluable resource. The microwave design engineer seeking rapid evaluations and optimizations of his
virtual prototypes can now find a complete solution in a software tool that, despite its new
power, has remained as user friendly and fun to
use as ever.
CST MICROWAVE STUDIO incorporates
not only the highly versatile Time Domain
Solver, but also an Eigenmode and a Frequency Domain Solver which serve to further augment its wide applicability. CST MWS is fully
integrated into the open CST design environment (CST DESIGN STUDIO™) and can be
tightly integrated with any other compliant
software through the COM interface. At MTTS 2002, Agilent and CST premiered the new
direct link between ADS and CST MWS.
This article reveals the key new features of
CST MWS Version 4 and specific application
areas addressed by the new features will be
shown. Regular users will come across many
other improvements as they start using the
software. The photo above shows a screen shot
of the single window interface of Version 4.
MESHING
CST MICROWAVE STUDIO is based on a
rectangular meshing scheme, which has been
shown in many studies to be the most efficient
time domain method available. In the scheme
field values are distributed around the edges
and face centers of mesh cells, but the software has always had the ability to arbitrarily
slice a cell into two regions — one dielectric
field region and one metal region through the
PBA® method. This has allowed for very good
geometrical representation of curved and inclined objects without a fine mesh or ‘staircasing.’
A first for time domain software, version 4
now has the ability to arbitrarily slice a single
mesh cell into three regions: two decoupled
dielectric field regions and a thin metal region
CST OF AMERICA® INC.
Wellesley Hills, MA
COVER FEATURE
DIELECTRIC
METAL
(a)
SINGLE MESH CELLS
DIELECTRIC
METAL
DIELECTRIC
(b)
▲ Fig. 1
Extension to the PBA method; (a) Version 3 original PBA
and (b) Version 4 extended PBA.
▲ Fig. 2
PBA split cells mesh thin curved regions exactly.
AUTODESK
ACIS
• INVENTOR
• MECHANICAL DESKTOP
STEP
IGES
STL
SAT
CST MWS
DASSAULT SYSTEME
ACIS, PARASOLID, CATIA
• SOLIDWORKS
• CATIA
• (ACIS)
▲ Fig. 3
CAD import routes.
SAT
STEP
IGES
STL
CATIA
STEP
IGES
STL
Pro/E
cutting through the cell. Figure 1 shows the extension to
the PBA method introduced in Version 4.
The result of this extension is that thin curved or inclined metal does not have to be meshed finely in order to
give accurate results. Because the time step in time domain solutions is related to the smallest mesh cell size, this
also keeps the time step high and reduces run time considerably. Application areas that will benefit from this extension are conformal antennas and signal integrity with thin
inclined IC parts. Figure 2 shows a thin metal cylindrical
shape modeled with no critical cells. Clearly the geometry
is matched exactly. Version 3 would work, but would fill up
sliced cell areas with “critical cells” or whole cells of metal.
Version 4 meshes exactly with no critical cells.
Other meshing improvements include changes to memory management so that an already efficient scheme uses
even less memory. It is now possible to run up to five million mesh cell problems on an off-the-shelf business PC.
CST MICROWAVE STUDIO’s expert meshing system,
where the software analyses a structure and its known electromagnetic properties to give a good mesh from the outset, has been improved further with the use of built-in project templates. For every new project, the user is offered a
selection of project types. These include IC packages, antennas with a ground plane and coaxial connectors. For
each template the software applies appropriate units,
boundary conditions, mesh properties and other settings.
All of this means that the user can confidently start building
a geometry straight off without having to consider the preliminary set-up and with little or no need for adaptive
meshing. Helping even further in this regard is the Version
4 extension to metal edge refinement. Due to field singularities at perfect metal edges it is necessary to refine the
mesh in those areas to correctly represent the fields. The
expert mesher has always done this for straight edges orthogonal to the mesh, but Version 4 extends this to include
all mesh edges again increasing the accuracy of the initial
solution.
