Filters Using Planar Transmission Lines

Microwave filter design proceeds using a methodical synthesis approach that merges the inherent properties of distributed structures such as coupled lines with a mathematical response characteristic. This chapter covers the most important prototype development procedures and the most important classes of planar transmission line structures. Most filter design begins with a low pass equivalent lumped‐element filter call a low pass prototype filter. There are two very important types, the Butterworth filter and the Chebyshev filter, that have desirable characteristics that can be expressed as a transfer function with Butterworth and Chebyshev polynomials, respectively. A bandpass microstrip filter is a cascade of resonators with each coupled to its neighbors. There are two principal approaches to planar coupling between adjacent resonators namely end coupling or parallel coupling. With Parallel‐coupled Line (PCL) filters, the resonator is approximately formed by one microstrip line, which is coupled to another microstrip line‐based resonator.