Modeling of Microstrip Lines with Vias

There are multiple ways to excite and terminate transmission lines using different types of port and lumped port features. In this example, transverse electromagnetic (TEM) type ports and a via type lumped port are used to simulate two adjacent microstrip lines. One via end is terminated as a metalized via while the other via end is probing an inflow signal. The computed S-parameters show the amount of crosstalk between the lines and the strength of the signal coupled through the cylindrical via.

Figure 1: Microstrip line circuit board modeled with TEM ports and a via type lumped port. The top air domain is not included for visualization purposes.

Model Definition

The model describes two 50 Ω microstrip lines adjacent to each other on a 60 mil substrate with the dielectric constant of εr = 3.38. One microstrip line is terminated with a via type lumped port and the other line is finished with a metalized via hole. The interior surfaces of metallic parts, including the patterned line on the top of the substrate and a metalized via, are defined using a transition boundary condition to capture the loss from a finite conductivity. The ground plane is located on the exterior surface of the model domain and characterized by an impedance boundary condition that is also used for modeling lossy conductive boundaries.

A transverse electromagnetic (TEM) type port boundary condition is used on each side of the cuboid model domain touching the microstrip line and ground plane. The TEM type port is completed by adding electric potential and ground subfeatures. The edge of microstrip line top trace on the TEM port boundary is set to electric potential while the edge of ground plane on the TEM port boundary is set to ground. The material above the circuit board is air.

Results and Discussion

After simulation, the default plot is modified to plot the electric field norm on the top surface of the circuit board as shown in Figure 2. It is observed that the input power to port 1 is partially coupled to the adjacent microstrip line connected to port 3.

Figure 2: Electric field norm plot. A fraction of input power is coupled to the adjacent microstrip line.

Figure 3 quantitatively shows the amount of coupling and crosstalk among ports. The computed S21 indicates that most of the input power is flown into the via-type lumped port (port 2). The far-end crosstalk in this circuit, S31, increases with the simulation frequency.

Figure 3: S-parameter plot describes impedance mismatching (S11), insertion loss to the via port (S21), and far-end crosstalk, FEXT (S31).

RF_Module/Transmission_Lines_and_Waveguides/microstrip_line_tem_via

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

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
tsub	20[mil]	5.08E-4 m	Substrate thickness

It is convenient to define parameters for frequently used values. Here, mil refers to the unit milliinch.

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 10.

In the text field, type 8.

In the text field, type tsub*10.

Locate the section. In the text field, type -4.

Click to expand the section. In the table, enter the following settings:


Layer name	Thickness (mm)
Layer 1	tsub

Work Plane 1 (wp1)

In the toolbar, click

Signal traces are patterned on this work plane.

In the window for , locate the section.

In the text field, type tsub.

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 7.5.

In the text field, type 1.13.

Locate the section. In the text field, type -0.565+0.8.

Work Plane 1 (wp1)>Circle 1 (c1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.8.

Locate the section. In the text field, type 7.5.

In the text field, type 0.8.

Work Plane 1 (wp1)>Union 1 (uni1)

In the toolbar, click

and choose .

Click in the window and then press Ctrl+A to select both objects.

In the window for , locate the section.

Clear the check box.

Click the

button in the toolbar. The wireframe rendering provides a better view of inside the box.

Cylinder 1 (cyl1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.4.

In the text field, type tsub.

Locate the section. In the text field, type 7.5.

In the text field, type 0.8.

The side surfaces of the cylinder will be configured as Via type lumped port and metalized via hole, later.

Rotate 1 (rot1)

In the toolbar, click

and choose .

Select the objects and only.

In the window for , locate the section.

Select the check box.

Locate the section. In the text field, type 180.

Click

Locate the section. In the text field, type 5.

Click

Electromagnetic Waves, Frequency Domain (emw)

Wave Equation, Electric 2

In the window, under right-click and choose .

Select Domain 1 only.

In the window for , locate the section.

From the list, choose .

The material properties for the substrate, that will imported from the RF material library, are defined based on the dielectric constant and loss tangent. It is necessary to choose the right constitutive relation for the substrate domain.

Transition Boundary Condition 1

In the toolbar, click

and choose . Lossy conductive surfaces can be modeled using for interior boundaries.

Select Boundaries 8, 11–13, 16, and 17 only.

In the window for , locate the section.

In the d text field, type 38[um].

Impedance Boundary Condition 1

In the toolbar, click

and choose . Lossy conductive surfaces can be modeled using for exterior boundaries. This can also be looked as geometrically thick conductive volume.

Select Boundary 3 only.

Port 1

In the toolbar, click

and choose .

Select Boundaries 1 and 4 only.

In the window for , locate the section.

From the list, choose .

Transmission lines supporting the (quasi-) TEM mode can be analyzed with the type port.

Ground 1

In the toolbar, click

and choose .

The exterior edges of the port boundaries are set to ground by default.

Port 1

In the window, click .

Electric Potential 1

In the toolbar, click

and choose .

Select Edge 9 only.

The electric potential edge is assigned on the signal trace.

Lumped Port 1

In the toolbar, click

and choose .

Click the

button in the toolbar.

Select Boundaries 18, 19, 22, and 23 only.

In the window for , locate the section.

From the list, choose .

Port 2

In the toolbar, click

and choose .

Select Boundaries 24 and 25 only.

In the window for , locate the section.

From the list, choose .

Ground 1

In the toolbar, click

and choose .

Port 2

In the window, click .

Electric Potential 1

In the toolbar, click

and choose .

Select Edge 57 only.

Now, make sure that Advanced Physics Options is enabled. Then, set the physics to use the constraint-based port formulation, that works best with TEM-type ports.

Click the

button in the toolbar.

In the dialog box, select in the tree.

In the tree, select the check box for the node .

Click .

In the window, click .

In the window for , click to expand the section.

From the list, choose .

Add Material

In the toolbar, click

to open the window.

Go to the window.

In the tree, select .

Click in the window toolbar.

In the tree, select .

Click in the window toolbar.

Materials

Copper (mat2)

In the window for , locate the section.

From the list, choose .

Select Boundaries 3, 8, 11–14, 16, and 17 only.

Add Material

Go to the window.

In the tree, select ® ™ .

Click in the window toolbar.

In the toolbar, click

to close the window.

Materials

RO4003C™ Laminates (mat3)

Select Domain 1 only.

Study 1

Step 2: Frequency Domain

In the window, under click .

In the window for , locate the section.

In the text field, type 1 3 5.

Mesh 1

In the window, under right-click and choose .

Definitions

Remove some boundaries from the view to inspect the mesh.

Hide for Physics 1

In the window, expand the node.

Right-click and choose .

In the window for , locate the section.

From the list, choose .

Select Boundaries 5 and 7 only.

Mesh 1

In the toolbar, click

Results

Multislice

In the window, expand the node, then click .

In the window for , locate the section.

Find the subsection. In the text field, type 0.

Find the subsection. From the list, choose .

In the text field, type tsub.

Locate the section. Click

In the dialog box, select in the tree.

Click .

Compare the reproduced plot to Figure 2.

Global 1

In the window, expand the node, then click .

In the window for , click to expand the section.

Find the subsection. From the list, choose .

The S-parameter plot is shown in Figure 3. This describe the impedance mismatching in the input port and signal coupling to the adjacent microstrip line.

Smith Plot (emw)