11 - Tuning and matching

Abstract

This chapter discusses how inductor–capacitor (L–C) circuits can be used for tuning frequencies and matching impedances. Resonant L–C tank circuits are used to tune radio receivers; it is these circuits that select the station to be received, while rejecting others. According to the superhet principle, the radio frequency being received is converted to another frequency, called the intermediate frequency, by being mixed with a local oscillator signal in a non-linear mixer stage. There are several L–C tank circuits present in this notional superhet radio. The antenna tank circuit is found at the input of the RF amplifier stage, or if no RF amplifier is used it is at the input to the mixer stage. Additional tank circuits are found in the IF amplifier section of the radio. These tank circuits are fixed tuned to the IF frequency, which in common AM broadcast band (BCB) radio receivers is typically 450 kHz, 455 kHz, 460 kHz, or 470 kHz depending on the designer's choices. Other IF frequencies are also seen, but these are most common.