Simulation can be expensive, especially when adding essential function modules to a suite of programs. But for those hoping to perform some basic linear circuit and system simulations, cost need not be an issue, especially when considering Ansoft Designer SV (Student Version) software from Ansoft Corp. (Pittsburgh, PA). The suite of linear-simulation tools provides a long list of capabilities, from a complete set of linear-component electrical models to filter synthesis and physics-based distributed models, all encompassed in a modern integrated design suite—and it's free.

Ansoft Designer SV features the same desktop design environment utilized by the company's commercially available high-frequency electronic-design software, Ansoft Designer and Version 9 of the electromagnetic (EM) simulator High-Frequency Structure Simulator (HFSS). The environment includes fully integrated schematic and layout editors, dynamic project and solution managers, advanced results graphing, post-processing, a three-dimensional (3D) viewer, and a scripted and parameterized footprint editor. The state-of-the-art solver technology includes a complete linear-circuit simulator, a complete set of physics-based linear distributed transmission-line models, discontinuities, commercial components, and ideal circuit elements. Circuit-simulation results includes S, Y, and Z parameters, VSWR, insertion and return loss, gain, stability circles, noise figure, and group delay.

The integrated schematic capture and layout editors in Ansoft Designer SV operate on a single database, which allows physical and symbol views of a design to be fully synchronized, and designers can work from either view. A change made to any component parameter is applied to the component and automatically updated in both views. This makes it possible to get a real-time understanding of a circuit's electrical performance and physical layout.

Ansoft Designer SV also provides a full set of transmission-line models and a utility to characterize them before placement into a circuit or system. Transmission lines and couplers can be analyzed and synthesized in seconds using the integrated transmission-line (TRL) utility. By entering the electrical properties, the tool will automatically synthesize the physical description or vice versa. The TRL utility includes microstrip, stripline, and coplanar waveguide mediums.

Like its commercially available version, Ansoft Designer SV offers a very large set of distributed models for common and obscure geometries required for RF and microwave circuit design. Mediums include microstrip, stripline, coplanar waveguide, grounded coplanar waveguide, and coaxial cables. The physics-based models perform over wider frequency ranges making them ideally suited for millimeter-wave applications. The software also uses Ansoft's approach to discontinuity modeling, which is derived from a solvable electromagnetic problem. In doing so, greater accuracy is assured.

The most accurate approach to characterizing full-wave, uniform, coupled transmission lines is by the spectral-domain method employed by Ansoft Designer SV. This method is fast and accurate and boasts a wider range of applications than other methods.

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The result is that Ansoft Designer and Ansoft Designer SV's multiple coupled-line model has become a workhorse in the analysis of interdigital microwave filters, coupled "hairpin" filters, edge-coupled filters, combline filters, and interdigital capacitors. The algorithm allows the distributed component model to provide a highly accurate characterization of up to 20 parallel transmission lines constructed in microstrip, stripline, and suspended stripline mediums. The technique accounts for the coupling that occurs between adjacent and non-adjacent lines for any arbitrary line widths and spacing and has been expanded to include lines on up to five different substrate layers.

The component library in Ansoft Designer SV contains more than 80,000 discrete linear and nonlinear surface-mount components based on manufacturer-supplied model data. Models can be added to this library by importing from vendors or by creating them using Ansoft's internal model development toolkit. All vendor parts include model data and symbol representation as well as package footprint data for direct layout implementation and floor planning. The component libraries are easily expanded by utilizing the component/library manager interface.

Ansoft Designer and Ansoft Designer SV's neutral model format (NMF) supports truth-table modeling and parametric S-parameters and links these tools to parametric results provided by HFSS. Ansoft Designer and Ansoft Designer SV can read and write industry-standard formats, such as the SnP or FLP formats. The linear table-based data addresses the need for transferability of linear network data, which can be arbitrarily parameterized, such as physical and material parameters. These formats offer multi-port network data (S-, Y-, Z-parameters, etc.) as a function of frequency. Supporting these formats allows HFSS or third-party tools to generate model data that can be incorporated into an Ansoft Designer SV simulation.

Ansoft Designer SV also includes the most powerful tool available for matching and network extraction. Through a Smith Chart tool called "Smith Tool," an exact match can be created and exported directly into the schematic. The tool plots critical design information, such as stability, gain, and noise circles, and then lets the user select ideal lumped and distributed components to interactively design an impedance transformation network that can be directly inserted into a hierarchical design. The Smith Tool works directly with the load-pull-analysis feature that enables matching networks based on large-signal circuit behavior to be created.

Physical layout also is available and lets the user draw patterns composed of simple geometric shapes, such as rectangles, polygons, circles, and arcs that can be placed on the circuit board or IC. Components in the design often consist of groups of basic shapes, called "footprints," which are the "stamp" of the component on one or more board/IC layers. If the footprint depends on component parameters such as transistor width, then it is a "parameterized footprint," which requires a capability beyond simple scaling to describe it. Ansoft Designer SV's layout tool eliminates the need for a user to program in C to manipulate footprint information. The tool embraces a standard interpretive language (Visual Basic® script or JavaScript®) and allows the script to be developed directly in the CAD environment so that changes can immediately be tested without any compilation or program restart.

Ansoft Designer SV is free of charge and available for download at www.ansoft.com/ansoftdesignersv. The program runs on a personal computer (PC) with the Windows 2000 or Windows XP operating system. Ansoft Corp., Four Station Square, Pittsburgh, PA 15219-1119; (412) 261-3200, FAX: (412) 471-9427, Internet: www.ansoft.com.