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Filter Characterized by Imported S-Parameters via a Touchstone File
Introduction
A Touchstone file describes the frequency responses of an n-port network circuit in terms of S-parameters. This can be used to simplify arbitrarily complex circuits. The Touchstone file can be obtained from numerical simulations or network analyzer measurements. The obtained file for a two-port network can then be included in simulations without building the complicated shape of the circuit. In this example, a low-pass filter between two coaxial connectors is modeled using a two-port network feature and imported S-parameters via a Touchstone file. The results include electric field distribution within the coaxial connectors and the S-parameters.
Figure 1: The circuit geometry inside a blue box frame is not included in the model but characterized by a Touchstone file.
Model Definition
The model consists of only two coaxial connectors as the circuitry between them is replaced with a two-port network. The exterior boundaries of the geometry are set to perfect electric conductor (PEC) by default. Since there is no wave propagation inside the center conductor of the coaxial connectors, its volume is removed and only the conducting metal surfaces are modeled. The modeling of metallic surfaces using the default PEC is valid when the operating frequency is not too high so the loss is negligible. The space between the inner and outer conductor is filled with lossless Polytetrafluoroethylene (PTFE) with a dielectric constant εr = 2.1. Each outer end of the coaxial connectors is terminated with the coaxial type of a lumped port that has a 50 Ω reference characteristic impedance. Both inner ends of the coaxial connectors are configured with a two-port network. By selecting the Touchstone file for the type of S-parameter definition, a simulated or measured S-parameter definition can be imported. The given Touchstone file in the model is generated from the first simulation part of A Low-Pass and Band-Pass Filter Using Lumped Elements in the RF Module Application Libraries. If the frequency range of the simulation is within that of the Touchstone file data but frequency sampling points are not exactly matched to each other, S-parameters will be interpolated based on cubic splines by default. If the frequency range of the simulation is outside that of the Touchstone file data, S-parameters will be extrapolated as a constant using the first or last value of the Touchstone file data. The subfeatures of the two-port network, two-port network ports define, where the port 1 and 2 are located.
Results and Discussion
In Figure 2, electric field distribution inside the coaxial connectors is visualized. Since 2 GHz is not within the passband, the input power at the excitation port is not delivered to the observation port.
Figure 2: The electric field distribution inside the coaxial connectors at 2 GHz.
Figure 3: S-parameter plot shows the frequency responses of a low-pass filter.
The computed S-parameters are plotted in Figure 3. The simulation with two coaxial connectors linked to the imported S-parameter data replicates the frequency response of the original lumped element low-pass filter, A Low-Pass and Band-Pass Filter Using Lumped Elements.
Application Library path: RF_Module/Filters/two_port_network_touchstone
Modeling Instructions
From the File menu, choose New.
New
In the New window, click  Model Wizard.
Model Wizard
1
In the Model Wizard window, click  3D.
2
In the Select Physics tree, select Radio Frequency>Electromagnetic Waves, Frequency Domain (emw).
3
Click Add.
4
Click  Study.
5
In the Select Study tree, select General Studies>Frequency Domain.
6
Click  Done.
Study 1
Step 1: Frequency Domain
1
In the Model Builder window, under Study 1 click Step 1: Frequency Domain.
2
In the Settings window for Frequency Domain, locate the Study Settings section.
3
In the Frequencies text field, type range(0.2[GHz],0.05[GHz],2[GHz]).
Geometry 1
1
In the Model Builder window, under Component 1 (comp1) click Geometry 1.
2
In the Settings window for Geometry, locate the Units section.
3
From the Length unit list, choose mm.
Cylinder 1 (cyl1)
1
In the Geometry toolbar, click  Cylinder.
2
In the Settings window for Cylinder, locate the Size and Shape section.
3
In the Radius text field, type 2.05.
4
In the Height text field, type 40.
5
Locate the Position section. In the y text field, type -20.
