Modeling of a CPW Using Numeric TEM Ports

This tutorial example shows how to set up port features in a physics interface when designing a coplanar waveguide (CPW) circuit that is useful for mmWave applications.

Figure 1: Coplanar waveguide (CPW) simulation model.

Model Definition

There are multiple ways to excite and terminate a CPW using ports or lumped ports. In this tutorial, a basic CPW circuit is modeled using type ports with the option. This configuration requires you to add a in the study and add an subfeature under a port feature to calculate the TEM mode characteristic impedance. The TEM mode characteristic impedance is calculated based on the power on port boundaries and voltage obtained from the user-defined integration through the abovementioned subfeature. The computed impedance scales the mode field, the solution of a boundary mode analysis, mapped to the port boundaries to excite or terminate the end cross section of the circuit.

Table 1: Key items to characterize a grounded coplanar waveguide
	Physics feature	Subfeature	Study step
Item in Model Builder	Port	Integration Line for Voltage	Boundary Mode Analysis
Notable configuration	Numeric type Analyze as a TEM field Include via edges	Set on a line geometry between two conductive boundaries

All conductive boundaries representing metalized or plated surfaces are defined as perfect electric conductors to simplify the modeling steps. If the loss due to the finite conductivity is assumed to be nonnegligible, these boundaries can be replaced by a transition boundary condition to take the loss in the model into account.

A scattering boundary condition is used to finish the outermost boundaries. That is a first-order absorbing boundary condition that by default absorb any possible radiation from the circuit. So, it will not reflect back and does not disturb the performance of, for instance, S-parameters, of the circuit.

Results and Discussion

The computed S-parameters indicate that the reflection due to the impedance mismatch is marginal (below −30 dB) and that the insertion loss is below 0.05 dB. When the computation is completed, three defaults plots are automatically generated. From the electric field norm plot, it is possible to see where the strong electric fields are confined; that is along the conductive edges around slots between the center conductor and a pair of ground planes. When a boundary mode analysis is performed for each port, it also generates the mode field plot with an annotation of the computed impedance value. See the Modeling Instruction section for more details.

RF_Module/Transmission_Lines_and_Waveguides/cpw_numeric_tem_port

Modeling Instructions

From the menu, choose .

New

In the window, click

Model Wizard

In the window, click

In the tree, select .

Click .

Click

In the tree, select .

Click

Study 1

Step 1: Boundary Mode Analysis

In the window, under click .

In the window for , locate the section.

In the text field, type 10[GHz].

Select the check box.

In the associated text field, type sqrt(12.9)/1.5.

Step 3: Boundary Mode Analysis 1

Right-click and choose .

Drag and drop below .

In the window for , locate the section.

In the text field, type 2.

Step 3: Frequency Domain

In the window, click .

In the window for , locate the section.

In the text field, type 10[GHz].

Global Definitions

Parameters 1

In the window, under click .

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Value	Description
w_c	200[um]	2E-4 m	CPW, center conductor width
w_s	w_c+2*125[um]	4.5E-4 m	CPW, center conductor and slot width
thickness	200[um]	2E-4 m	Wafer thickness

Geometry 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

Block 1 (blk1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 6000.

In the text field, type 4000.

In the text field, type 2000.

Locate the section. In the text field, type -500.

Block 2 (blk2)

In the toolbar, click

In the window for , locate the section.

In the text field, type 6000.

In the text field, type 3000.

In the text field, type thickness.

Locate the section. In the text field, type 1000-thickness.

Click

Click the

button in the toolbar.

Work Plane 1 (wp1)

In the toolbar, click

In the window for , locate the section.

From the list, choose .

On the object , select Boundary 4 only.

Work Plane 1 (wp1)>Plane Geometry

In the window, click .

Work Plane 1 (wp1)>Rectangle 1 (r1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 6000.

In the text field, type w_c.

Locate the section. From the list, choose .

Work Plane 1 (wp1)>Rectangle 2 (r2)

Right-click and choose .

In the window for , locate the section.

In the text field, type w_s.

Work Plane 1 (wp1)>Difference 1 (dif1)

In the toolbar, click

and choose .

Select the object only.

In the window for , locate the section.

Find the subsection. Click to select the

toggle button.

Select the object only.

Right-click and choose .

Materials

Material 1 (mat1)

In the window, under right-click and choose .

In the window for , locate the section.

In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Relative permittivity	epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0	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

Material 2 (mat2)

Right-click and choose .

Select Domain 2 only.

In the window for , locate the section.

In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Relative permittivity	epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0	12.9	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