Ansys HFSS Tutorial: Microstrip Transmisison Line

 
This tutorial will go through the process for building and simulating a section of microstrip transmission line in Ansys HFSS.

If you are a UNCC student, and this is your first time using HFSS, please see Initializing HFSS.

Design of a Microstrip Transmission Line

HFSS_microstrip/Microstrip.png
The characteristic impedance Zo of a microstrip transmission line is determined by the dielectric constant of the material separating the trace from the ground plane, the width w of the trace, and the thickness d of the dielectric.  The relevant equations can be found in the textbook Microwave Engineering, 4th ed., by David M. Pozar, on pages 148 and 149, and are also presented here. If we wish to create a microstrip line with characteristic impedance Zo of 50 ohms, and choose to build our microstrip on teflon (which has a relative permittivity of 2.08), with a thickness d of 1.5mm, we can calculate that the appropriate width value is 4.788mm.  For the purposes of this example, we will arbitrarily choose to let the simulated line have a length of 200mm.  We will also let the width of the simulation space be 60mm , and its height be 30mm.  We will discuss the determination of minimum width and height later in this tutorial.  Finally, we will choose to use a perfectly conducting metal with a thickness of 0.03mm for all conducting surfaces.

Now we are ready to build our geometry.

Creating the Geometry

The geometry of this model will consist of six objects.
  1. A rectangular box for the ground plane.
  2. A rectangular box for the dielectric.
  3. A rectangular box for the trace.
  4. A rectangular box in which we want HFSS to solve for the fields (an air box).
  5. and 6. Two sheets, one at either end of the transmission line, on which we will define the waveport excitations.
Let's begin by drawing the ground plane.  To do this, click the "Draw Box" icon at the top of the page.
HFSS_microstrip/DrawBox.png
If you do not get a pop-up dialog box, hit F4 on your keyboard.  In the dialog box that comes up, in the "Position" field, enter the following comma-separated triplet:
-xbox/2, -ybox/2, -mt-zdielectric
This position is stated in terms of four undefined variables, so the next thing HFSS will do is ask you to define those four variables.  Go ahead and hit Enter on your keyboard.  In the pop-up "Add Variable" window that appears (asking you to provide a value for xbox), enter the value of the length we chose for the box (200mm), and click "OK."
HFSS_microstrip/xbox_def.png
In the next "Add Variable" pop-up that appears (asking you to provide a value for ybox), enter the value of the width we chose for the simulation space (60mm).  Click "OK."
In the next "Add Variable" pop-up that appears (asking you to provide a value for mt), enter the thickness for the metal (0.03mm).  Click "OK."
In the next "Add Variable" pop-up that appears (asking you to provide a value for zdielectric), enter 1.5mm (the thickness we chose for the dielectric).  Click "OK."
Back in the Create Box dialog box, in the XSize field, enter 'xbox', and in the YSize field, enter 'ybox'.  In the Zsize field, enter the variable mt, and hit Enter on your keyboard. At this point, your CreateBox dialog should look like this:

HFSS_microstrip/CreateBox_mt.png
Click "OK".
Click the "Fit All" icon at the top of the screen to make your entire model visible in the model viewing window.
HFSS_microstrip/FitContents.png

Next, let's create the rectangular box for the dielectric.  To do this, click the "Draw Box" button again, and this time enter the following in the field boxes:
"Position" : -xbox/2, -ybox/2, -zdielectric
"XSize"    : xbox
"YSize"    : ybox
"ZSize"    : zdielectric

HFSS_microstrip/CreateBox_dielectric.png

Click "OK."
Now, by default HFSS assumes that any undefined space is filled with perfect electric conductor (PEC).  So at this point, the only parts of space that are not considered PEC by the simulator are the two boxes we've created.  We must create an open space (a box filled with vacuum) in which HFSS can solve for the fields over the microstrip trace.  So the next thing we will create is the airbox.  To do this, click the "Draw Box" button again, and enter the following in the position and size fields:
"Position": -xbox/2, -ybox/2, -zdielectric-mt 
"XSize"    : xbox
"YSize"    : ybox
"ZSize"    : zbox
In the pop-up Add Variable dialog box that appears, enter a value of 30mm for zbox, and click "OK."

HFSS_microstrip/CreateBox_airbox.png

Click "OK."
In order to be able to see your model more clearly, you may want to adjust the transparency of the air box.  To do this select "Box3" in the project tree, click "Transparent" in the Properties window, and use the slider that pops up to adjust the transparency of the box.  Click "OK."

HFSS_microstrip/adj_transparency.png

The next geometric element we want to create is the microstrip trace itself.  To do this, click the "Draw Box" button once more.  Enter the following data in the fields:
Position: -xbox/2, -ustripwidth/2, 0
XSize    : xbox
YSize    : ustripwidth
Zsize     : mt

When asked, enter a value of 4.788mm for ustripwidth.

