The Changing Ways Of RF Design

Designing circuits to match a transistor to 50 Ohms used to require sheets of paper preprinted with a Smith Chart, and then finding input and output matches. But design is "paperless" now, and the world has gone to using computer-aided-engineering (CAE) software not only for the matching networks but for anything else needed in a circuit or system.

High-frequency designers rely on a variety of CAE tools, for linear and nonlinear circuits, electromagnetic (EM) simulation, and even system-level simulation. If any trend in design software emerged from the IEEE MTT-S Symposium & Exhibition in Boston last month, it was that EM simulators are no longer "plug-ins" to other software programs. They are essential tools for RF/microwave design that can calculate not only the electrical behavior of active and passive components, but the packages around them.

As the RF Primer on EM simulators in this issue explains, these programs use Maxwell's equations to calculate the current flow and EM fields around a planar or 3D structure. Because the equations are solved at every frequency point, this once required more than a few hours of computation time and the largest workstation available. But computers (and code writers) have been catching up to Maxwell in recent years, and the latest EM simulators can deliver calculations in a fraction of the time, and with better accuracy, of their counterparts of a few years ago. With the availability of multiple-core microprocessors, laptop computers can now perform EM analysis.

Design engineers concerned with an efficient "design flow" would at one time pore over schematic and layout drawings to ensure that an engineering concept would translate effectively into production. Now the design flow is captured within a single suite of CAE tools that allow cross checking between EM and circuit simulations and layout tools. Process requirements can be programmed to check for design-rule violations, helping to eliminate many design iterations.

Of course, a concern is that the software is "designing" rather than the engineer. In truth, software is a tool, requiring someone with skills and knowledge for the best results. Just as this word processor is not writing this article, microwave CAE software does not complete a high-frequency circuit or system design without the operator. Because it is faster than manual design, the software allows many different looks at a circuit without having to actually fabricate the circuit for glimpse into its performance. The old ways may have provided great opportunities for learning, but the new ways are faster and more efficient, and are really the only way to compete in a modern world where time is money.

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