Programming the Right Simulator for the Task

Programming the Right Simulator for the Task

Software simulation tools are available with a wide range of functions, from simple filter design tools to integrated suites of linear and nonlinear circuit simulators and EM simulators.

Simulation and modeling have become ways of life for many RF/microwave design engineers, with designers choosing to predict performance changes in design iterations on a computer screen rather than building and testing different versions of a prototype circuit. In fact, the wide variety of RF/microwave commercial software simulators currently available allows designers to experiment with many different possibilities, such as mixers with different diodes or low-noise amplifiers (LNAs) with different types of transistors. Whether modeling at the device, component, or system level, modern software simulation tools help fuel the design imagination while also paying for themselves after just a few design cycles that take the place of building and testing several different prototypes.

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A recent trend in high-frequency computer-aided-engineering (CAE) software has been the integration of multiple functions, such as linear circuit simulation, nonlinear circuit simulation, and electromagnetic (EM) simulation, into common platforms. Still, some designers prefer the focus and simplicity of “single-purpose” CAE software programs, such as simulators for designing filters or amplifiers.

The popular S/FILSYN program from ALK Engineering, for example, is devoted to the design of RF/microwave filters. The S/FILSYN software is capable of the design, synthesis, and analysis of both active and passive low-frequency and high-frequency filters, helping to create all major filter types, including lowpass, highpass, band-reject, and bandpass filters. The software can design infinite-impulse-response (IIR) and finite-impulse-response (FIR) filters and is available (in different versions) for mainframe computers and PCs.

The same company also offers the PCFILT filter design program with many built-in functions and reference designs to use as starting points to help speed the filter design process. For example, it can scale normalized reference designs to create filters that are closely matched to a software user’s set of requirements. The PCFILT software can also perform fast analysis, using Fourier analysis in the time domain, and can import filter files from S/FILSYN and other design software programs for analysis.

Similarly, MultiMatch was developed as a dedicated amplifier design program by the South African software development firm AMPSA Ltd. The company eventually collaborated with AWR Corp. (which in turn would eventually be acquired by National Instruments) to integrate MultiMatch as a design function module into the Microwave Office suite of CAE design tools. MultiMatch allows designers to work with different transistors and impedance-matching networks, both lossy and without loss, to create power amplifiers with targeted performance parameters.

The amplifier design software simplifies matters for users by largely automating the creation of circuit schematic diagrams and layouts, and can automatically transfer designed amplifier networks into the Microwave Office program for further analysis and optimization or even design interaction with other designed RF/microwave components, such as attenuators and filters, that might be connected to a power amplifier in a high-frequency system. The program was developed as a departure to the tedious approach of using Smith Charts to find matching elements and impedance networks for different transistors along the way to creating the input and output matching networks needed for a power amplifier.

Keeping Pace

As high-frequency circuits have grown in complexity and integration, simulation software has kept pace by expanding the number of models and modeling functions that can be handled under one platform, with probably the two most widely used suites of simulation programs being Microwave Office from National Instruments (NI) AWR and the Advanced Design System (ADS) suite of simulation software tools from Keysight Technologies. It is not a coincidence that both suites of software tools are backed by companies with considerable expertise and experience in the design of high-frequency test equipment and measurement techniques. Measurements and simulations go hand in hand, with measurement tools providing the test data to build new device and component models and simulation accuracy judged by how it compares to actual measurements of a design prototype represented by those simulations.

The suppliers of these two leading software simulation suites allow a user to specify the amount of simulation power needed for a facility adding software modules, such as EM simulation, as needed and operating with only as much modeling power as required by a company. The modular architecture of these software platforms enables them to model a wide range of devices, components, and circuits, including analog and digital circuits, and even to progress to system-level simulations as needed. These two software suites have grown through the years by means of internal development as well as through acquisition, with one example being APLAC Corp. and its analog and radio-frequency-integrated-circuit (RFIC) circuit simulators being acquired by AWR.

Both software simulation platforms are quite accurate and quite capable of modeling circuits from RF through millimeter-wave frequencies, with ample support for other software programs in terms of links and the compatibility of saved files. Perhaps the simplest comparison of tradeoffs between single-function programs like a MultiMix and a suite of CAE tools such as Microwave Office or ADS is that as simulation power and complexity grows, so too must the memory size and processing power of a PC or mainframe computer (along with the computer simulation time needed to solve a problem).

It should be noted that accurate models are extremely important to the success of any RF/microwave circuit simulator. Although not a supplier of a commercial CAE simulator, Modelithics holds a strong position in the area of high-frequency circuit and system simulation with its large complement of active and passive device and component models for numerous different simulators, including ADS and Microwave Office. Modelithics even offers substrate libraries for simulating circuits and studying the effects of different commercial PCB materials, with available substrate libraries for Keysight Genesys, Keysight ADS, and NI/AWR Microwave Office.

Do the Math

While many engineers rely on suites of simulation tools such as ADS and Microwave Office, many have also found that the EM simulators within these suites, and standalone EM simulators, are quite useful for modeling different circuit phenomena, especially for passive circuit elements. EM simulators were once considered more or less scientific curiosities because of their large memory and computer processing requirements compared to linear circuit simulators. But with advancements in the software and in computers, EM simulators are very much now mainstream circuit simulation tools for RF/microwave designers and have proven to be quite versatile modeling tools for high-frequency simulations.