HFSS_microstrip/CreateBox_ustrip.png


Click "OK." 

The next thing we need to do is create the two waveport excitations, which will consist of 2D sheets defining the area on each "wall" of the simulation structure through which energy may enter and exit the system.  These will be located at the two ends of the microstrip trace, and must be large enough to contain all of the critical microstrip field lines in that area.  To find out how big that should be, perform the following steps:
HFSS_microstrip/Help.png
HFSS_microstrip/WaveportSize.png
HFSS_microstrip/DrawRectangle.png
HFSS_microstrip/waveport1.png
HFSS_microstrip/Rectangle2.png
At this point, the geometry of your model is complete.  The next step is to assign the materials and boundaries.

Assigning Materials and Boundaries

By default, HFSS assumes that all the space internal to your model (in this case, Box1, Box2, and Box3) are vacuum, and everything outside that space is perfect electric conductor.  In other words, your model is currently a vacuum-filled cavity in an infinite conductor.  To convert it into a microstrip transmission line, we will need to make some material and boundary assignments.

To begin, select Box1 in the project tree.  While holding the "Ctrl" key on your keyboard, click Box4 as well, so that they are both selected.  Right-click in the project viewing window, and click "Assign Material..."
HFSS_microstrip/Assignmaterial.png
In the dialog box that opens, type "pec" in the "Search by Name" field.
HFSS_dipole/selectmaterial.png
With "pec" selected in the list, click "OK."  What we have just done is assigned the material of the ground and the microstrip trace to be perfect electric conductor.  When actually building a microstrip trace, the conductors will both need to be composed of a more realistic material, like copper.  However, copper has a high enough conductivity that, for the purposes of this tutorial, the results will not be much affected, and the use of PEC rather than copper can help to streamline the simulation.

We will also need to assign the dielectric.  To do this, select Box2 in the project tree, right-click in the project viewing window, and click "Assign Material...".  This time, type "teflon" in the Search by Name field, and select teflon_based from the materials returned by the search.  With "teflon_based" selected, click "OK."

Now our model consists of a dielectric slab with a microstrip trace on its top side and a rectangular conductor on its bottom side, suspended inside a cavity in an infinite conductor.  The next thing we need to do is assign the boundaries of Box3 so that HFSS knows we expect the dielectric slab and ground plane to continue infinitely in the x and y directions.   The way we do this is to assign a "Radiation Boundary" on  the top face and all four side faces of Box3.  A radiation boundary tells HFSS that we expect the geometry present on the face of the boundary to continue infinitely in the normal outward direction.  To make this assignment,  perform the following steps:
HFSS_microstrip/Radboundary.png

To check that the boundary was properly assigned, go to the Project Manager window (expand it,  if necessary), expand the "Boundaries" list, and click on your new radiation boundary, Rad1.  You should be able to tell in the project viewing window that all five relevant faces of the box are now assigned to the radiation boundary.  You can rotate the model, if you like, to make sure.
HFSS_microstrip/Radboundary2.png

By default, the bottom face of the model is defined as PEC (perfect electric conductor).  This is an appropriate assignment, so we'll leave it as it is.  At this point, all your materials are properly set, and your boundaries are properly assigned.

Assigning Excitations


The next task is to assign the waveport excitations.  To do this, perform the following steps.
HFSS_microstrip/waveport_excitation.png
  • In the pop-up dialog box that appears, under "Integration Line", click the field that says "None", and select "New Line."
  • Click the "Fit Selected" icon at the top of the screen, to zoom in on Rectangle1.
HFSS_microstrip/FitSelected.png
HFSS_microstrip/waveport.png
  • Note: you have just defined the two ends of the expected electric field lines - the geometry across which we expect a voltage difference.
  • In the pop-up window, under "Integration Line", you should now see the word "Defined."  If you do not, something went wrong with your assignment.  Choose "New Line" again, and repeat the definition process.
HFSS_microstrip/AssignedWaveport.png
  • Click "Next"
  • Click "Finish"
You have just completed the assignment of the first waveport.  We will repeat the same process on the second waveport.  To do this, perform the following steps:
  • In the project tree, click Rectangle2 to select it.
  • Right-click in the project viewing window, and click "Assign Excitation" -> "Wave Port"
  • In the pop-up dialog box that appears, under "Integration Line", click the field that says "None", and select "New Line."
  • Click the "Fit Selected" icon at the top of the screen, to zoom in on Rectangle2.  You may also rotate the model, if you wish, to get a better viewing angle.
  • Click on the center of the bottom edge of Rectangle2 (the icon will turn into a triangle when you are hovering over the exact center)
  • Click on the center of the bottom edge of the microstrip trace (again, the icon will turn into a triangle when you are hovering over the exact center).
  • Note: you have just defined the two ends of the expected electric field lines - the geometry across which we expect a voltage difference.
  • In the pop-up window, under "Integration Line", you should now see the word "Defined."  If you do not, something went wrong with your assignment.  Choose "New Line" again, and repeat the definition process.
  • Click "Next"
  • Click "Finish"
You have just completed the assignment of the second waveport.  Your model is now complete.

