Cavity Resonators: Basics and Functionality

A cavity resonator is essentially a short segment of a waveguide that functions as a high-Q resonant circuit. Waveguides themselves are fundamental microwave devices used in a variety of applications. Sections of waveguides are used as directional couplers, hybrids, duplexers, circulators, and, as we’re discussing, cavity resonators.

As illustrated in the figure above, a simple cavity resonator can be constructed from a short piece of waveguide that’s approximately 1/2 wavelength in size. A small probe, positioned at the center, is used to inject microwave energy. Both ends of this waveguide section are shorted using metallic pieces.

Because of these shorts at both ends, any signal injected via the probe will bounce back and forth between the shorted positions. Even without continuous injection, the wave will continue to resonate until it dissipates due to waveguide losses. This phenomenon is known as resonance, and the resulting circuit behaves like a parallel resonant circuit.

Cavity resonators boast very high Q factors, often reaching up to 30,000. This characteristic makes them ideal for designing microwave filters and other resonant circuits.

There are several types of cavity resonators, including circular and rectangular designs. In addition to the fixed cavity resonator described above, there are also tunable versions.

In a tunable cavity resonator, one wall of the cavity is movable. The figure above depicts a cylindrical cavity with adjustable disks. An adjustment screw controls the position of the end wall, allowing you to fine-tune the resonant frequency. Generally, a smaller cavity size corresponds to a higher operating frequency. Tunable cavity resonators are also available with adjustable plugs.