Short note: Superheterodyne receiver

A superheterodyne receiver converts every received station frequency to a fixed intermediate frequency (IF) (455 kHz for AM broadcast, 10.7 MHz for FM) before the main amplification and detection. It is used in almost all radio and TV receivers.

Ant->[RF amp]->(Mixer)->[IF amp]->[Detector]->[AF amp]->Spkr
       ^          ^        455k        |
       |        [LO]                  AGC
       +--gang tuning--+   <-----------+

Working:

1. RF stage: tunes to the wanted station $f_s$, amplifies it and rejects the image frequency.
2. Mixer and local oscillator: the LO is ganged with RF tuning so that $f_{LO}=f_s+f_{IF}$. The mixer gives $f_{LO}-f_s=f_{IF}$ for every station.
3. IF amplifier: fixed-tuned, high-gain stages with steep skirts provide most of the gain and selectivity.
4. Detector: an envelope detector recovers audio; it also gives a DC level for AGC, which controls RF/IF gain to keep output steady.
5. AF amplifier and loudspeaker reproduce the sound.

Advantages: uniform gain and selectivity over the whole band, good adjacent-channel rejection, high sensitivity, stable fixed-frequency amplifiers. Disadvantages: image frequency $f_{si}=f_s+2f_{IF}$ interference (e.g. station at 1000 kHz has image at 1910 kHz), extra mixer noise and more complex circuitry.