ANSYS HFSS - Tips, Tricks, and Troubleshooting

In this article, we plan to provide some in-depth information and best practices you'll need to know for solution setup and meshing in ANSYS HFSS (part for ANSYS Electronics Desktop) in Nimbix. But, just in case you're here because of a specific error, we want to provide some tips and tricks around common errors and troubleshooting first. After that, we'll take a deeper dive into the solution setup and meshing mode.

Tips and tricks for resolving ANSYS HFSS errors and mesh refinement

1. HFSS solver failed to start while performing a frequency sweep.

A common problem in running HFSS using ANSYS AEDT is the failure to run a frequency sweep even though the main solution solves.

The problem resides in the HPC settings and the license you may have purchased. The discrete frequency sweep fails if you do not have the license required to do a 'distributed solve' - where multiple frequencies are solved in parallel. The HPC settings in ANSYS AEDT are found under Tools > Options > HPC and Analysis Options. The default settings in NIMBIX are automatic (notice the checked box in front of the "Use Automatic Settings" option that you can access clicking the "Edit" button in the HPC and Analysis Options window).

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The solution is to uncheck 'Auto-HPC' from the HPC settings and make sure only 1 task is set and re-run the frequency sweep. It will take a much longer time to do a discrete sweep, so it’s recommended to use an interpolated sweep instead of a discrete sweep.

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2. Port refinement, process hf3d error. Port “1” eliminated.

An error that has been encountered upon augmenting an already solved HFSS design relates to port elimination. The solver throws the following error in the message manager window (located at the bottom of the window):

[error] Port refinement, process hf3d error: Port '1' eliminated.  Likely causes are Material Override or faulty material assignment.

This problem can have several causes:

  • Port “1” is overlapping a material (other than PEC) or dielectric materials are overlapping in your design (effect of material or object intersection)
  • Boolean operators such as Unite or Subtract did not perform the Boolean operation as intended
  • Construct a nonmaterialistic boundary or structure around the outer design and then select port.

One possible solution is to enable the material to overwrite in the HFSS to Design Settings if the option is not already selected. PEC will take priority over the dielectric.

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You should also investigate checking your analysis and design by using HFSS Modeler to Model Analysis and Modeler to Model Preparation. This allows you to verify misalignments, surface mesh, etc.

Note: Port “1” is used here generically for the port ID in the HFSS design affected by this generic error.

3. AEDT: Unable to open some designs from the project file.

A watch out for storing AEDT/HFSS files post-completion for further reference. If the AEDT throws the following error:

Unable to pen some designs from the project file. This may be caused by insufficient licenses for the types of designs in the project, or by a corrupted project file.

The problem resides missing project folders. Check your /data/Project_Folder folder to make sure you have the .aedb (in some versions) and .aedtresult folders in your Project_Folder. If the .aedb (in some versions) folder is not present in the project directory, the model will get corrupted and will be unable to open.

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The solution is to open the projects and re-run the analysis. For future references, archive the folder post-completion for storing. Use File to Archive option to archive your files for future use. Both models and solutions/results will be available for future use.

NOTE: The path /data/Project_Folder will be different for every user and is used here as an example of the current directory in your /data folder.

4. AEDT HFSS mesh refinement tips and tricks.

A few pointers on generating a high-quality mesh:

  • For slender (thin and long) geometry, insert a mid-surface in order to add another mesh refinement to the model. The mesh generator will be forced to add nodes on that mid-surface and therefore almost double the number of elements on the thickness
  • Always mesh at a higher frequency than the working frequency. A high frequency will generate a finer mesh than a low frequency. For a frequency sweep analysis, specify the mesh to be at the highest frequency (the higher the frequency, the finer the HFSS mesh)

HFSS – ADVANCED MESHING AND SOLUTION SETUP

Let’s talk about solution setup details and the background work related to a successful HFSS analysis. There are two aspects of the typical HFSS solution: meshing and the frequency sweep. In ANSYS HFSS, the two choices for the solution setup are “Auto” and “Advanced”. The options can be accessed either from the Simulation tab (HFSS Solution Setup) or from the Analysis option in the project manager.

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In the HFSS Solution Setup Auto option, both the meshing and the frequency sweep are specified on the General tab of the Driven Solution Setup dialog box.

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The slider (more about the mesh than anything else) allows the user to tradeoff between speed and accuracy and eventually sets up the HFSS meshing parameters.  The sweep type impacts the solution points. Using a Discreet sweep instructs the solver to compute the solution at all discreet frequencies. Interpolating sweep type does not compute the solution at every frequency This is again a tradeoff between solution accuracy and speed. For large numbers of frequency points, interpolating sweep will run much faster than discrete sweep.

