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Lecture 3-1: HFSS 3D Design Setup

Introduction to ANSYS HFSS

Boundaries for 2D Sheets or Faces

2D Sheets/Faces Assign Boundaries

How to model conductors (3D vs 2D)?

• Conductor Thickness
  • In many applications, the thickness of a conductor on a PCB is not critical to the performance of a device. The efficiency of the solution problem can be increased by removing the thickness of conducting layer. The following slides compare examples of using a 3D and 2D conductor for a patch antenna and an edge coupled filter.
  • In summary, the patch antenna shows no difference in performance for a 3D vs 2D conductor because the edge effect of the conductor does not impact the device performance. In the case of the coupled line filter, the edge effects of the trace thickness make a noticeable difference and needs to be included. Conductor as a 3D Solid (^) Conductor as a 2D Sheet

Trace Thickness Effects

• Conducting traces often modeled as 2D objects for electromagnetic simulations
  • More computationally efficient since fewer meshing surfaces required
  • Good approximation for many structures operating in skin depth regime
• Planar filters which use edge coupling to operate require 3D objects (finite thickness) for modeling

conducting traces

• Applications whose performance depends upon closely-coupled traces
• End-coupled, parallel-coupled, hairpin filters, etc. Patch antenna modeled with 2D sheet Patch antenna modeled with 3D object Frequency response of both models Edge-coupled filter modeled with 2D sheets Edge-coupled filter modeled with 3D objects Frequency response of both models

Material Properties for 2D sheets/faces

• Surface Loss Modeling
  • Finite Conductivity – A Finite Conductivity boundary enables you to define the surface of an object as a lossy (imperfect) conductor. HFSS applies this boundary for lossy metal materials. To model a lossy surface, you provide loss in Siemens/meter and permeability parameters. Loss is calculated as a function of frequency. It is only valid for good conductors. Forces the tangential E-Field equal to Zs(n x Htan). The surface impedance (Zs) is equal to, (1+j)/(), where: -  is the skin depth, (2/())0.5^ of the conductor being modeled,  is the frequency of the excitation wave,  is the conductivity of the conductor,  is the permeability of the conductor Manually define Conductivity (Default is Copper) Use Material Definition to define Conductivity Adjust the Conductivity based on conductor thickness

Surface Approximations for Manufacturing/Component Definitions

• Surface Loss Modeling
  • Layered Impedance – Multiple thin layers in a structure can be modeled as an impedance surface. See the Online Help for additional information on how to use the Layered Impedance boundary.
  • Lumped RLC – a parallel combination of lumped resistor, inductor, and/or capacitor surface. The simulation is similar to the Impedance boundary, but the software calculate the ohms/square using the user supplied R, L, C values.

Which Radiation Surface should I use?

• Radiation Surface Selection
  • The following slides cover details about each boundary and are really intended for those doing Antenna, Array, or RCS simulations. Effects such as incident angle to the radiation surfaces are compared so a user can discern the differences in performance and select the appropriate boundary for the application.
  • For everyone else, below is a simple selection table for choosing Radiation Surfaces. In general, a Radiation Boundary is the easiest to setup and is a good starting point.

Antenna/EMI

Radiation

PML

FE-BI

Array/RCS

PML

FE-BI

SI

Radiation

Radiation Boundary

• Mimics continued propagation beyond boundary plane
  • Absorption achieved via 2nd order radiation boundary
  • Absorbs best when incident energy flow is normal to surface
  • Distance from radiating structure
    • Place at least /4 from strongly radiating structure
    • Place at least /10 from weakly radiating structure
  • Must be concave to all incident fields from within modeled space Boundary is /4 away from horn aperture in all directions

Perfectly Matched Layer (PML)

• Perfectly Matched Layer (PML)
  • Fictitious lossy anisotropic material which fully absorbs electromagnetic fields
  • Two types of PML applications
    • “PML objects accept free radiation” if PML terminates free space
    • “PML objects continue guided waves” if PML terminates transmission line
  • Guidelines for assigning PML boundaries
    • Use PML setup wizard for most cases
    • Manually create a PML when base object is curved or inhomogeneous PML Setup Wizard Parameters to Define: 1. Define PML Thickness 2. Specify Minimum Frequency (From Sweep) 3. Specify Minimum Distance from Antenna to PML (Measure in Model)

PML: Incidence Angle Dependency

PML functions well for incident angles less than 65 °- 70 ° (^) Better absorption leads to better consistency in the patterns

PML

PML

Excitations for 2D Sheets or Faces

2D Sheets/Faces Assign Excitations

Excitations

• Excitations
  • There are many choices and important details regarding excitations that can make HFSS seem difficult to use. Most of the challenges faced with selecting and apply excitations result from the ability to create excitations that would not be possible in the physical world. Because this is just an introduction to HFSS, the following discussion will simplify the choices to establish a reliable and repeatable simulation process. If you need to solve designs outside of this introductory discussion, see Appendix 6.3 3D Excitations for an advanced discussion on the powerful excitation options available within HFSS.
• Port Selection: When to select a Wave Port or Lumped Port?
  • Wave Ports: Use for closed port definitions. Examples of closed structures are Coax and Waveguides.
  • Lumped Ports : Use for all other port definitions.
    • An exception to this guideline would be to use Wave Ports for Stripline
• Solution Type: When to select Driven Modal or Driven Terminal?
  • Driven Terminal is typically faster to use because HFSS automates much of the setup. In general Driven Terminal should be used for any TEM/Quasi-TEM structures such as coax, microstrip, stripline, or co-planar waveguide. In the case of a design that only uses Lumped Ports, the resulting s-parameters will be identical in Driven Terminal and Driven Modal, therefore the ease of setup is usually the determining factor.
  • Driven Modal is required for Waveguides.
    • In some cases, the fields post-processing might be easier when the excitations are defined in terms of Power (Driven Modal) instead of Voltage (Driven Terminal).

HFSS Excitation Methods

Driven Modal

• Fields based transmission line interpretation
• Port’s signal decomposed into incident and

reflected waves

• Excitation’s magnitude described as an incident

power

Driven Terminal

• Circuit Based transmission line interpretation
• Port’s signal interpreted as a total voltage (Vtotal

= Vinc + Vref)

• Excitation’s magnitude described as either a total

voltage or an incident voltage

• Supports Differential S-Parameters

Modal Propagation

• Energy propagates in a set of orthogonal modes
• Modes can be TE, TM and TEM w.r.t. the port’s normal
• Mode’s field pattern determined from entire port geometry
• Each Mode has its own column and row in the S, Y, and Z parameters

Terminal Propagation

• Each conductor touching the port is considered a terminal or a ground
• Energy propagates along each terminal in a single TEM mode
• Each Terminal has its own column and row in the S, Y and Z parameters
• Does not support symmetry boundaries or Floquet Ports

Excitations: Modal vs. Terminal

T1 (^) T Integration Line Mode 1 (Even Mode) Integration Line Mode 2 (Odd Mode) Terminal Transformation SPICE Differential Pairs Modal Port1 Port Port1 (^) Terminal Port T T T T 2 Modes 2 Modes