Coplanar Waveguide Bandpass Filter

Coplanar waveguide (CPW) bandpass filters can be realized using interdigital capacitors (IDCs) and short-circuited stub inductors (SSIs). Such a filter can readily be implemented on a GaAs wafer. The presented model is compact in relation to its resonant frequency and provides a relatively high Q factor compared to capacitively coupled microstrip line model designs.

Figure 1: A coplanar waveguide bandpass filter on a 200 μm GaAs wafer composed of an interdigital capacitor and short-circuited stub inductors.

Model Definition

The structure shown in Figure 1 can be realized by etching a pattern in a thin gold layer on a high dielectric (εr = 12.9) GaAs substrate. In this model, the gold layer is treated as an infinitely thin layer of perfectly conducting material. Two lumped ports represent a coplanar waveguide coupling into, and out of, the device. The lumped port applies a voltage difference between the center conductor and the ground planes. This voltage difference is applied through the multielement uniform type of a lumped port that equally divides the signal between two ground planes. The line width and coupling gap on the comb in the interdigital capacitor is 100 microns, which is wide enough to account for etching tolerances. Series and parasitic SSIs are added to generate a bandpass frequency response.

The model space consists of the GaAs wafer, with the pattern on the surface, and an air box surrounding the entire structure. The air box, in turn, is bounded by a perfect electric conductor boundary representing a die packaging placed far enough from the CPW so as not to introduce any unwanted coupling.

Results and Discussion

The structure is simulated over a range of frequencies from 7.1 GHz to 8.1 GHz. The simulation results show bandpass filter characteristics around 7.65 GHz as presented in Figure 2.

Because the filter is enclosed by the PEC package, it is effectively a grounded coplanar waveguide circuit and there is a parasitic reactance between the circuit and the bottom ground plane. This reactance loading produces wider frequency responses than those generated from the circuit itself. The model without the package as well as the bottom ground plane provides sharper frequency responses.

It is generally recommended to add air bridges around SSIs when structures of this type are used for very high frequencies in order to suppress any potential radiation over the slots.

Figure 2: The frequency response of the coplanar waveguide filter shows bandpass characteristics around 7.65GHz.

RF_Module/Filters/cpw_bandpass_filter

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: Eigenfrequency

In the window, under click .

In the window for , locate the section.

In the text field, type 7.1[GHz].

Step 2: Frequency Domain, Modal

In the window, click .

In the window for , locate the section.

Click

In the dialog box, type 7.1[GHz] in the text field.

In the text field, type 2[MHz].

In the text field, type 8.1[GHz].

Click .

Geometry 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

Import 1 (imp1)

In the toolbar, click

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file cpw_bandpass_filter.mphbin.

Click

Click the

button in the toolbar.

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 toolbar, click

to close the window.

Materials

GaAs

In the window, under right-click and choose .

In the window for , type GaAs in the text field.

Select Domain 2 only.

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

Definitions

In the window, expand the node.

Before setting up the physics, hide the air domain to get a better view and easier access to the CPW circuit geometry.

Hide for Physics 1

In the window, expand the node.

Right-click and choose .

Select Domain 1 only.

Electromagnetic Waves, Frequency Domain (emw)

Perfect Electric Conductor 2

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

Select Boundary 9 only.

Excite and terminate each end of the CPW line using multielement uniform lumped ports.

Lumped Port 1

In the toolbar, click

and choose .

Select Boundaries 10 and 11 only.

In the window for , locate the section.

From the list, choose .

For the first port, wave excitation is by default.

Locate the section. Click

In the dialog box, Create a set of selections for use in the study settings.

type Lumped Port 1 in the text field.

Click .

Uniform Element 1

In the window, expand the node, then click .

In the window for , locate the section.

Click

Select Boundary 11 only.

Locate the section. Specify the ah vector as


0	x
1	y
0	z

Uniform Element 2

In the window, click .

Select Boundary 10 only.

In the window for , locate the section.

Specify the ah vector as


0	x
-1	y
0	z

Lumped Port 2

In the toolbar, click

and choose .

Select Boundaries 27 and 28 only.

In the window for , locate the section.

From the list, choose .

Locate the section. Click

In the dialog box, type Lumped Port 2 in the text field.

Click .

Uniform Element 1

In the window, expand the node, then click .

In the window for , locate the section.

Click

Select Boundary 28 only.

Locate the section. Specify the ah vector as


0	x
1	y
0	z

Uniform Element 2

In the window, click .

Select Boundary 27 only.

In the window for , locate the section.

Specify the ah vector as


0	x
-1	y
0	z

Mesh 1

In the window, under right-click and choose .

The default frequency and material-dependent physics-controlled mesh is not fine enough to accurately evaluate the reactance of the comb structures in the IDCs. Refine the mesh settings manually.

In the window for , locate the section.

From the list, choose .

Size 1

In the window, right-click and choose .

In the window for , locate the section.

From the list, choose .

Click

In the dialog box, type 13-26 in the text field.

Click .

In the window for , locate the section.

Click the button.

Locate the section.

Select the check box. In the associated text field, type 80.

Click

Click the

button in the toolbar.

Study 1

Step 2: Frequency Domain, Modal

In the window, under click .

In the window for , click to expand the section.

Find the subsection. From the list, choose .

Under , click

In the dialog box, in the list, choose and .

Click .

In the toolbar, click

Results

Electric Field (emw)

Since the results of the analysis are stored only on the lumped port boundaries, this default E-field norm plot does not provide useful information. Switch the dataset to the solution of the analysis.

In the window for , locate the section.

From the list, choose .

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 0.

In the toolbar, click

S-parameter (emw)

In the window, under click .

In the window for , locate the section.

From the list, choose .

The S-parameter plot is shown in Figure 2.

Smith Plot (emw)