Passive microwave circuits — Filters and cavities

The problem of designing microwave filters is complicated by the fact that the elements we use have distributed parameters; there is no such thing as a completely general synthesis procedure. Indeed, the frequency behavior of microwave circuit elements (transmission lines, cavities) is complex, making it impossible to develop a complete, general synthesis procedure.

However, despite these additional microwave complications, a number of techniques have been developed to design microwave filters. The case of narrow-band filters is exemplary, as many microwave elements have frequency characteristics in a narrow frequency band that resemble those of ideal inductive or capacitive reactances.

Microwave filters can be used in conjunction with other passive elements or devices, as in the multiplexers or diplexers often used in telecommunications. Microwave filters are also used in active circuits such as amplifiers, oscillators, mixers and so on.

Electromagnetic cavities, or microwave resonators, are used in a wide range of applications, including filters, oscillators, frequency meters, tuned amplifiers and microwave sensors. The operation of microwave resonators is, in many respects, similar to the operation of tuned circuits with localized elements in Kirchhoff's circuit theory. That's why it's useful to keep in mind the fundamental properties of series and parallel RLC resonant circuits.

We will describe the operation of microwave resonators based on distributed elements such as transmission lines, rectangular and circular waveguides and dielectric cavities.