RF Circuit Design, 2nd Edition

Chapter 19

Tunable Filter

19.1 Tunable Filter in A Communication System

There are two kinds of filters in electrical circuit design: the fixed filter and the tunable filter. Intuitively, a fixed filter's passband is fixed and well defined, while a tunable filter's passband can be tuned over a certain frequency range.

Theory for a fixed filter design has been comprehensively developed many decades ago. The engineering developments of passive filters, including the LPF (low pass filter), HPF (high pass filter), BPF (band pass filter), and BRF (band reject filter), have been formulized and tabulated for the design with discrete parts. This simplifies the design procedures: the only thing remaining is how to convert the values of parts into reasonable ones through the so-called Δ − ^* or π − T transformation.

In past years, the frequency bands assigned for a communication system rose higher and higher. It became more and more difficult to implement filters by discrete parts because the required values of either capacitors or inductors were too small, which are unacceptable for the production line. Many types of filters with distributed parameters were developed, such as the crystal filter, ceramic block filter, microstrip line filter, SAW (surface acoustic wave) filter, and so on.

The theory for a tunable filter design was developed many decades ago. Its implementation, however, faces some barriers. This is one of the reasons why this, and not another type of filter, is selected to ...