Ansoft HFSS Tutorial: Stripline

Dr. Ryan S. Adams

March 12, 2008

This tutorial introduces the interface of Ansoft’s HFSS, and walks the student through anexample problem of creating, simulating and evaluating the response of a standard striplinestructure.

1 Starting HFSS

At UNCC, the HFSS application resides on a Linux based server by the name of “hertz”which is part of the Reconfigurable Computing System (rcs). The best way to access thisserver is through the Mosaic system.

1.1 Logging Into Mosiac Linux Server

When you are logged in to one of the Mosaic windows computers, you first need to log in toExceed Linux. This is done by selecting the start menu => All Programs => MOSAIC XP=> Unix Connectivity => Exceed Linux, as shown in Figure 1.1. This should bring you toa screen that looks something like Figure 1.1. In the box, lxs-sm1 and lxs-sm2 are identicallinux servers that are maintained by Mosaic. Highlight whichever one you prefer and clickOK. You should then be presented with a login screen for Red Hat Enterprise Linux 5; useyour normal Mosaic login ID and password to login to the linux server.

1.2 Logging into the Reconfigurable Computing System Network

At this stage, you should be looking at a linux desktop (probably with a red background). Tolog in to the RCS network, start from a command prompt. To do this, right click anywhereon the desktop and click “Open Terminal” in the pop–down menu that appears. In theterminal that appears perform the following commands:

1. type ssh -C -Y rcs you should now be prompted to log into the rcs system –do not use your mosaic password, use your rcs password instead.

2. type ssh -C -Y hertz you should now be prompted to log into hertz – use thesame password that you used to log into the rcs network.

3. type hfss and wait a few moments for the gui to appear.

1

Figure 1: Path in Windows Mosaic to access Exceed Linux servers.

Figure 2: Selecting a Linux server.

Note: The first time you log into hfss, it will prompt you for certain file locations. Usethe defaults and follow the prompts until the gui appears. You may find yourself back at acommand prompt. If this happens, just retype hfss and the gui should come up.

2 The HFSS Interface

The main HFSS interface is shown in Figure 2, which illustrates the main components of thegui. They are summarized as follows:

• 3D Modeler Window This is the area where you create the model geometry. Thiswindow consists of the model view area (or grid) and the history tree as shown inFigure 2. The history tree documents the actions that have been taken in the modelview area, and provides an alternative way to select objects in the model view area.

• Project Manager with Project Tree The project manager window displays detailsabout all open HFSS projects. Each project ultimately includes a geometric model, itsboundary conditions and material assignments, and field solution and post processinginformation. An expanded view of the project manager is shown in Figure 2

• Properties Window The properties window consists of two tabs. The command tabdisplays information about an action selected in the history tree that was performedto either create an object or modify an object. The attribute tab displays informationabout the material and display properties of a selected object.

• Progress Window This window is used when a simulation is running to monitor thesolution’s progress.

• Message Manager This window displays messages associated with a project’s devel-opment (such as error messages about the design’s setup)

3 Setting up HFSS

Before you can use HFSS for the first time, there are a couple of items that need to beconfigured for efficient and accurate operation.

1. On the Tools menu, select Options => General Options ..., click the Default Units taband ensure that Length is set to mm. Click OK.

2. On the Tools menu, select Options => HFSS Options..., ensure the Include ferritematerials check box is checked. Click the Solver tab, set the number of Processors to4, Desired RAM Limit (MB) to 6000 and the Maximum RAM Limit (MB) to 8000.Click OK.

You should now be ready to use HFSS.

3D ModelerWindow

ProjectManagerwithProjectTree

MessageManager

ProgressWindow

PropertyWindow

Figure 3: Main screen of HFSS.

4 An Example: A Simple Stripline Circuit

To begin to appreciate the functionality of this simulation tool, we will create and simulatea simple stripline transmission line. Before we can begin to work through the simulation, weneed to design the circuit on paper. We wish to design a 50 Ω stripline transmission line thatis 10 cm long using teflon (εr = 2.08, tan δ = 0.0004). The overall material thickness we’llchoose to be 1 cm. Therefore,

√eprZ = 72.11 < 120 and the following equation should be

used to compute the trace width of this transmission line:

W

b=

30π√εrZ

− 0.441. (1)

With our choice of material thickness, W = 0.866 cm.Now that we have the geometry completely defined, we are ready to open HFSS and

build the model. We’ll do this in the 3D modeler window.

4.1 Creating the Dielectric

The first component of the geometry that we’ll draw is the dielectric. This is a solid rectangleof teflon, which can be drawn by selecting Draw => box in the file menu. To make the boxthe correct size, you can either

1. input the x, y, z coordinates of each component of the box into the appropriate fieldsat the bottom of the 3D model window, or

HistoryTree

ModelViewArea

Figure 4: 3D Modeler Window, which consists of the model view area and the history tree.

