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Microwave and millimetre integrated circuits and components are currently being used in a wide and ever expanding variety of electrical equipment. Design of high frequency circuit components (up into the terahertz range) is a challenging activity which relies on computational models to reduce costly manufacture and test development cycles.
At high frequencies planar transmission line structures are required for passive component design and microstrip components are frequently used in MMIC applications. The majority of microstrip CAD is based on methods such as quasi-static approaches, equivalent wave-guide models or semi-empirical methods. These methods are computationally efficient but unfortunately they cannot account for coupling between components, radiation and surface wave effects. Understanding of these effects is essential for effective circuit design.
Open microstrip elements are frequently used and are free to radiate into space and into surface wave modes. Furthermore, impedance discontinuities, which constitute the circuit elements of the device, represent significant sources of radiation Moreover, circuit elements may be designed to radiate, or example patch antenna structures. In these cases the full understanding of the network impedance and loss characteristics can only be achieved using a full wave solution technique.
Over the past decade a wide range of full-wave methods have been developed. These methods are broadly categorised into variational methods such as finite elements, integral equation approaches such as method of moments, or difference methods such as FDTD.
The advantages of the FDTD method for this application are many. It is a time domain method and so can model non-linear device characteristics, also it can obtain extremely wide bandwidth information from a single calculation. The method itself is simple and robust yet can provide a great deal of information about the circuit under study.
Some examples of simple circuit elements are presented in the following pages which illustrate the application of the method.
Continue to Example 1 - Single Layer Microstrip Patch Antenna