An SMA Connector on a Grounded Coplanar Waveguide

SMA connectors are popularly used on printed circuit boards (PCB) for testing. This example shows how to excite an SMA connector on a microwave substrate and how to terminate a grounded coplanar waveguide (GCPW) with 50 Ω using a lumped port and an air-bridge.

Figure 1: An SMA connector is soldered onto a GCPW line. The other end of the GCPW line is terminated with 50 ohms using a lumped port.

Model Definition

The purpose of this example is to show how to excite a grounded coplanar waveguide with the signal from a coaxial cable through an SMA connector. The model also illustrates a particular 50 Ω passive termination constructed with and without an air-bridge.

The signal in the coaxial cable is carried by the TEM mode at 1 GHz. Inside the cable, the electric field of the TEM mode points radially between the inner and outer conductors. (In the cross-section of the cable, the electric field is the same as the electrostatic field with a DC voltage between the conductors.) This field passes through the SMA connector and excites a symmetric mode of the coplanar waveguide. The symmetric electric field in the coplanar waveguide points from the center conductor outward to the conductors on either side, or depending on the phase, inward from the outer conductors toward the center conductor.

The ground plane on the bottom of the PCB with εr = 3.38 is connected to the outer conductors of the CPW with metalized vias to prevent unwanted surface waves.

The domain between the inner and outer conductor of the SMA connector is filled with PTFE and all metal parts including the SMA connector, CPW, metalized vias and air-bridge are modeled as perfect electric conductors. The entire modeling domain is bounded by scattering boundaries that represent an open space except the ground plane.

Figure 2: Three ways to add a port on a CPW

This example evaluates the passive 50 Ω termination on the GCPW in three ways (Figure 2). The first approach is extending the lumped port horizontally from the center conductor to the outer conductor of the GCPW. The second approach is using an air-bridge which extends above the substrate and connects the outer conductors while maintaining an air gap between itself and the center conductor. In this case, the lumped port extends vertically from the center conductor to the air-bridge. The third approach is using a multi-element uniform lumped port that terminate/excites two slots equally.

All domains are meshed by a tetrahedral mesh with maximum element size of five elements per wavelength so that the wave is well-resolved. The parts in SMA connector and vias are meshed more finely to provide good resolution of the curved surfaces.

Results and Discussion

The norm of electric fields is plotted in Figure 3. The plot setting is modified to show the fields on xy-plane confined between the center conductor and outer conductor (ground plane) of the GCPW. The evaluated S-parameters for all passive termination methods are better than -20 dB.

Figure 3: The plotted electric field distribution is symmetric due to the geometry set up.

RF_Module/Transmission_Lines_and_Waveguides/sma_connectorized_gcpw

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

Geometry 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

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	2.8[mm]	0.0028 m	CPW, center conductor width
w_s	5.8[mm]	0.0058 m	CPW, center conductor and slot width

Geometry 1

Block 1 (blk1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 60.

In the text field, type 40.

In the text field, type 20.

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

Click the

button in the toolbar.

Block 2 (blk2)

In the toolbar, click

In the window for , locate the section.

In the text field, type 60.

In the text field, type 30.

In the text field, type 1.524.

Click

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.

It might be easier to select the correct boundary by using the window. To open this window, in the toolbar click and choose . (If you are running the cross-platform desktop, you find in the main menu.)

Click

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

In the toolbar, click

In the window for , locate the section.

In the text field, type 60.

In the text field, type w_c.

Locate the section. In the text field, type -30.

In the text field, type -w_c/2.

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

In the toolbar, click

In the window for , locate the section.

In the text field, type 60.

In the text field, type w_s.

Locate the section. In the text field, type -30.

In the text field, type -w_s/2.

Work Plane 1 (wp1)>Rectangle 3 (r3)

In the toolbar, click

In the window for , locate the section.

In the text field, type w_s.

Locate the section. In the text field, type -25.

In the text field, type -w_s/2.

Part Libraries

In the toolbar, click

and choose .

In the window, click .

In the window, select in the tree.

Click

Geometry 1

SMA Connector, Square Flange with Four Holes 1 (pi1)

In the window, under click .

In the window for , locate the section.

Find the subsection. In the text field, type 15.

In the text field, type 1.524+0.635.

This is the sum of the substrate thickness and the inner radius of the imported SMA connector part.

Locate the section. In the table, enter the following settings:


Name	Expression	Value	Description
l_dielectric	8[mm]	8 mm	Length of dielectric
l_pin	2[mm]	2 mm	Length of pin from flange

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


Name	Keep	Physics	Contribute to
Dielectric	√	√	None

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


Name	Keep	Physics	Contribute to
Conductive surface	√	√	None

Click

Cylinder 1 (cyl1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 1.524.

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

In the text field, type 7.

Array 1 (arr1)

In the toolbar, click

and choose .

Select the object only.

In the window for , locate the section.

In the text field, type 11.

In the text field, type 2.

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

In the text field, type 16.

Difference 1 (dif1)

In the toolbar, click

and choose .

Select the objects and only.

In the window for , locate the section.

Find the subsection. Click to select the

toggle button.

Select the objects , , , , , , , , , , , , , , , , , , , , , and only.

Click

The vias are not part of the model domain. By removing from the model domain, the exterior boundaries of those vias are set to PEC by default.

Work Plane 2 (wp2)

In the toolbar, click

In the window for , locate the section.

From the list, choose .

In the text field, type 54.

Click

Work Plane 2 (wp2)>Rectangle 1 (r1)

In the toolbar, click

In the window for , locate the section.

In the text field, type w_s.

Locate the section. In the text field, type 1.524.

In the text field, type 12.1.

Work Plane 2 (wp2)>Rectangle 2 (r2)

In the toolbar, click

In the window for , locate the section.

In the text field, type 1.6.

In the text field, type 6.8.

Locate the section. In the text field, type 1.524.

In the text field, type 11.6.

Work Plane 2 (wp2)>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.

Work Plane 2 (wp2)>Rectangle 3 (r3)

In the toolbar, click

In the window for , locate the section.

In the text field, type 2.8.

Locate the section. In the text field, type 1.524.

In the text field, type 13.6.

In the window, right-click and choose .

The finished geometry should look like this.

Electromagnetic Waves, Frequency Domain (emw)

Perfect Electric Conductor 2

In the window, under right-click and choose the boundary condition .

In the window for , locate the section.

Click

In the dialog box, type 68 69 77 89 210 211 212 in the text field.

Click .

Perfect Electric Conductor 3

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Lumped Port 1

In the toolbar, click

and choose .

Select Boundary 4 only.

In the window for , locate the section.

From the list, choose .

For the first port, wave excitation is by default.

Lumped Port 2

In the toolbar, click

and choose .

Select Boundary 203 only.

Scattering Boundary Condition 1

In the toolbar, click

and choose .

Select Boundaries 62–66, 73, 90, 91, 93, 94, 215, and 216 only.

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 .

In the window for , locate the section.

From the list, choose .

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	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

Material 3 (mat3)

Right-click and choose .

Select Domain 6 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	3.38	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