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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

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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

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2

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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