RF Filters

Filters are essential circuits in all RF and microwave systems. This chapter introduces the concepts of RF filter design using lumped components. It shows how the lumped designs can be extended into distributed formats, and how modern fabrication processes permit the design of compact, high‐quality planar filters. There are six main parameters that determine the performance of a filter at a particular frequency, namely insertion loss, return loss, bandwidth, phase response, group delay, and roll‐off. The relative values of the inductors and capacitors in a multi‐element configuration determine the precise frequency response of the filter. The Butterworth and Chebyshev filters are very common, and these are discussed in more detail to illustrate filter design principles. Whilst the use of stepped impedances to create a microstrip low‐pass filter provides a straightforward design, the filter can sometimes occupy a significant area of substrate particularly at lower frequencies.