CAD IMPORT/EXPORT
The range of CAD exchange formats has been substantially increased in Version 4. The universal STEP format is
added to the existing IGES and STL capabilities. The industry’s two main kerEDS-PLM
nels, ACIS and Parasolid, are addressed
PARASOLID, IDEAS
— directly in the case of ACIS, as this is
• IDEAS (EX SDRC)
CST MICROWAVE STUDIO’s native
• SOLIDEDGE
• UNIGRAPHICS
format, and through one of the universal
• (PARASOLID)
interchange formats in the case of Parasolid. CATIA® and Pro/E® files can now
be imported directly. Furthermore,
ACIS
STEP and 2D DXF have been added to
the list of CAD formats that can be exported. Figure 3 shows the main
commercial CAD vendors and their
PTC PARAMETRIC TECH.
products and how CST MWS can deal
with them. The grey boxes show the ven• Pro/E
dor in black, the kernel they use in red
• CADDS
• ICEM
and their main products. The yellow
• (Granite One)
boxes show the interchange formats that
can be used to transfer data to CST
MWS.
COVER FEATURE
∇ × H = jωD + J
= jωεE + σE
Jtotal = jωε'E + (ωε" + σ)E
Jtotal
jωε'E
δ
σeffE
RESPONSIBLE FOR
DISPLACEMENT
CONDUCTIVITY LOSS
CURRENT DENSITY
RESPONSIBLE FOR DIELECTRIC LOSS
STORED
▲ Fig. 5
tanδ =
σeff
ωε'
LOSS-HEAT
Material equations.
required frequen▲ Fig. 4 Electric field slice for an STL imported car with sheet
cies in the pulse.
meshing.
This can give rise to
CST MICROWAVE STUDIO Verdifficulties in setting correct material
sion 4 now has a direct link to Agiproperties for every frequency. Genlent’s ADS circuit simulator. CST
erally the user can specify conductiviMWS can import an ADS model or
ty or a tangent δ at a particular frealternatively ADS can export a model
quency creating a fixed “effective
directly to CST MWS for field simulaconductivity” across the band. Figure
tion.
5 shows the equations involved where
To address the significant problem
the effective conductivity is ωε˝ + σ.
of differing requirements for meIn order to maintain the fixed efchanical and electromagnetic CAD,
fective conductivity the imaginary
healing capabilities have also been
part of the permittivity (ε˝) must
improved in CST MWS Version 4.
change across the band, giving rise to
Automatic healing with individual
a non-physical dispersion effect.
part reporting is set by default on imMoreover, the tangent delta also
port of external geometry.
changes across the band.
Version 4 addresses this area with
THIN SHEET MESHING
more material options. It is now posFor large thin metallic bodies with
sible to define a constant tangent
many details such as a car, direct
delta across the band or a “dispersion
CAD import and subsequent meshfit” tangent delta where the user can
ing may become impractical due to
specify a table of tangent delta values
the large number of unnecessary deagainst frequency. Figure 6 shows
tails and materials. The ideal solution
the fraction of a decibel accuracy now
to this is to use a sheet meshing
possible with a coplanar line above a
scheme as implemented in Version 4.
lossy substrate.
Here metal materials are represented
Version 4 allows many other materby two-dimensional (2D) internal
ial options to be specified, including
surface conditions, not full three-dimagnetic conductivity, dielectric dismensional (3D) cells. This technique
persion models such as Debye, Drude
allows very fast runs for apparently
and Lorentz, and magnetic dispersion
complex objects such as an STL immodels, including gyrotropic. Among
ported BMW car with a monopole
other things, these new material opcommunications antenna. Figure 4
tions allow plasmas and ferrites to be
shows the car plus antenna in CST
included so that for example isolators
MICROWAVE STUDIO with reand circulators can now be modeled.
sults. This was solved in 1 hour and
TRUE DE-EMBEDDING
38 minutes on a 1.9 GHz P4 and used
1.6 M mesh cells.
Inhomogeneous ports are modelled
with high accuracy in CST MWS VerMATERIALS — FREQUENCY
sion 4. An inhomogeneous port is a
DEPENDENT LOSSES
2D port driving a problem that has
AND DISPERSION MODELS
more than two materials in it — for
Time domain methods are inherexample, a microstrip line where there
ently broadband so that a shaped
is metal, air and a dielectric substrate.
time pulse excites a geometry with all
At such a port a quasi TEM mode is
Fig. 6 High accuracy with new
material options. ▼
created where, as well as the transverse field components, some longitudinal field components also exist due
to the differing permittivities of the dielectrics. The quasi TEM mode created at an inhomogeneous port will vary
slightly at different frequencies and
that difference will increase the more
inhomogeneous the port is.
