s-96 3175 mws lecture
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05.02.2010
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General purpose solver 3D-volume
Transient
large problems
broadband
arbitrary time signals
Frequency
Domain
narrow band / single frequency
small problems
periodic structures with Floquet port modes
Special solver 3D-volume: closed resonant structures
Eigenmode strongly resonant structures, narrow band
cavities
FD Resonant strongly resonant, non radiating structures
Special solver 3D-surface: large open metallic structures
Integral Equation
(based on MLFMM)
large structures
dominated by metal
CST MICROWAVE STUDIO®
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Transient Solver Introduction
• hexahedral mesh only
• time and frequency domain
results
• all frequencies in one simulation
• Begin with no energy inside calculation domain.
• Inject energy and step through time.
• As time progresses, energy inside calculation domain decays.
• When energy decays “far enough,” the simulation stops.
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Overview
• An arbitrary input signal can be used.
• Inject energy and watch it dissipate.
• Solve for unknowns without matrix inversion.
• Hexahedral mesh: broadband meshing and results with a single
solver run.
• Simulation is performed on a port-by-port basis.
• smaller mesh cells = longer simulation runtime
• Energy storage for high Q structures prolongs simulation time.
Transient Solver
The transient solver is very robust
and can handle most applications.
Well suited applications: broadband,
electrically large structures.
Highly resonant, electrically small
structures may be better suited to
the frequency domain solver.
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Frequency Domain Solver
• Simulation performed at single frequencies.
• “Broadband Frequency Sweep” to achieve accurate S-parameters.
• Very robust automatic mesh refinement (easy to learn).
2nd general purpose solver (besides time domain)
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Eigenmode Solver
The eigenmode solver is a very specialized tool for closed
cavities. No S-parameters are generated, only eigenmodes
which are single frequency results.
Well suited applications: narrow band, resonant cavities.
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Integral Equation Solver
Areas of application
S-parameter calculation
far field & RCS calculation
reflector antennas
for electrically large problems
Excitations
plane wave excitation
discrete face ports
waveguide ports
far field source excitation
current source
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General Performance
Tuning
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General Performance Tuning
To obtain the most efficient simulation times:
1. Delete all unnecessary field monitors.
2. Stimulate only the ports necessary for the results of interest.
3. Consider structure symmetries.
4. Use the combined strength of CST DESIGN STUDIO™ and the 3D CSTsolver modules.
5. If applicable, use field sources to reduce model complexity.
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Field Monitors
3D field monitors need resources and slow down the simulation.
Thus, do not define field monitors you do not really need.
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Excitation Ports
Excite only the ports of interest.
Example: Only excitation of
mode 1 for port 1 is of interest.
Only the desired
port/mode is excited.
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Symmetry Planes (I)
Electric and magnetic symmetry conditions are available.
To use symmetry conditions the model and the excitation must
meet this symmetry.
Electric
Magnetic
Computational resources (memory and
simulation time) can be reduced by a
factor of 4 for this example!
Computational Domain
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Symmetry Planes (II)
Electric Field Magnetic Field
Electric symmetry plane (i.e. tangential electric field vanishes)
Magnetic symmetry plane (i.e. tangential magnetic field vanishes)
To use symmetry conditions the model and the excitation must
meet the symmetry specifications.
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Performance Tuning for
Transient Simulations
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Transient Simulation - Behind the Scenes
Port 1 Port 2
Excitation Time Signal Output Time Signal
Numerical time integration
of 3D Maxwell equations
The simulation duration depends on:
1. Duration of input signal (determined by frequency range selected)
2. Duration of output signal (determined mainly by the size and the
resonances of the model under study)
3. Time step width for numerical time integration (determined by the
mesh used to discretize your model)
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Performance Tuning -Transient Simulations
To obtain the most efficient simulation times:
1. Decrease the duration of the excitation signal by choosing a properfrequency range.
2. Split the simulation frequency range into several portions forstructures with multiple resonances such that only one resonance isleft in each frequency band.
3. Use online AR-Filter for resonant structures (for S-parameter calc.).
4. Increase time step by increasing the size of the smallest mesh cell.
5. Use subgridding if applicable.
6. Use the combined strength of CST DESIGN STUDIO™ and CST MWS.
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Frequency Range (I) – Excitation Signal
IDFT
IDFT
DFT
DFT
frequency
frequency
time
time
Frequency Range = Resulting Excitation Signal
The duration of the excitation pulse for the T-solver is proportional to .
The broader the frequency range the shorter the excitation signal.
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Frequency Range (II) – Excitation Signal If DC is included ( ) then the duration of the excitation pulse decreases again
compared to the case .
Frequency Range =
Frequency Range =
IDFT
IDFT
DFT
DFT
Time
Time
Resulting Excitation Signal
Frequency
Frequency
If the excited mode has no cutoff frequency (TEM or Quasi-TEM),
include DC in the simulation bandwidth.
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Frequency Range (III) - Resonances
Be very careful when changing the frequency range as this may lead to the inclusion of a
resonance that was previously excluded. This will increase the amount of transient
activity (duration of output signals) and slow down the energy decay leading to a longer
simulation time.
Higher upper frequency limit increases
simulated time interval by >10 ns.Resonance is included in frequency
range causing the slow energy decay.
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Frequency Range (IV) - Resonances
A method to decrease transient activity (duration of the output signals) is to break up the
simulation bandwidth into intervals. This has the effect of only simulating one resonant
point at a time.
Frequency Range 1 Frequency Range 2
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Online AR-Filter (I)
Resonant structures capture the EM energy which
leads to a slow energy decay and thus, to a long
simulation time.
