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During the “dark ages” of EM simulation, it was necessary to redraw the structure for the EM simulator and to transfer EM-generated S-parameters back into the circuit simulator manually. This process led to many opportunities for errors. Modern simulators treat an EM-simulated structure directly as a circuit-simulator element that is then analyzed in a manner no different from any other S-parameter block. No copying or redrawing is necessary and, in fact, the EM structure is copied directly into the layout tool as well. This prevents “translational” errors or unnecessary, time-consuming repeated effort.

EM simulation is not just for microwave circuits. For example, in a cellular handset power amplifier, the load impedance is only a few ohms. Even at 1800 MHz, the inductance of the chip’s output bond wires cannot be ignored, as it is a large fraction of this value. EM simulation is really the only accurate way to predict that inductance at design time. EM simulation is also useful for finding areas of dangerously high current density or, in high-power circuits, high-field regions that could lead to arcing.

In the days when most high-frequency design was performed in the aerospace industry, system simulation was largely divorced from the circuit-design process. In wireless systems, for example, individual circuits can have strong effects on system performance, so system design must be concurrent with circuit design. This is especially true for power amplifiers, where amplifier nonlinearities must be carefully monitored to meet adjacent-channel-power (ACPR) specifications. Tight integration between circuit and system simulators, in which a circuit from the circuit simulator can be treated automatically as a block in the system simulator, can be an enormous aid to the design process.

In a modern EDA system, each circuit element has a single representation in the EM, layout, system, and circuit simulators. Because it has only one instance in the simulation system, changes in its circuit parameters are immediately reflected in the layout. Similarly, changes in the layout, such as a change in microstrip line length or width, are immediately seen in the circuit simulator. No “back-annotation” or other such processes are needed, and the need for layout-versus-schematic (LVS) checking is greatly reduced. In this way, a designer sees the consequences of a change immediately and can modify the circuit if something is not practically realizable. And indeed, this is how it should be: Layout is part of the engineering design and should not be off-loaded onto a layout technician.

Evaluate EDA Software For A Wireless World, Fig. 3

Evaluate EDA Software For A Wireless World, Fig. 4

Advanced Layout Views

A graphics system used for circuit layout must be clear and easy to follow. Visual fatigue and lack of clarity lead to errors. Computers are now available with powerful graphics capabilities, and it makes sense to put such capabilities to use as part of the RF/microwave design process. Figure 3 shows an example of a circuit layout of a power-amplifier chip, showing the clarity and detail possible. Figure 4 shows a three-dimensional (3D) view of the layout, which can be rotated in all axes. Such graphic capabilities are invaluable in ensuring that a high-frequency layout is created as intended.

EDA tools have come a long way in the past few decades. They have evolved from individual tools with limited functions to systems that provide more accurate designs, smooth the design flow, and greatly reduce design costs. These advantages come from the use of modern software technology as well as improved technical capabilities. Evolving products, such as Virtual System Simulator (VSS) from AWR, make it easy to move from system-level design and behavioral modeling to detailed component-level design and ultimately performance evaluation. Today’s EDA systems are powerful and versatile, and a huge advantage over those of even the recent past.

Dr. Stephen Maas, Chief Scientist

AWR Corp., a National Instruments Co., 1960 East Grand Ave., El Segundo, CA 90245; (310) 726-3000.

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