Chapter 5 - Front-End Filtering

Abstract

The filtering used on the front end of a radio receiver determines some important characteristics of the radio. Two forms of filtering are used: single frequency and a frequency band. The chapter also discusses the L-C inductor resonant tank circuits. While using an inductor (L) and a capacitor (C) together in the same circuit, the combination forms an L-C resonant circuit, which is also sometimes called a tank circuit or resonant tank circuit. These circuits are used to select one frequency, while rejecting all others. There are two basic forms of L-C resonant tank circuit: series and parallel. The series resonant circuit, like other series circuits, is arranged so that the terminal current (I) from the source (V) flows in both components equally. The parallel resonant tank circuit is the inverse of the series resonant circuit. The line current (I) from the source splits and flows in the inductor and capacitor separately. Many of the resonant circuits used in RF circuits, and especially radio receivers, actually are transformers that couple a signal from one stage to another. Typically, the tuned RF/IF transformers built for radio receivers are wound on a common cylindrical form and surrounded by a metal shield can that prevents interaction of the fields of coils in close proximity to each other. The IF and RF transformer holds a high potential for intermittent problems in radio receivers.