Tuner

A tuner is a subsystem that receives radio frequency (RF) transmissions, such as FM broadcasting, and converts the selected carrier frequency and its associated bandwidth into a fixed frequency that is suitable for further processing, usually because a lower frequency is used on the output. Broadcast FM/AM transmissions usually feed this intermediate frequency (IF) directly into a demodulator that converts the radio signal into audio-frequency signals that can be fed into an amplifier to drive a loudspeaker. More complex transmissions like PAL/NTSC (TV), DAB (digital radio), DVB-T/DVB-S/DVB-C (digital TV) etc. use a wider frequency bandwidth, often with several subcarriers. These are transmitted inside the receiver as an intermediate frequency (IF). Subcarriers are then processed like real radio transmissions, but the whole bandwidth is sampled with an analog-to-digital converter (A/D) at a rate faster than the Nyquist rate (that is, at least twice the IF frequency). A tuner can also refer to a radio receiver or standalone audio component that is part of an audio system such as a hi-fi or stereo, to be connected to a separate amplifier. The verb tuning in radio contexts means adjusting the receiver to detect the desired radio signal carrier frequency that a particular radio station uses. The simplest tuner consists of an inductor and capacitor connected in parallel, where the capacitor or inductor is made to be variable. This creates a resonant circuit which responds to an alternating current at one frequency. Combined with a detector, also known as a demodulator (diode D1 in the circuit), it becomes the simplest radio receiver, often called a crystal set. Older models would realize manual tuning by means of mechanically operated ganged variable capacitors. Often several sections would be provided on a tuning capacitor, to tune several stages of the receiver in tandem, or to allow switching between different frequency bands. A later method used a potentiometer supplying a variable voltage to varactor diodes in the local oscillator and tank circuits of front end tuner, for electronic tuning.