6
Locate the Axis section. From the Axis type list, choose y-axis.
Cylinder 2 (cyl2)
1
In the Geometry toolbar, click  Cylinder.
2
In the Settings window for Cylinder, locate the Size and Shape section.
3
In the Radius text field, type 0.635.
4
In the Height text field, type 40.
5
Locate the Position section. In the y text field, type -20.
6
Locate the Axis section. From the Axis type list, choose y-axis.
Block 1 (blk1)
1
In the Geometry toolbar, click  Block.
2
In the Settings window for Block, locate the Size and Shape section.
3
In the Width text field, type 10.
4
In the Depth text field, type 30.
5
In the Height text field, type 10.
6
Locate the Position section. From the Base list, choose Center.
Difference 1 (dif1)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Difference.
2
Select the object cyl1 only.
3
In the Settings window for Difference, locate the Difference section.
4
Find the Objects to subtract subsection. Select the  Activate Selection toggle button.
5
Select the objects blk1 and cyl2 only.
6
Click  Build All Objects.
Materials
Material 1 (mat1)
1
In the Model Builder window, under Component 1 (comp1) right-click Materials and choose Blank Material.
2
In the Settings window for Material, locate the Material Contents section.
3
In the table, enter the following settings:
Property
Variable
Value
Unit
Property group
Relative permittivity
epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0
2.1
1
Basic
Relative permeability
mur_iso ; murii = mur_iso, murij = 0
1
1
Basic
Electrical conductivity
sigma_iso ; sigmaii = sigma_iso, sigmaij = 0
0
S/m
Basic
Electromagnetic Waves, Frequency Domain (emw)
Lumped Port 1
1
In the Model Builder window, under Component 1 (comp1) right-click Electromagnetic Waves, Frequency Domain (emw) and choose Lumped Port.
2
Select Boundary 3 only.
3
In the Settings window for Lumped Port, locate the Lumped Port Properties section.
4
From the Type of lumped port list, choose Coaxial.
For the first port, wave excitation is on by default.
Lumped Port 2
1
In the Physics toolbar, click  Boundaries and choose Lumped Port.
2
Click the  Wireframe Rendering button in the Graphics toolbar.
3
Select Boundary 8 only.
4
In the Settings window for Lumped Port, locate the Lumped Port Properties section.
5
From the Type of lumped port list, choose Coaxial.
Two-Port Network 1
1
In the Physics toolbar, click  Boundaries and choose Two-Port Network.
2
Select Boundaries 4 and 7 only.
Two-Port Network Port 1
1
In the Model Builder window, expand the Two-Port Network 1 node, then click Two-Port Network Port 1.
2
Select Boundary 4 only.
Two-Port Network Port 2
1
In the Model Builder window, click Two-Port Network Port 2.
2
Select Boundary 7 only.
Two-Port Network 1
1
In the Model Builder window, click Two-Port Network 1.
2
In the Settings window for Two-Port Network, locate the Two-Port Network Properties section.
3
From the Type of port list, choose Coaxial.
4
From the Type of S-parameter definition list, choose Touchstone file.
5
Click Browse.
6
Browse to the model’s Application Libraries folder and double-click the file two_port_network_touchstone.s2p.
7
Click Import.
8
Click the  Zoom Extents button in the Graphics toolbar.
Mesh 1
1
In the Model Builder window, under Component 1 (comp1) click Mesh 1.
2
In the Settings window for Mesh, locate the Physics-Controlled Mesh section.
3
From the Element size list, choose Coarser.
4
Click  Build All.
Study 1
In the Home toolbar, click  Compute.
Results
Electric Field (emw)
Compare the created plot to Figure 2.
S-parameter (emw)
See Figure 3 to check the S-parameter plot.
Smith Plot (emw)
Smith plot showing the reflection coefficient S11. The color legend shows the frequency, ranging from 0.2 GHz to 2 GHz.