Software-based models of electronic devices and components for use in RF/microwave designs have long dealt exclusively with linear behavior. But with the increased use of multitone signals and advanced modulation formats in high-speed, high-capacity communications systems, the nonlinear behavior of some components has required the use of computer-aided-engineering (CAE) models and software that can account for the nonlinear effects.

At one time, simulators were all standalone tools, with different simulations needed to analyze a circuit or structure with an electromagnetic (EM) simulator. Modern simulation software is more typically a suite of tools that can link various forms of simulations together to analyze a circuit design.

Circuit simulators employ various mathematical techniques to provide the analysis power needed to predict the nonlinear behavior of a circuit. Under certain conditions, an active circuit (requiring bias) may generate excessive levels of distortion as a result of changes in bias energy, input power, or temperature. Modern communications systems, for example, must channel multi-tone signals used for digitally modulated transmissions. The presence of two or more tones through an active component such as an amplifier can lead to intermodulation distortion (IMD), and a nonlinear circuit simulator offers a circuit designer the opportunity to explore different "trouble spots" for a design.

A nonlinear circuit simulator also requires the use of nonlinear models to predict the behavior of different circuit elements, such as transistors or integrated circuits (ICs), under changing input power, temperature, and other conditions. these models typically come from semiconductor foundries, although they may also be created by CAE software suppliers and simulator users.

Earlier this year, for example, AWR Corp. made available a model library from Mitsubishi Electric for its Microwave Office suite of simulation tools. The model library includes both gallium arsenide (GaAs) and gallium nitride (GaN) devices, such as high-power and low-noise transistors and ICs, allowing designers to understand the behavior of their circuits at different power levels. This facilitates better optimization of designs for linearity and efficiency. AWR's circuit simulators also work with a wide range of nonlinear device models, including GaAs heterojunction-bipolar-transistor (HBT) IC amplifiers from TriQuint Semiconductor and unmatched GaN-on-silicon-carbide (SiC) power transistors from RF Micro Devices.

Although CAE tools such as awr's microwave office and CST studio suite from Computer simulation technology (CST), with its built-in APLAC for CST Design Studio simulator for linear and nonlinear simulations, provide effective nonlinear simulations, the most extensive set of models and nonlinear modeling capability may be found in the Advanced Design System (ADS) suite of CAE tools from Agilent Technologies. Agilent is a pioneer in what it calls X-parametersessentially, nonlinear S-parameters for modeling and analysis purposes.

Through the tireless work of Agilent's model developer, David Root, and contributors such as Larry Dunleavy, President of Modelithics, ADS has become a standard tool for nonlinear simulation. Modelithics recently announced support for the latest version of ADS (8.0), but also supports other commercial nonlinear simulators. The current Modelithics nonlinear device library includes PIN, Schottky, and varactor diodes from Avago Technologies, Infineon Technologies, Microsemi, Skyworks Solutions, and others.

ADS also offers the model W2305 X-parameter generator, which can create X-parameter files from circuit schematics. Plus, the circuit simulator works with models from Process Design Kits (PDKs) created by semiconductor foundries, affording foundry customers the opportunity to model custom ICs and devices. For more information on nonlinear modeling and simulators, AWR Corp. offers a free white paper written by Malcolm Edwards (available at www.awrcorp.com).