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Guidelines for Meshing in Ansoft HFSS

Topics Covered Adaptive Meshing Overview

Description Choosing Adapt Frequency

Driven Solution Mesh Guidelines Frequency and Adaptation Criteria Mesh Settings

Seeding and Manual Refinement

Eigenmode Solution Mesh Considerations

Examples

Adaptive Meshing Overview Adaptive meshing is performed at a single

frequency specified by the user Model behavior is explored systematically

by solving gradually denser meshes Mesh density is added where necessary,

not indiscriminately Solution progress is evaluated after each

adaptive mesh is solved based on the convergence criteria

Criterion 1: Number of Passes Criterion 2: Maximum Delta-S

Maximum Delta-S is the worst-case vector magnitude change of any S-parameter’s solution from Pass N as compared to its solution from Pass (N-1).

Per-Parameter criteria also available 95% of HFSS Project Setups Should Use

at Least Some Adaptation!!!

Note how mesh density is greater in the region between filter posts, where wave energy is superposed by reflections

Filter Posts

Overview: Tetrahedral Refinement

Tetrahedral Refinement is based on percentage of the prior mesh

This maintains a consistent “lever arm” for solution changes

If the mesh grows too quickly, subsequent solutions may be accurate, but take excessive computer resources and time

If the mesh grows too slowly (e.g. a fixed tet growth count, rather than percentage), the “lever arm” shrinks with respect to the problem, and solutions may appear to converge before an accurate solution is reached

The default Tet. Refinement value is 20%. This is adequate for the vast majority of HFSS projects.

Convergence data below is shown for a model usingthe default 20% tetrahedral refinement criterion. Had the number of new tetrahedra been kept level, the solution would likely have exited on or around Pass 5.

Choosing Adapt Frequency Proper adaptive frequency selection is

very important to solution accuracy Initial mesh and subsequent adaptation

are in part wavelength dependent Despite convergence, the mesh may be

too coarse for good results at higher frequencies with a significantly smaller wavelengths

Adaptive frequency recommendations: For single-frequency or narrow-band

solutions (insignificant change in ): Adapt at frequency of interest

For wide-band solutions: Adapt between the middle and high ends of the band (smaller wavelength)

Caution: If you want to view behavior over a specific band, but the device’s response is more narrow, adapt within the device’s bandwidth

For this band-pass filter, adapting here will result in mesh refinement inside the filter structure, capturing its behavior...

...while adapting here may only permit tetrahedral refinement at the ends of the filter, where the energy is beingrejected.

Driven Solutions: Mesh Selection

Solutions can begin with several different meshes For the first solution performed, the initial mesh is

generally used Initial mesh uses lambda refinement by default,

and can also utilize seed refinement If adaptive passes have been solved (at one or more

adaptive frequencies) the current or previous mesh can be used

Current allows continuation of the adaptation process if the desired number of passes was reached before the desired delta-S value.

Previous allows the user to take a step back from an overly large mesh

For the first or subsequent solution(s), the user may elect to create a manual mesh

The Mesh Options button allows further definition for either the Initial or the Manual meshes

Initial Mesh Options: Seeding

Lambda Refinement is the default initial mesh setting

The mesher will assure that tetrahedral edge lengths are on the order of /4

The Define Seed Operations button accesses the graphical meshing interface

Here, the user selects objects, object faces, or box subregions within the model to apply mesh seeding

Seeding is application of additional ‘vertices’ with a specified spacing within the model

Seeding is acted upon when the solution process begins meshing the project

Initial Mesh Options: Seeding Method

Select an object, face, or combination thereof in which a seeding parameter is desired.

From the Seed menu, select Object to seed the volume, or Object Face to seed surfaces

Box does not require prior object selection; the interface will prompt for the box location in which to apply seeding.

Define whether seeding should be by (tet edge) length, (triangular face) area, or (tetrahedral) volume.

Define seed dimension (in the active drawing’s units) and tetrahedral count restraints

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Manual Mesh Options

To create a Manual Mesh, first select the mesh to use

Options will be the same as those available for solution, depending on the project’s current status

The Define Manual Mesh button activates the graphical meshing interface

This is the same interface used for mesh seeding. However, in Manual Mesh mode the seeding operations are disabled

The interface is used to directly generate a mesh based on user inputs

Manual meshing options are identical to the seeding options

Manual Mesh Options: Procedure

Select an object, object face, or combination thereof

From the Refine menu, pick whether you want to refine the mesh in the object volume or on its faces

Box requires no prior geometry selection

Select whether to refine by length, area, volume, etc.

Provide refinement dimension criteria and mesh growth limit

Continue through all objects to be manually refined

NOTE: Since you are generating a MANUAL mesh, Lambda Refinement will NOT be performed on any objects you neglect!

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When to Seed or Manual Mesh

Some model types do not solve efficiently using adaptive refinement alone; these options can speed convergence

High Dielectric Constants: Materials with high dielectric constants solve better if seeded or manually meshed due to their smaller effective wavelength

Locally Strong Field Gradients: Some structures, such as the capacitively-loaded cavity on the lower left, have features whose influence on the behavior is not wavelength-driven

Extreme Aspect Ratios: Models with very high aspect ratios are harder to mesh with ‘high quality’ tetrahedra; manual or seed-based assistance can improve the mesh quality and resultant matrix condition

Cylindrical Dielectric Resonator in Cavity. Dielectric Puck has r = 90. Wavelength is only 1/10 that in surrounding air volume! Seeded to /4 in the material to compensate.

Cavity structure at right has post extending from bottom to almost touch the top. The narrow capacitive gap between the post end face and the cavity end itself has a virtual solid defined to allow manual meshing where fields will be very strong

Eigenmode Meshing Options Eigenmode Solution

Starting Mesh selection and Initial and Manual Mesh Options are identical to those for driven solution

Initial Mesh Options accesses the mesh seeding interface

Manual Mesh Options accesses the graphical mesher for direct user-refined meshing

Seed coupling structures that are << Seed/manually mesh reactive regions

Example 1: Reactive Coupling

Initial mesh for lead-frame that has high aspect ratio objects

Example 2: Lead-Frame

Example 2: Lead-Frame (detail)

Initial mesh shows long, skinny surface triangles

May need seeding or manual refinement to fully capture field behavior

Note: It is always a good idea to try adaptive solutions to evaluate if user modification of the mesh is required.

Example 3: Flat Panel LCD

Many thin objects closely packed and stacked

Example 3: Flat Panel LCD (detail)

Example 4: Spiral Inductor

8 turn spiral 2 micron thick traces 0.8 micron gap 200 micron square

Example 4: Spiral Inductor

Initial mesh is sparse on traces (in gap region also)

Seeding/Manual meshing may be needed to characterize spiral properly

Due to small electrical size, may need:

Skin Depth meshing Solve Inside conductors

If electrical size << Create large dense mesh Use ZERO_ORDER_MODE

for the solution