Simulation software is now a normal component of a well-balanced design process. Modern computer-aided-engineering (CAE) software tools are essentially suites of different programs, employing painstakingly constructed models of circuit elements, transmission lines, and active devices to predict the performance of a circuit, structure, or system before it is fabricated. But engineers who have worked in this industry more than a couple of decades can remember a time when CAE software for RF/microwave design was more or less an oddity, slightly more advanced than a desktop calculator.

Les Besser (Fig. 1) is often called "the father of microwave computer-aided-design (CAD) software" for his early persistence in creating RF/microwave design aids to run on a computer. Besser spent time at Fairchild Semiconductors, Farinon Electric Co., and Hewlett- Packard Co., writing programs in BASIC that would help, for example, solve for circuit-element values in transistor impedance matching networks. His first software program, SPEEDY, was completed while at Fairchild, as a promotional device for the company's transistors.

SPEEDY was a simple solver and lacked any optimization capabilities. Those would come when Besser released a program in 1973 called COMPACT, an acronym formed of the phrase "computerized optimization of microwave passive and active components." Both SPEEDY and COMPACT were written on and for the computers of that time, which were time-sharing systems with limited memory and processing speed. Besser would start a company to market the software, first called Compact Engineering and later renamed Compact Software, and COMPACT quickly became the RF/microwave computer-simulation program of choice for the industryor at least, until it was superseded by a next-generation version called SuperCOMPACT.

Along the way, Compact Software would merge with Communication Satellite Corp. (COMSAT) in 1980, and Besser would remain on board long enough to see SuperCOMPACT become the CAE standard of an industry, running on either time-share computers or installed at some companies within their in-house computer mainframes. Besser would leave COMSAT to form a company called Besser Associates, devoted to furthering education in the microwave industry. And RF/microwave CAE technology would soon become much more widespread, with the growth of the personal computer (PC).

Ulrich Rohde, Chairman of Synergy Microwave Corp. (Paterson, NJ), and President of Communications Consulting Corp. (Upper Saddle River, NJ), would eventually buy Compact Software from COMSAT. As President of Compact, he would make many improvements to both UNIX and PC versions of the software. Advances included adding harmonic-balance (nonlinear) and electromagnetic (EM) analysis capabilities to a version of the software called Microwave Harmonica. Rohde would eventually sell the improved code to Ansoft, where it would become the Ansoft Designer program.

Although COMPACT (and then SuperCOMPACT) were embraced by hundreds of companies for high-frequency circuit design, some designers chose to use a public-domain program developed at the University of California at Berkeley called SPICE. It was created by student Lawrence Nagel with help from his research advisor, Professor Donald Pederson. It used nonlinear differential equations to predict the behavior of the various circuit elements and transmission lines within a circuit. It was often criticized as being a low-frequency modeling tool and with limited accuracy for some of its components, such as inductors. But the fact that it was available from the university as a public-domain tool invited many engineers to find ways to apply it to their own projects. Eventually, commercial versions became available, such as HSPICE, which is now supported by Synopsis, and PSPICE, which is now owned by Cadence Design Systems.

But the growth of CAE software tools would very much be tied to the advancement of the personal computer (PC) rather than to the increased availability of time-share computers. Early computers that were bundled as "personal computers," such as the 9100A from Hewlett-Packard Co. in the 1960s, were actually technical computers with limited processing and memory resources. It would not be until later in the 1970s (1977, to be precise), that products more closely resembling computers that an individual or person might own would appear on the market. These early personal computers included the Personal Electronic Transactor (PET) from Commodore, the TRS-80 produced by Tandy Corp. and sold by its chain of Radio Shack electronics stores, and the 8-b Apple II home computer from Apple Computer (now Apple, Inc.). Admittedly, these were humble machines: the Apple II was based on a microprocessor running at a 1-MHz clock speed, with 4 kB of random-access memory, and programs that were loaded by means of an audio cassette drive interface. But, if anything, these early personal computers (PCs) would change the thinking about time-sharing computers, and clear the way for an eventual PC with RF/microwave CAE software on the desk of any engineer who wanted it.

As PCs became available and popular, high-frequency engineers created design tools that could run on a PC rather than on a workstation, often for extremely low cost. For example, in 1984, Jim Lev started Microwave Software (San Clemente, CA) with five low-cost software tools written for the Apple II PC: SmithMatch, Sceptre, MStrip+, OptiMatch, and Utilities+. These programs would be translated two years later (in 1986) to MS-DOS format for use on IBM PCs and their derivatives, and a sixth program called SData+ would be added in 1991. MStrip+, for example, which has been detailed in Microwaves & RF (see May 1984 and January 1985 issues), provides accurate analysis and synthesis of single or coupled microstrip transmission lines, analysis and synthesis of single or coupled stripline, and even analysis and synthesis of suspended-substrate transmission lines. The company is still in operation (www.microwavesoftware.com), still run by Jim Lev, still offering the same programs, and still at low cost. Each program sells for $29.50 for a single-user license, except for Utilities+, which is only $19.50 for the single-user license.

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Around the same time (1985), filter designer Randy Rhea would start his company CIRCUIT BUSTERS (Stone Mountain, GA) with a first software product called STAR. It was another low-cost program for the IBM PC, aimed at filter synthesis and selling for only $99. It was supplied on a single 5-1/2-inch floppy disk but without a security key to prevent unauthorized users from making their own copies of the code. Following STAR, the company's second-generation version of the program was SUPER-STAR. The name of the company was changed to Eagleware Corp. in 1991 and the company would go on to develop a powerful but affordable suite of programs known as GENESYS (Fig. 2) in 1999, after six years of development.

