Emissions Prediction by
Cascading
EMC Workflow Example
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Background - USB Differential Channel
 A USB differential channel including the PCB, connector and cable is investigated in this
workflow example. The channel contains intended differential mode signals and unintended
common mode signals
 Several potential coupling paths exist that may lead to increased radiated emissions.
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USB Differential Channel
CM choke
Driver to choke
Choke to connector
pins
Virtual Reference
Plane
USB channel
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Agenda
PCB and Connector
• Import PCB
• PCB port set-up during import
• Import connector
• Align Connector
• Create virtual reference plane
• Connector ports set up in 3D
• 3D Broadband Simulation
• Export Touchstone
Simplified Cable Model
• Import touchstone of PCB and connector
• Define schematic
• Define transient task for signals
• Define AC task with combine results
• Radiation spectrum at probes
• Compare to results with and without common mode choke
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Workflow: Section 1
Section 1
Transition from PCB to
Connector
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Section 2
Radiation from
cable shield
CST Project Template
Select “Create Project” to begin and
click “Next” to confirm each step of
the Configuration Wizard
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CST Project Template
Verify the project settings before
completing the project template
definition. Note that this newly defined
template can be directly applied to
future projects.
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CST Microwave Studio
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CST EDA Import
Select “Modeling: Exchange->Import/Export->2D/EDA Files->CST Layout Database” and
specify the path for the “usb_pcb.ldb” file. This opens the CST EDA Import dialog.
Set units of the import to um
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CST EDA Import
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CST EDA Import
Select these nets
1
2
Check dashed line in the preview window
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Port Definition
Define six single-ended ports as shown and
select the GROUND net as the port reference.
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Coarsen the Mesh
For a first analysis we keep the mesh settings coarse. They can be easily set to
medium or fine in this menu
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Microwave Studio Model
After clicking OK to import the PCB, your model should look like this. Time to save it.
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Simulation Settings
“Simulation: Settings->Background”
“Simulation: Settings->Boundaries”
We use this model for S-Parameter calculation, only. We do not calculate the
radiation, therefore we do not need to use open boundaries.
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Import the Connector
Drag and Drop the file “EMC_Training_USB_Connector.sat” into the 3D window. The
connector should be perfectly aligned with the PCB.
PCB materials do not
need the update
The *.sat file does not contain EM
material properties. We need to
update them from the material
library.
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Create Virtual Reference
Pick the top two faces of the connector
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Create a brick
Create Virtual Reference
Select the created shape. Right click and select transform. Apply a
translation as in the screenshot below.
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Define Ports at Virtual Reference
Select edge of connector pin and select face
of the virtual reference.
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Define Ports at Virtual Reference
Repeat for the remaining Pins
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Define Ports at Virtual Reference
Pick Point on the connector chassis and the plane to define
port for the screen.
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Time Domain Solver
We would start the solver
with the default settings.
However, the model is too
large to run during the
training time.
The results of the
broadband 3D simulation
have been exported into
a touchstone file.
We will use this precalculated file.
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Workflow: Section 2
Section 1
Transition from PCB to
Connector
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Section 2
Radiation from
cable shield
Modeling Approach - Information
 In order to simplify the analysis, only the cable shield will be modeled since we are
mainly interested in capturing the coupling to the shield and the subsequent radiation
effect which is generally the dominant radiated emissions mechanism in this type of
application.
 To validate this modeling approach, CST Cable Studio was used to model the real cable
cross-section as shown below. Using the bi-directional coupling feature, both the effect of
leakage through the cable shield from the signal wires along with the radiation from the
cable shield can be accurately captured. As can be observed from the 3 meter farfield
results, the emission levels due to the shield are an order of magnitude higher thereby
justifying the modeling approach.
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CST Project Template
Select “Create Project” to begin and
click “Next” to confirm each step of
the Configuration Wizard
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CST Project Template
Verify the project settings before
completing the project template
definition. Note that this newly defined
template can be directly applied to
future projects.
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Model Creation – Ground Plane
Create the ground plane
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Model Creation - Wire
Create the wire
Wire is placed 5 cm above ground plane. The ground plane is not ideal.
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Model Creation - Ports
Repeat on other side
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Run the Solver
Run the solver with default settings
Watch the results evolve. Especially
the far field probes.
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Design Studio
Switch to Design Studio. Drag and Drop the Touchstone from previous calculation
(USB_model.s10p).
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Re-Arrange the Ports
Double Click the Touchstone go to Layout and re-arrange the ports. This makes
set-up much more convenient. The screenshot depicts the port set-up in 3D
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Connect the Schematic
Create the schematic as shown below:
The signal traces are matched, the screen pin connect to the 3D model of the
750mm long wire.
Hint:
Probe direction can be changed by right click.
Probes can be re-named in the navigation tree.
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Define Transient Task
Port 1: +3.3V / Port 2:-3.3V
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Visualize Results
Re-name the task to symmetric
Voltages
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Current in the screen
Define Post Processing
We calculate the spectrum of the input signal and apply combine results to the model.
By doing so, we obtain the radiated spectrum.
Repeat for In_P
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Excitation Spectrum
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Define the AC Task
We calculate the spectrum of the input signal and apply combine results to the model.
By doing so, we obtain the radiated spectrum.
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Define the Combine Results
Re-name the AC task in the DS tree
Double click the task
With automatic naming we
make sure that other tasks do
not overwrite the results in the
3D tree
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Radiated Spectrum at 3m Distance
Switch back to 3D. Select 1D/results/probes/E-Farfield. Select the probes from
Combine Results. This is the radiated spectrum at 3m distance in dBuV.
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Add Skew to Signal
Duplicate the 3 tasks and rename them accordingly.
Add a skew to the signal at port 2
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Add Skew to Signal
Update the post processing and the AC Task to use the results from the
transient task with the skewed excitation.
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Radiated Spectrum at 3m Distance
Go Back to the 3D model and compare the radiated spectrum:
Skew has a strong impact on the radiation
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Common Mode Choke Coil
In the last step we will study the effect of a common mode choke on the signals.
Reference: www.murata.com/products/emc/knowhow/pdf/26to30.pdf
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New Schematic
We can create a second schematic for the definition of the model containing the
common mode choke.
This new schematic can also perform combine
results on the shared 3D model!
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New Schematic
All tasks defined in the original schematic are taken over to the new one. This is very
convenient.
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New Schematic
Drag and Drop the file “Choke.s4p” into the schematic and connect it as shown below.
Updating all tasks calculates automatically both sequences, combines the results and
adds the radiated spectrum into the 3D navigation tree.
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Compare Results
Copy and Paste the current at the screen for the four scenarios into a folder.
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Compare Results
Switch to the 3D model and multi select the probe results for the different scenarios.
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Compare Results
Combine Results is also applied to the 2D/3D Monitors. Plots show E-Field at 0.7 GHz
choke - symmetric
No choke - skew
choke - skew
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Notes
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