Analysis

To perform the simulation, we need to set up the analysis options.  Do the following steps:
1. Right click on Analysis in the Project Manager, and select "Add Solution Setup"

2. Under the General tab:
    (a) Set the solution frequency to 4 GHz,  This is the frequency at which HFSS will refine the field solution. For a simulation, like this one, that is not expected to be resonant, the solution frequency should always be as high or higher than the highest frequency of the planned the planned frequency sweep.  In other words, for this solution frequency, we will not expect our solutions to be valid at any frequencies above 4 GHz.
    (b) Set the maximum number of passes to 30
    (c) Set maximum Delta S to 0.01 (this sets an upper limit on the uncertainty of our solution).
3. Under the Options tab:   
    (a) Set the Maximum Refinement per pass to 20 %
    (b) Set the Order of Basis Functions to Second Order
4. Under the Advanced tab:
    (a) uncheck the "Save Fields" box.
Click `OK'.

Perform the following steps to set up the frequency sweep:
1. Under the Analysis item in the Project Tree (expand it, if necessary), right-click on Setup1.
2. Select Add Frequency Sweep.
HFSS_dipole/adfreq.png
3. In the dialog box that pops up, set "Sweep Type" to Discrete.
4. Set "Distribution" to Linear Step.
5. Set start frequency to 3 GHz.
6. Set end frequency to 4 GHz (note: the upper limit of the sweep should never be higher than the solution frequency, unless the solution frequency is exactly the same as the resonant frequency.).
7. Set the step size to 0.05 GHz.
HFSS_microstrip/FreqSweep.png 
8. Click "OK."



Final Checks and Running the Simulation

Save the project by clicking on the save icon at the top of the screen.
Select HFSS => Validation Check... to ensure the project is prepared for simulation (click close).
HFSS_dipole/ValidationCheck.png
Right-click Setup1 under Analysis in the project tree and select Analyze to begin the simulation. At this point the progress window should show the progress of the simulation, beginning with the mesh generation.

Check for Proper Simulation Resolution

Once the simulation completes, there are a few things you should check before looking at the results, to make sure that you can expect the results to be accurate.  
1. Check that the final message in the Message Manager says "Normal completion of simulation on server:...."
HFSS_dipole/MessageManager.png
2. In the Project Manager, under "Analysis", right click on "Setup1" and click "Convergence".  In the pop up window, change the view from "Table" to "Plot"
HFSS_microstrip/convergence.png


3. Check that the simulation is marked as "CONVERGED"
4. Check that the plot ends with a downward trend (as shown above).  It may be jagged in the beginning (on the left), but if it is jagged at the end (jumps suddenly from a high value to a low value), this may indicate a false convergence.  If this happens to your simulation, you will need to modify your simulation as follows:
    • In the Project Manager, under "Analysis", right-click on "Setup1" -> "Properties".
    • Click on the "Options" tab, and change the "Minimum Converged Passes" to 2 (or 3).
5. Re-analyze the simulation, and repeat the checks above.


View the Simulation Results

  Now you are ready to view the simulation results.

To view the S-parameters of the transmission line, 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 S-parameter -> S(1,1)->'dB' and click New Report
4. Select the S-parameter -> S(2,1)->'dB' and click Add Trace
5. Click Close.
HFSS_microstrip/Sparams.png
Notice that there are no abrupt changes in the S-parameters.  This indicates that the structure is non-resonant (at least at these frequencies) and supports the decision we made in the beginning to set the solution frequency to the highest frequency of the sweep.

You may also check that the characteristic impedance of your microstrip matches your design, by performing the following steps:
1. Right-click on `Setup' in the project manager
2. Click `Matrix data'
3. Check the box marked `Zo'
4. Under the column `Port Zo', check that the values marked for the characteristic impedance Zo are approximately what you calculated it to be. Note that the two Zo values correspond to the two ports of the network.
HFSS_microstrip/portimpedance.png

Note: Before using a plot in a report, ensure that:

  • All axes are labeled, with appropriate text sizing (unlike the images above, which are screenshots of the default graphs produced by HFSS)
  • All plots have reasonable axis limits (using the wrong axis limits can greatly affect how your results are perceived)


Copyright 2021, Kathryn Leigh Smith.  All rights reserved.
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