The upshot of all this from a time
domain software point of view is that
the Q-TEM mode is only 100 percent
accurate at a single frequency and
this is usually set at the center of the
band of interest. For highly inhomogeneous ports where the difference
in dielectric constants is large, the accuracy of the overall 3D solution can
suffer, particularly at the extremities
of the band. Traditionally, running
several narrower band problems or
shifting the “mode calculation frequency” can rectify this problem.
CST MWS Version 4 addresses
this issue head on by using a unique
de-embedding technique that decouples the port region from the 3D solution region. A single check box is
checked for de-embedding and a
number of samples chosen. The number of samples determines the number of port calculations and directly
influences the achievable accuracy.
Figure 7 shows the CST MWS Version 4 results for a particular microstrip device. The return loss is below 60 dB across most of the band.
COVER FEATURE
S-PARAMETER
MAGNITUDE (dB)
0
−20
−40
−60
−80
−100
0
5
10
15
FREQUENCY (GHz)
20
▲ Fig. 7
Accuracy improvement using
de-embedding.
OTHER MAJOR CHANGES
A new optimizer is included in the
default configuration of CST MWS.
Based on the interpolated quasi Newton technique, it gives convergence to
the desired result with many fewer
runs than previously possible. It is also
possible to create much more complex
goal functions. Target functions (for
example, S11) can be allocated an operator (for example, minimum), a target value, a frequency range and a
weight. With multiple target functions
it is possible to build up complex goals
such as a filter response. All intermediate results from optimization runs
can now be stored in a cache for later
analysis if required. This also applies
to parametric studies.
To reduce the number of runs
necessary for complete S-matrix multiport devices, it is now possible to
define port symmetries. If S-Parameter symmetries exist, then groups of
S-Parameters can be defined so that
only one of the group is calculated.
For simultaneous port excitation, it
was previously possible to combine
solutions as a post processing step to
see the combined far field effect. It is
now possible to simultaneously excite
ports during the analysis, which may
give some further insight due to coupling effects and also allows coupled
near fields to be determined.
The Eigenmode Solver has received particular attention in CST
MWS Version 4 and now makes use
of the Frequency Domain Solver (no
additional license required) to achieve
a first solution. Whereas the old
Eigenmode Solver may have required
100 eigenmodes to get a good solution
for resonances and S-Parameter levels, the new solver can achieve equal
or better accuracy with the one frequency domain solution and four
eigenmodes. Periodic boundaries with
arbitrary phase have been introduced
into the Eigenmode Solver and also
the Frequency Domain Solver.
Post processing has improved in
CST MWS Version 4 to include better
visualization of plane waves with field
vectors automatically orthoganalized
to the propagation direction. Far field
plots compliant with electromagnetic
compatibility (EMC) regulations have
been implemented so that a reference
distance can be set along with appropriate units such as dB-µv/m. EMC
current and voltage sources can be
defined when the model is being
built. Near field values can now be
plotted based on a user-defined curve,
which includes a 3D curve in Version
4. Line integral values are shown and
a one-dimensional entry is created in
the navigation bar to show the field
variation result.
CONCLUSION
CST MICROWAVE STUDIO
Version 4 is a major upgrade to the
existing software with many new
features based on both structured
long term development and user requests. The new features are implemented with the microwave design
engineer in mind. The aim throughout has been to add throughput
benefits while retaining the unique
user-friendliness of the software.
Although there are many new features, both existing and new users
will find the software very approachable. For the casual user, less
input is required due to the improvements in the systems and
large number of new CAD imports
available. For the expert user, new
features will provide faster and
more accurate virtual prototyping.
CST of America Inc.
Wellesley Hills, MA
(781) 416-2782
www.cst-america.com