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CST MICROWAVE STUDIO® General purpose solver 3D-volume Transient Frequency Domain
large problems broadband arbitrary time signals narrow band / single frequency small problems periodic structures with Floquet port modes
Special solver 3D-volume: closed resonant structures Eigenmode FD Resonant
strongly resonant structures, narrow band cavities strongly resonant, non radiating structures
Special solver 3D-surface: large open metallic structures Integral Equation (based on MLFMM)
large structures dominated by metal
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Transient Solver Introduction
• 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.
• hexahedral mesh only • time and frequency domain results • all frequencies in one simulation
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Transient Solver 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. 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. 3
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 CST solver 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. 10
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Symmetry Planes (I) Electric and magnetic symmetry conditions are available. Magnetic
Computational Domain
Electric
Computational resources (memory and simulation time) can be reduced by a factor of 4 for this example!
To use symmetry conditions the model and the excitation must meet this symmetry. 11
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. 12
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Performance Tuning for Transient Simulations
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Transient Simulation - Behind the Scenes Output Time Signal
Excitation Time Signal
Numerical time integration of 3D Maxwell equations
Port 1
Port 2
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) 14
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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 proper frequency range.
2.
Split the simulation frequency range into several portions for structures with multiple resonances such that only one resonance is left 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 The duration of the excitation pulse for the T-solver is proportional to
.
Resulting Excitation Signal
Frequency Range = IDFT
DFT frequency
time
IDFT
DFT frequency 16
time
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 =
Resulting Excitation Signal IDFT
DFT Time
Frequency
Frequency Range = IDFT
DFT Frequency
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Time
If the excited mode has no cutoff frequency (TEM or Quasi-TEM), include DC in the simulation bandwidth.
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
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"Ringing" behavior of time signals
Inaccurate S-parameters (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. Signals which can be described by a weighted sum of exponentially decaying sine functions of different frequency can be handled.
3D simulation in this range.
analytical description possible in this range
If such an analytical description of the time signal can be found, the ARfilter can produce accurate S-parameter results. This can shorten the simulation time. 21
Increase the Time Step For stability, the time step is determined by the smallest mesh step. Increasing the smallest mesh step will increase the time step. Mesh line ratio limit > 30 allows fine meshing.
tiny t: slow
Thin Wires
t
Thin Microstrip Line
Mesh line ratio limit < 5 generates one mesh line.
big t: fast
t w t 22
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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.
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BUT: Be aware of critical cells when coarsening the mesh!
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
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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.
Workflow Example Horn 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. 27
Cylindrical Horn Antenna 8 – 12 GHz 1 0.5 0.5 dia=2, rad=1
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zlength=2 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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Project Template At the beginning choose
“File” -> “New”
For an existing project you may choose
to create a new project.
“File” -> “Select Template”.
The project templates customize the default settings for particular types of applications. 29
Project Template background material
Antennas should be modeled with vacuum as background material.
PEC is very practical for closed structures. (e.g. waveguides, connectors, filters)
The project templates customize the default settings for particular types of applications. 30
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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.
Define a cylinder (outer radius: 1.0 in, height: 0.25 in) made of PEC. 32
Pick face. Align the WCS with the face.
Move the WCS by 2.0 inches.
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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. Select multiple objects (ctrl or shift + left mouse button).
shell solid: 0.01 in (outside)
Pick two faces.
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Port Definition Pick point inside corner.
Define a waveguide port.
Pick edge.
Define the port on the internal profile. 35
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 steadystate monitor. The simulation is finished when the electromagnetic energy in the computational domain falls below this level.
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Analyze 1D Results port signals
S-parameter
energy
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Analyze 2D/3D Results
port information: • cut-off frequency • line impedance • propagation constant 43
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 phi=90
phi=0
Create a new folder “Comparison” to compare different 1D results. 46
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Parameterization Optimization
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Parameterization (I)
r1 outer radius r1 = variable goal: maximize gain
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Parameterization (II)
outer radius r1
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Result Processing Templates (Shift+P) 1D results Define gain(theta) at phi=0.
Postprocessing templates provide a convenient way to calculate derived quantities from simulation results. Each template is evaluated for each solver run. 50
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Result Processing Templates (Shift+P) 0D results
Define max of gain(theta).
Read the online help to learn more about the postprocessing in CST MWS. 51
Result Processing Templates (Shift+P) Alternative solution: The maximum gain can be computed using the “Farfield” template in “0D Results”.
Define max of gain(theta).
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Parameter Sweep - Settings 1
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Parameter Sweep - Settings Add a S-parameter watch.
The results will be automatically listed in the “Tables” folder. 54
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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.
Define the goal function.
Template based postprocessing 0D results can be used to define very complex goal functions. 58
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Automatic Optimization Choose the “Classic Powell” optimizer.
Follow the optimization.
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Automatic Optimization - Results parameter values
1D results
goal: maximize gain
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