Eagleware was acquired by Agilent Technologies in 2005, and supports the latest version of GENESYS, which combines circuit and EM simulators, in fact as many as seven different simulation modules, and runs on a standard PC. The software suite is ideal for performing simulations on multifunction microwave integrated circuits (MICs), monolithic MICs (MMICs), and system-on-chip (SoC) designs.

Another major microwave software development during the 1980s was the formation of Ansoft (Pittsburgh, PA) in 1984 by Zoltan Cendes and his brother Nicholas, a company that would commercialize EM simulation in the form of its High-Frequency Structure Simulator (HFSS) software introduced two years later. Based on finite-element analysis (FEA), HFSS could predict the behavior of EM fields around a complex structure, such as an antenna or coupler. FEA methods can be applied in two-dimensional (2D) or three-dimensional (3D) approaches. The 3D approaches, such as HFSS, are more accurate but much more computationally intensive than 2D approaches, requiring a great deal of computer power. After numerous marketing partnerships with Hewlett-Packard (later as Agilent Technologies), Ansoft was acquired by Ansys which continues to support HFSS.

A major proponent of EM simulation advancement in the 1980s and beyond was Jim Rautio, who founded Sonnet Software (Liverpool, then North Syracuse, NY) in 1983. With time spent at General Electric, as well as teaching at Syracuse and Cornell Universities, and inspired by the life and work of James Clerk Maxwell, Rautio embarked on developing "EM simulation programs for the masses." He proceeded to develop the Sonnet Suites of 3D planar EM simulation software tools. The software products are ideal for analyzing single-layer and multilayer circuits and antennas. (To this day, Rautio offers a scaled-down version of his Sonnet EM simulation software in the form of Sonnet Lite, available for free download from the Sonnet website at www.sonnetsoftware.com.) The company also supports the CST STUDIO SUITE of full-wave 3D EM simulation tools from Computer Simulation Technology (CST).

In addition to playing a role in Compact Software, COMSAT would also indirectly lead to the creation of one of the industry's most important software companies, EEsof, Inc. COMSAT had added to its collection of companies by acquiring amplifier manufacturer, Amplica, with Chuck Abronson president of Amplica. Bill Childs, who had been with Compact Software when COMSAT acquired that company, had been moved to Amplica to help manage amplifier product development. Childs and Abronson (Fig. 3) would combine their engineering and management talents, respectively, to form a microwave-software-only company, EEsof.

EEsof's first PC software product was called TOUCHSTONE, which provided an early form of microwave CAE tuning by using two display screensone for entering a circuit's netlist and the other for displaying results. As the value was changed for a capacitor, for example, in an amplifier's input matching network, the amplifier's response with frequency could be seen to change.

EEsof would bring many more innovations to RF/microwave simulation software, including a closer link between actual test data and simulation models. The company developed a program called ANACAT which could work with one of the commercial microwave vector network analyzers (VNAs) of that time, such as the HP 8510A from Hewlett-Packard Co. (Palo Alto, CA) or the model 360 from Wiltron Co. (Mountain View, CA), and convert the data format from those instruments into a configuration that could be read into TOUCHSTONE for performing circuit simulations and developing models.

TOUCHSTONE simulated the performance of linear RF/microwave circuits, although many high-frequency circuits, such as amplifiers, are often nonlinear in nature. Because of this, Childs and Abronson realized that further development was needed and would eventually introduce a nonlinear simulator called Libra, which incorporated harmonic-balance techniques to provide insights into the nonlinear behavior of high-frequency circuits, such as mixers and large-signal amplifiers. Harmonic-balance methods use voltage and current sources during a simulation to create a set of discrete frequencies at every node in the circuit under analysis. The circuit's linear components can be modeled in the frequency domain, but nonlinear circuit elements are treated in the time domain, using Fourier transforms to translate solutions from the frequency domain to the time domain.

Libra was a powerful simulator, but it differed from TOUCHSTONE in that it was not written for conventional DOS-based PCs but for UNIX-based PCs and workstations. Users were required to invest in the UNIX operating system for Libra simulations. This reliance on the UNIX operating system would also plague a microwave design program initially developed for internal use at Hewlett-Packard Co. and called the Microwave Design System (MDS). It was written for workstation computers, such as UNIX machines from Hewlett-Packard Co., Apollo, and Sun Microsystems, as well as for UNIX PCs, which prevented its widespread adoption.

EEsof would be acquired by Hewlett-Packard Co. in 1993, but not before the company had also developed a system-level simulator called OmniSys and a powerful layout tool called Academy. When Hewlett-Packard Co. spun out its test-related businesses in 1999 as Agilent Technologies, the microwave software group went as well, becoming known as Agilent EEsof. Although Hewlett-Packard Co. had won over some supporters for its MDS software, the company would eventually abandon the UNIX-based code in favor of developing a newer simulation platform called the Advanced Design System (ADS) which remains one of the industry's most powerful integrated software suites, with linear and nonlinear circuit simulation, EM simulation, and layout tools (Fig. 4).

The concept of integrated simulation tools was also demonstrated with the introduction of the Microwave Office suite of software tools by AWR Corp. in 1998. The company, founded in 1994 as Applied Wave Research by Dr. Joseph Pekarek, Ted Miracco, and Dr. Stephen Maas, understood the need for multiple simulation tools. In addition to the Microwave Office software, the company developed the AXIEM EM simulator and the Visual System Simulator (VSS) system simulator. Most recently, AWR Corp. was acquired by instrument maker National Instruments, providing a further level of integration between simulation and test.