2. click randomly in the 3D model window three times to create a box and edit the sizein the properties box. To do this, click the command tab in the properties box andinput the correct start position, and xsize, ysize and zsize for our box.

For our stripline circuit, we want to box to be 200mm in the x-direction, 40mm in they-direction and 10mm in the z-direction. The box should start at the location (-100mm,-20mm, -5mm) with XSize = 200mm, YSize = 40mm and ZSize = 10mm. The materialparameters are set by clicking on the attribute tab in the properties window. Click on theblock that says Vacuum and a window should open titled “Select Definition.” HighlightTeflon in the materials list and click OK. If you wish to change the color or transparency ofthe box you may also do so in the attribute tab.

At this point, the box should cover the entire screen. To view the entire box, click thebutton with a picture of a magnifying glass with a white square in the middle. This button“fits all the contents in the view.” All that remains is to create the center trace of thestripline.

4.2 Creating the Center Trace

We will create the center trace as an infinitesimally thin strip. To do this, we select Draw=> rectangle in the file menu. We can define the size and location of the rectangle in thesame manner as for the dielectric above. Our strip width is 8.66mm, and 1000mm long, sothe position is (-100,-8.66/2,0) and XSize = 200mm, YSize = 8.66mm. Since the trace hasno thickness, we do not apply any material attributes to it. We will make it into a conductorusing the boundary conditions.

Figure 5: Project Manager window illustrating the boundery conditions, excitation, etc. ofthe current model.

4.3 Boundary Conditions

The following boundary conditions must be applied to this device:

• Perfect E boundary on the top of the dielectric (simulates a metal layer)

• Perfect E boundary on the bottom of the dielectric (simulates a metal layer)

• Perfect E boundary on the trace (simulates a metal layer)

• Radiation boundary on the long sides of the dielectric (simulates the material extendingto infinity in that direction)

To apply these boundary conditions, right click in the 3D modeler window and click “SelectFaces”. Now, select the top face, bottom face and the strip using the control key. Note: toselect faces beneath what you can see, click the face you want and press the “b” key until thedesired face is selected. With these three items selected, right click in the “white–space” ofthe 3D modeler window and highlight “Assign Boundary,” and click “Perfect E...”. Click OK.Next, select the long sides of the dielectric using the control key. Right click in the white–space of the 3D modeler window and highlight “Assign Boundary,” and click “Radiation...”Click OK.

To check that the boundaries are created correctly, expand the “Boundaries” item in theProject Tree, and you should see “PerfE1” and “Rad1” or similar. Highlight PerfE1, andyou should see hash marks indicating which regions of the model apply to this boundary.Highlight Rad1 to see the same information for the radiation boundary.

4.4 Excitations

We will create a “Waveport” excitation at each end of the circuit. To do so, we perform thefollowing steps:

1. Select the face of the nearest end of the circuit

2. right click in the 3D modeler window and select Assign Excitation => Wave Port.

3. Click Next

4. Under “Integration Line,” click the word None, and select New Line...

5. In the 3D modeler window, click the center of the bottom of the selected face (a trianglewill appear)

6. Click the center of the trace

7. Clck Next

8. Click Finish

The same steps should be performed to create a wave port at the other end of the circuit.

4.5 Analysis

Perform the following steps to set up the analysis options:

1. Right click on Analysis in the Project Tree, and select “Add Solution Setup”

2. Under the General tab:

(a) Set the solution frequency to 20 GHz

(b) Set the maximum number of passes to 30

(c) Set maximum Delta S to 0.01

3. Under the Options tab:

(a) Set the Maximum Refinement per pass to 20 %

(b) Set the Order of Basis Functions to Second Order

Perform the following steps to set up the frequency sweep:

1. Under the Analysis item in the Project Tree, right-click on Setup1

2. Select Add Frequency Sweep...

3. Set start frequency to 1 GHz

4. Set stop frequency to 20 GHz

5. Set step size to 0.25 GHz

6. Click OK

4.6 Final Checks and Running the Simulation

Select HFSS => Validation Check... to ensure the project is prepared for simulation (clickclose).

Save the project by clicking on the save icon at the top of the screen.Right-click setup1 under Analysis in the project tree, select Analyze to begin the sim-

ulation. At this point the progress window should show the progress of the simulation,beginning with the mesh generation.

4.7 Simulation Results

To view the results of the simulation, perform the following steps:

1. Right click on the results item in the Project Tree

2. Click Create Modal Solution Data Report => Rectangular Plot

3. Under the trace tab, select the first S-Parameter that you wish to view and click NewReport

4. Add additional S-Parameters by highlighting them and clicking Add Trace

5. Click Close