Example: Waveguide Iris Filter
"Ringing" behavior of
time signals
Inaccurate S-para-
meters (ripples) due
to truncation error
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Online AR-Filter (II) Such a resonant behavior of the time signals, can easily be predicted using
an analytical model. This is what the AR-Filter does.
analytical description
possible in this range
3D simulation
in this range.
If such an analytical description of the time signal can be found, the AR-
filter can produce accurate S-parameter results. This can shorten the
simulation time.
Signals which can be described by a weighted sum of exponentially decaying
sine functions of different frequency can be handled.
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Increase the Time Step
Thin Wires Thin Microstrip Line
Mesh line ratio limit > 30allows fine meshing.
t
tiny t: slow
Mesh line ratio limit < 5generates one mesh line.
big t: fast
wt
For stability, the time step is determined by the smallest mesh step.
Increasing the smallest mesh step will increase the time step.
t
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Smallest Mesh Step
The smallest mesh step in a model can be visualized in the mesh
view.
To increase the smallest mesh step the
following settings can be adjusted:
1. Decrease the ratio limit in the global
mesh settings or specify the minimal
size of the smallest mesh cell.
2. Switch off fixpoints of less important
model parts.
BUT: Be aware of critical cells when coarsening the mesh!
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Staircase Cells
Cells which contain more than two metallic
material boundaries are completely filled
with PEC (staircase cells).
A warning is shown by the
solver to inform you of this
modification.
Staircase cells are shown in the
mesh view.
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Online Help – PBA and TST
PBA TST
Whenever a mesh cell cuts more than two metallic material
boundaries the cell is filled with PEC material (staircase cell).
Quite often such cells do not influence the simulation result
much, but if they introduce shortcuts (as shown on the last slide)
this might be critical.
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Workflow ExampleHorn Antenna
Purpose : Optimize the
aperture of the horn
antenna such that the gain
is maximized at 10 GHz.
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CST MWS - Standard Workflow
Choose a project template.
Create your model.
parameters + geometry + materials
Define ports.
Set the frequency range.
Specify boundary and symmetry conditions.
Define monitors.
Check the mesh.
Run the simulation.
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units: inch
waveguide: 1.0 in x 0.5 in x 0.5 in
aperture radius: 1.0 in, length: 0.25 in
shell thickness: 0.01 in (outside)
monitors: E-field, H-field & far field at 10 GHz
1
0.5
0.25
zlength=2dia=2, rad=1
0.5
Cylindrical Horn Antenna 8 – 12 GHz
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Project TemplateAt the beginning choose to create a new project.
For an existing project you may choose
“File” -> “New”
“File” -> “Select Template”.
The project templates customize the default settings
for particular types of applications.
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Project Template
The project templates customize the default
settings for particular types of applications.
PEC is very practical for closed structures.
(e.g. waveguides, connectors, filters)
Antennas should be modeled with
vacuum as background material.
background material
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Change the Units
Define units.
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Horn Antenna – Construction (I)
Define a brick (1.0 x 0.5 x 0.5 in)
made of PEC.
Pick face.
Align the WCS with the
face.
Move the WCS by 2.0
inches.
Define a cylinder (outer radius: 1.0 in,
height: 0.25 in) made of PEC.
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Horn Antenna – Construction (II)
Pick two opposite faces. Perform a loft.
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Horn Antenna – Construction (III)
Perform a
Boolean add.
Pick two faces.
Select multiple objects
(ctrl or shift + left mouse button).
shell solid: 0.01 in
(outside)
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Port Definition
Pick point
inside corner.
Pick edge.
Define a waveguide port.
Define the port on the internal profile.
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Set the Frequency Range
Set the frequency range.
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Boundary Conditions and Symmetry Planes
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3D Monitors
Add field monitors for E-field, H-field, and far field at 10 GHz.
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Mesh View (I)
mesh properties
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Mesh View (II)
TST at work!
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Transient Solver: Start Simulation
The accuracy defines the steady-
state monitor.
The simulation is finished when
the electromagnetic energy in the
computational domain falls below
this level.
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Analyze 1D Results
energy
port signals
S-parameter
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Analyze 2D/3D Results
port information:
• cut-off frequency
• line impedance
• propagation constant
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Electric Field at 10 GHz
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Far Field at 10 GHz
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Polar Plot for Far Field at 10 GHz
Create a new folder “Comparison” to compare different 1D results.
phi=90 phi=0
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Parameterization
Optimization
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Parameterization (I)
outer radius r1 = variable
goal: maximize gain
r1
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Parameterization (II)
outer
radius
r1
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Result Processing Templates (Shift+P)
Define gain(theta) at phi=0.
1D results
Postprocessing templates provide a convenient way to calculate
derived quantities from simulation results.
Each template is evaluated for each solver run.
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Define max of gain(theta).
0D results
Result Processing Templates (Shift+P)
Read the online help to learn more
about the postprocessing in CST MWS.
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Define max of gain(theta).
Result Processing Templates (Shift+P)
Alternative solution:
The maximum gain can be computed using
the “Farfield” template in “0D Results”.
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Parameter Sweep - Settings
1
2
3
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The results will be automatically listed
in the “Tables” folder.
Parameter Sweep - Settings
Add a S-parameter watch.
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Parameter Sweep – Table Results
Right click on plot
window and select
“Table Properties…”.
Choose the result curve for each
parameter value with the slider.
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Parameter Sweep – Table Results
parameter values
parameter values
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Automatic Optimization
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Automatic Optimization
Define the parameter space.
Template based postprocessing 0D results can be
used to define very complex goal functions.
Define the goal function.
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Automatic Optimization
Choose the “Classic Powell” optimizer. Follow the optimization.
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goal:
maximize gain
parameter values
1D results
Automatic Optimization - Results
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