TABLE OF CONTENTS Filters have one basic function: to pass a particular thing and reject everything else. That is true of a car's oil filter (pass the oil and reject, or trap, dirt particles), a furnace filter (pass the warm air and trap the dust and dirt), or a pool filter (pass the water and trap dirt, leaves, twigs). Each of those filters provides a clean operation in some form; all are filters we encounter every day. The idea behind those filters also applies to RF and microwave filters: provide a clean signal for the system. By "clean" signal, we mean the one and only desired signal or band of signals required for the system's proper operation. The filters reduce or eliminate spurious signals or harmonics.
As we will show, filters are an important part of RF and microwave systems. They are, however, often overused because they do such an excellent job of cleaning up the signals. It must be understood that whenever you add a filter to a circuit, you are adding loss, VSWR from the filter, ripple (described in Section 4.7.1), and even some delay in the circuit. So, the use of filters should be considered carefully; in some cases, there may be an alternative means of cleaning up the circuit or system.
This section covers three types of RF and microwave filters: bandpass, lowpass, and highpass filters. The response curve of each type of filter is presented, in order to introduce the filters as separate components. The common...
