Electromagnetic (EM) circuit simulation has grown steady in importance to RF/microwave designers. Long an accurate method of predicting high-frequency behavior, early EM simulators were nonetheless difficult to use because of limited design functions. The latest version (Version 9.0) of the High-Frequency Structure Simulator (HFSS) from Ansoft Corp. (Pittsburgh, PA), however, takes EM-based design a long stride forward to increase design productivity, starting with a new highly-integrated architecture, Windows-type user environment, greater automation, and a long list of new features.
The most noticeable change to Version 9.0 of HFSS is its new Windows-oriented design environment, the Ansoft Desktop. Nearly every function in the process of design capture, analysis, or post-processing can be invoked from this desktop. Design variables are available both globally and locally, seamlessly linking multiple designs within a single project. The desktop is inherently parametric, allowing parameters to be easily applied to material properties, boundary conditions, and solution control. As a result, design modifications are simplified since there is no need to redraw a design or specify solution setup information with every small change. Any design or solution parameter can be examined before or after analysis, and graphical animation allows a parameterized geometry to be visualized when changing physical design parameters. The same parametric display, including field visualization, also is available after analysis is performed.
Design capture is more complete and easier to perform in Version 9.0. In addition to 3D geometry generated within the Ansoft Desktop environment, designs created in either a two-dimensional (2D) electronic-design-automation (EDA) layout tool or a 3D mechanical computer-aided-design (CAD) package can be imported, provided the files are industry standard file types (SAT, STEP, and IGES). Ansoft offers an add-on product called AnsoftLinks that allows 2D EDA layout designs to be imported, creating a direct link to EDA tools from Cadence, Mentor Graphics, Synopsys, and Zuken.
The solid-modeling capabilities in HFSS v9 now include geometric object translucency (see figure) along with a "Windows-standard" color palette. Objects can be selected by clicking on them in the solid-modeling window or through an associated history tree, which is a file browser-type display of the geometry, its hierarchy, and its history. The user also has the ability to spin a geometry, giving the model a constant angular rotation so that all sides can be seen.
The local coordinate system of an object is easily associated with an existing face in the geometry in Version 9.0, allowing designers to intuitively draw "stacked" objects. When an object is selected, its properties may be edited in a window or dialog box. These properties include materials, color, and geometric parameters.
Version 9.0 of HFSS allows multiple objects to be selected and their common attributes edited in a single operation. The geometric copy-paste function allows groups of objects to be copied within a design or between separate designs. This operation transfers all user-defined material properties and design variables. All associated boundary conditions and port definitions (including definitions for modes, lines, de-embedding, etc.) as well as entire designs may be copied between projects.
A variety of solution setup options are available to designers in Version 9.0, since designers may want to generate analyses with various levels of accuracy, analyses that are adaptively refined at specific frequencies, or with appropriate seeding densities. A design can include any number of solution setup specifications, and they can be launched as one analysis or as separate analyses. Each setup may have multiple frequency-sweep specifications. For example, a localized frequency sweep may be augmented by a few discrete frequency samples outside this range to sample the high- or low-frequency behavior of a band-limited design.
Analysis setup and control are intuitive, and all configuration and control parameters are accessible from within the Ansoft Desktop. The total number of adaptive solution refinement passes can be specified as well as a minimum number, or the number of consecutive adaptive refinement passes at a particular accuracy level that defines adequate convergence.
To accommodate the broadband behavior of circuit-board materials such as FR4, Version 9.0 of HFSS allows arbitrary frequency dependence to be specified. The same is true for thin-film resist materials, which are handled with frequency-dependent boundary condition specifications. The ability to add frequency dependence to existing surface-based thin layer material approximations makes it possible to determine the broadband behavior of thin dielectric layers or coatings.
Ansoft has also integrated its Optimetrics parametrics and optimization engine into the Ansoft Desktop. Optimetrics performs statistical analyses, parametric sweeps, optimization, and sensitivity studies. Statistical analysis along with sensitivity studies provide insight into the performance of fabricated designs, which is well suited to automated design refinement and design-for-manufacturing approaches.
Version 9.0 of HFSS offers expansive post-processing capabilities. General reports plot frequency-swept data and parameter-swept responses as S-parameter results, characteristic impedance and propagation constants, and Y- and Z-parameters. Data is easily manipulated by built-in equations to generate derived quantities that are easily specified prior to analysis and are available for use as optimization goals. Arbitrary families of traces may be displayed on the same plot. Two-dimensional (2D) data can be extracted from arbitrary n-dimensional, swept-parameter data and displayed as families of traces or as 3D color-shaded plots. All 2D reports may be dynamically updated as solutions are performed, which allows arbitrary convergence data to be viewed as analyses are performed.
Version 9.0 allows local and radiated field solutions to be visualized; the program includes many built-in quantities for display of field data, such as magnitudes, individual scalar components, and specific absorption rate (SAR). The displays are all available at individual points, along lines, on surfaces, and within volumes, and new display types and control mechanisms have been added as well. Version 9.0 can also animate any swept variable, frequency or material parameters, and simultaneous geometric and field animations are supported.
Version 9.0 of HFSS makes it simple to export results, including circuit parameters (S-, Y-, and Z-parameters), and impedance and propagation constants, to industry-standard file formats. The model export ability of Ansoft's Full-Wave Spice model (an add-on for HFSS) has been significantly improved, and SPICE models are available for HSpice, PSpice, and general SPICE analysis engines. Convolution-based and pole-zero models, partial fraction expansions, and lumped equivalent circuits are available as well. Ansoft Corp., Four Station Square, Pittsburgh, PA 15219-1119; (412) 261-3200, FAX: (412) 471-9427, e-mail: [email protected], Internet: www.ansoft.com.