The HFSS Solution Setup Advanced option separates the meshing and the frequency sweep into two distinct parts: Driven Solution Setup and Frequency Sweep setup.

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The Driven Solution setup offers several options to deal with tradeoffs between solution and accuracy as well as how to approach non-linear solver. The HFSS solver uses adaptive meshing (mesh is refined/tuned to capture the model’s electrical performance). The “Maximum Delta S/Energy” parameter is used to measure the model error. The Driven terminal solution will use the S parameter and the driven modal solution will be assessed in terms of energy. The maximum number of passes ensures that the solution finishes (non-convergence can lead to infinite loops and waste of valuable time and resources). Pay attention to the Message Manager at the bottom of the HFSS GUI as well as the Profile dialog box available by right-clicking on Setup in the Project Manager.

Touching a bit on the frequency sweep nuances, ANSYS HFSS offers several options and distributions for the frequency sweep:

  1. Interpolating: Good for wideband -no fields (runs faster than discreet but does not always provide you with the solution at desired points)
  2. Discrete: Generates and can save fields at every frequency.
  3. Fast (ALPS - WARNING): Recommended only for the very narrow frequency sweeps when field information is necessary for many frequency points.

There are several distributions for frequency sweep that one can use: Linear step (frequency between steps), Linear Count (total number of frequencies, spread over the entire specified frequency range), Log Scale, Single Point, and Single Point Sweep.

Driven Modal and Driven Terminal solution setups in HFSS

Before creating your model, as a user, you need to specify the solution type you want HFSS to compute. There are 4 solution types available in ANSYS HFSS with the first 2 being the most used:

  1. Driven Terminal: recommended for models and simulations that deal with signal integrity such as transmission lines. Choose the Driven Terminal solution when you want HFSS to calculate the terminal-based S-parameter of single and multiple conductor transmission line ports. The S-matrix solution will be expressed in terms of voltages and currents on the terminals.
  2. Driven Modal: most used in HFSS. Choose the Driven Modal solution type when you want HFSS to calculate the modal-based S-parameter of passive, high-frequency structures such as microstrips, waveguides, and transmission lines. The S-matrix solutions will be expressed in terms of the incident and reflected powers of waveguide modes.
  3. Transient
  4. Eigenmode
  5. Characteristic mode
  6. SBR+

Access the solution type from the HFSS to Solution Type with the Network Analysis being the default for the Driven solution.

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Regardless of the type of solution you chose, there are a couple of advanced options to control your convergence (tradeoffs between speed and accuracy): maximum Delta S (main factors), the maximum number of passes (controls the number of steps to reach convergence, and avoids infinite loop). These two parameters will determine when HFSS stops the adaptive solution process. Maximum refinement per pass is another parameter used in the solution process but has no impact on terminating the solution.

HFSS MESHING EXPLAINED – TIPS AND TRICKS

ANSYS HFSS on NIMBIX uses tetrahedral adaptive meshing to create and refine the mesh used in finite element simulations. In this process, the mesh is refined iteratively for each simulation step and is localized to regions with high electric field solution error. ANSYS HFSS uses a tetrahedral mesh (default). The adaptive solution meshing process follows the following structure:

  • HFSS generates a geometrically conforming, tetrahedral mesh automatically.
  • HFSS's iterative meshing algorithm solves the fields of the model and intelligently refines the mesh until S-parameters converge below a user-defined threshold, Maximum Delta S.
  • The user defines frequency or frequencies at which adaptive meshing is performed.
  • After each solution, tetrahedral elements are “graded” for their accuracy to Maxwell's Equations.
  • The user defines the percentage of “bad” tetrahedral elements to be refined after each pass (30% Default).

There is little interaction required from the user, but the software allows you to refine your mesh using the slider during the setup. You can also use the mesh settings slider or inserting per-object settings (length based or curvature) by right-clicking on the mesh and using either Initial Mesh Settings or Assign Mesh Operations:

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In addition to Single frequency meshing, HFSS offers Broadband and Multi-Frequencies meshing. If you want to improve the reliability of broadband devices and get more accurate solutions, you can specify the Broadband option for the solution frequency setup. Access the other meshing modes by Expand Analysis node to Right-click Setup to Properties.

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This setup allows HFSS to determine the appropriate frequencies where to adapt the mesh. You need to specify the band limits (Low and High) and the solver will best choose the frequencies where to adapt the mesh (minimum 3 frequencies are chosen within the specified range). Adaptive meshing at additional frequencies requires the HPC feature to unlock additional computational resources.

Parting thoughts, how do you inspect the surface mesh in HFSS: The easiest way is from the model window: Select the object/surface you are interested in, to Right-click on the surface à Plot Mesh. You will be prompted to enter a name for this mesh plot (be specific if you want to retrieve later) and your mesh will be plotted on the selected object.

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