Computer-aided-engineering (CAE) software tools can take time to use properly. But they also can save an enormous amount of time in a design cycle by predicting the effects of different operating conditions on a circuit or a system. Such tools are no longer a novelty in RF/microwave design, but instrumental in efficiently and cost effectively meeting a customer's design requirements. Knowing the types of available CAE software tools is the first step in learning how to use them to the greatest benefit.
The roots of RF/microwave CAE tools can be traced back some four decades. Because of his work with a program called "Speedy" at Fairchild's Microwave Division in 1970, and later with COMPACT (an acronym for "Computer Optimization of Microwave Passive and Active Circuits"), Les Besser is often credited as the "Father of Microwave Software." Numerous companies were experimenting with the use of scattering (S) parameters and software to help design RF/microwave circuits, including Texas Instruments with its CAIN-01 program.
These early software tools were written for time-share computers with many different users vying for a turn. Introduced in the early 1980s, the IBM personal computer (PC) changed the way that software would be developed for the microwave industry. COMSAT, which acquired COMPACT in 1980, would develop SuperCOMPACT-PC for the PC, along with a layout program called AUTO-ART. Ulrich Rohde, Chairman of Synergy Microwave Corp., would later buy COMPACT and Super-COMPACT from COMSAT, greatly improving both products. Additional software companies would emerge, including EESof, Inc. of Westlake Village, CA and Circuit Busters of Stone Mountain, GA. EESof, for example, would develop the Touchstone microwave design software specifically for the IBM PC and similar computer platforms.
Before a high-frequency engineer can design an RF/microwave circuit, there must be some understanding of the qualities of the "building materials" for that circuit, including the printed-circuit boards (PCBs), integrated circuits (ICs), and passive circuit elements. Software tools can contain large libraries of circuit elements, ICs, and even extensive choices in PCB materials, but they cannot point to the secret combination of ingredients that will result in delivering a customer's demanding set of performance requirements. PCB materials for high-frequency circuits, for example, can vary widely in terms of key electrical and mechanical parameters, such as relative dielectric constant, dissipation factor, and coefficient of thermal expansion (CTE). Knowing the limits of industry-standard circuit building blocks can ease the way towards effectively using high-frequency CAE software tools.
For a design team starting out, the choices in RF/microwave design software can seem overwhelming. Software tools exist for linear and nonlinear circuit analysis and design, for electromagnetic (EM) analysis and design, and even for system-level design. Modern commercial CAE software packages tend to be well equipped with device models, including active devices like packaged and chip discrete diodes, transistors, and packaged ICs. Nevertheless, a useful software tool must include a means for importing data (from, for example, a microwave vector network analyzer ), in order to create a model from S-parameter data for a device that may be new or otherwise not included in the software's device library.
Different types of design software offer different capabilities. While a circuit simulator or a complete suite of design tools might be well suited for traditional microstrip or stripline circuit design, a planar three-dimensional (3D) electromagnetic (EM) simulator will provide more insight into a circuit's behavior when used as an antenna or part of an antenna array. A full 3D EM simulator is capable of analyzing the EM fields for all three dimensions of an item of interest, such as a coaxial connector or a radiating cable (that is sometimes used as an antenna).
In fact, a number of different firms, including Sonnet Software, offer stand-alone EM simulator programs. Others provide EM simulators as part of software tool suites, including Agilent Technologies, ANSYS, Computer Simulation Technology (CST), and Mentor Graphics. The technologies employed in these EM simulators vary, from method-of-moments (MoM) analysis that helps speed two-dimensional (2D) planar antenna simulations in Mentor Graphics' IE3D EM simulator to the full 3D simulations possible with the finite-element-method (FEM) analysis of ANSYS' frequency-domain-based High-Frequency Structure Simulator (HFSS) EM simulator. CST's Microwave Studio suite also offers 3D EM analysis capability by means of time-domain-based perfect boundary approximation (PBA) analysis.
Some commercial software design packages are very focused. The firm AMPSA, for example, focuses on the design of amplifiers and impedance-matching programs. The Ampsa Amplifier Design Wizard (ADW) represents a departure from traditional design optimization programs, in that it relies on circuit-element synthesis to more quickly find the values of required circuit elements for a target level of amplifier performance. The software performs a number of systematic searches for best-fit component values, and then performs an optimization across the circuit elements it has found.
Shoring Up Support
A growing trend in RF/microwave software design tools is extended support for design software from other manufacturers. For example, Sonnet Software recently announced the release of a software version (Version 13.56) which extends Sonnet's Application Program Interface to support the Advanced Design System (ADS 2011) suite of design software tools from Agilent Technologies. The new link allows an ADS 2011 user to activate Sonnet's three-dimensional (3D) planar EM simulation engine from an ADS layout. When working within the ADS 2011 environment, the Sonnet EM extraction process creates a new model cell with layout, schematic, and layout look-alike symbol views.
The Sonnet software can also work with the Cadence Virtuoso IC design software from Cadence Design Systems and Microwave Office software from AWR Corp. Shawn Carpenter, Vice President of Marketing for Sonnet, explains: "With Sonnet, you have one solver capable of accurate model extraction from kilohertz through terahertz frequencies.RF designers can now reach for this powerful and accurate EM model extraction seamlessly through our automated APIs within Cadence Virtuoso, Agilent ADS 2011 and AWR Microwave Officewithout ever having to open our own project editor."
Of course, not all RF/microwave design tools are expensive. The MicroWorks Design package from Microwave Software is priced at only $149.95 (USD) for its variety of software design tools. It features six different design functions, including SmithMatch for designing matching networks with an interactive on-screen Smith chart; Sceptre for designing complete two-, three-, and four-port networks; and MStrip+ for designing microstrip circuits.
Don't Forget Free
When trying to learn more about different CAE tools, it can make sense to take advantage of "freeware" programs or free trial versions. Many of the suppliers of full-fledged RF/microwave CAE software suites offer trial versions to help users get started, such as AutoCAD 2D/3D circuit layout software from Autodesk which can be downloaded as a 30-day free trial. The software is available in versions for Macintosh computers and PCs. AWR Corp. offers a trial version of its popular Microwave Office suite of circuit and EM design tools.
One of the better-known free software utilities that has repeatedly proven its worth over the years is AppCAD (currently available from www.hp.woodshot.com). Originally developed by Hewlett-Packard's Microwave Semiconductor Division (now Avago Technologies), AppCAD included a useful set of calculators that could be applied to circuit and system design using discrete components or ICs.
In some cases, device manufacturers may even offer design software for some of their products, such as Analog Devices for its digitizers and phase-lock-loop (PLL) devices. The latest version of the company's ADIsimRF design tools supports engineers creating RF signal chains with RF-to-digital function blocks. In addition, the ADIsimPLL Version 3.4 provides a series of design tools for the firm's lines of PLLs. Both software programs can be downloaded from the company's website free of charge.