Mathematical analysis programs such as MATLAB have been used as circuit simulators capable of predicting different circuit parameters, such as the scattering (S) parameters for impedance matching of input and output ports for active and passive circuits and components. Mathematical modeling is also the basis for the versatile MapleSim physical modeling system offered by Maplesoft.

For engineers involved in studying structures in which the physical composition is as important as the electrical behavior, such as waveguide sections, MapleSim can quickly create models for physical structures and automatically generate equations for those models that can be used in other software programs, such as MATLAB or LabView from  National Instruments. To simplify simulations, MapleSim includes a library of functions for dynamic system modeling, including frequency responses and time responses. The MapleSim software can be used to compute such things as LaPlace transforms, transfer functions, and frequency-domain representations of high-frequency component and circuit functions, such as mixer frequency translations. It is suitable for modeling different physical systems and interacting with system-level software (see “Simulators Predict System-Level Behavior”) to provide models for those system simulators.

As commercial applications incorporate RF/microwave circuits at ever-higher frequencies, such as the microwave and millimeter-wave radar systems that are being used in commercial automobiles for collision-avoidance and protection purposes, the economy provided by modeling with CAE software tools, especially EM simulators, becomes invaluable. Because of the complexity and the small-scale dimensions of the circuits employed in these higher-frequency radar systems, EM simulators provide effective tools for studying the interaction of different circuit components with transmission lines, such as various microstrip and stripline structures. The software can also model the effects of different dielectric PCB materials on those transmission-line structures, especially as the dimensions of the transmission lines and the interfaces with the dielectric materials become more critical at millimeter-wave frequencies.

The free-of-charge SonnetLite EM simulation software was used to design this edge-coupled microstrip bandpass filter. (Courtesy of Sonnet Software)

EM simulators are available within the leading CAE software simulation suites but also as standalone software tools such as the Sonnet Suites of 3D planar EM simulation software tools from Sonnet Software. Sonnet’s EM simulation software provides an effective means of modeling different high-frequency circuit structures in microstrip, stripline, and coplanar-waveguide transmission-line formats (see the figure). It can even account for the thickness of metal conductors on PCBs and the surface roughness of the metal conductors and how these variables affect circuit performance. The EM simulator has been used to design a wide range of passive components and structures, including spiral inductors, branchline couplers, filters, resonators, and stub tuners.

Although it is a standalone EM simulator, the Sonnet Suites also feature straightforward interfaces to other leading high-frequency CAE tools, including ADS, Microwave Office, and Virtuoso from Cadence Design Systems. The EM simulator can also import and export files in standard file formats, including .DXF, Gerber, and GDSII file formats. Sonnet provides model extraction for simulation results in both time- and frequency-domain formats, and can produce S-, Y-, and Z-parameter files in Touchstone, Cadence, and Keysight MDIF formats for use in different simulators.

Sonnet Software also provides a painless introduction to EM simulation through its SonnetLite feature-limited version of its SonnetSuites collection of EM simulation tools, available for free download from the company’s website. The free software can be used to design planar high-frequency circuits from 1 MHz past 1 THz, albeit with only a fraction of the features and flexibility of the Sonnet Suites. In fact, SonnetLite can be used to view any project developed with the “full-strength” version of Sonnet planar EM simulation software. It is easy to learn and can be freely integrated with ADS and Microwave Office.

The CST Studio Suite of EM simulation software from Computer Simulation Technology is another powerful collection of EM simulation tools for the 3D time-domain EM simulation of passive circuits and components, including antennas, filters, and couplers. The software suite includes the CST Microwave Studio EM simulator; the CST EM Studio software for the design and analysis of static and low-frequency applications such as motors, sensors, and actuators; the CST Cable Studio for the EM analysis of cable assemblies; and the CST PCB Studio software for the EM analysis of PCBs.

Remcom, another supplier of a powerful EM simulation tool, the XFdtd EM simulation software, offers a free trial version of the software from its website. The firm recently posted an excellent white paper on the use of EM simulation software for modeling microwave and millimeter-wave automotive radar systems, “Benefits of Time-Domain Electromagnetic Simulation for Automotive Radar.” It explores the modeling of such phenomena as the ground-plane current distribution within these automotive radar systems at 25 and 77 GHz and the effect of an automotive radar radome on the performance of a PCB-based radar system at 77 GHz. The white paper also points out XFdtd's ability to support very large and complex problems due to the efficient scalability of the FDTD algorithm combined with the increased performance of GPU-based computing.

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This file type includes high resolution graphics and schematics when applicable.

Of course, this is but a fraction of the software tools available for simulation of different device, component, and circuit functions at RF and microwave frequencies. Many more software programs, for example, are commercially available for EM simulation as well as for system-level simulation. The wide array of available software tools simply points out the growing dependence that high-frequency designers have placed on the act of simulation and on using the computer and software to save the time and expense of multiple prototyping cycles.

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