Superheterodyne

The superheterodyne technique in radio communications converts information at a low frequency to a high frequency suitable for transmission. On reception, it reverses the process. Typically, there is one or more intermediate frequencies between the transmission and user information frequencies.

It uses a principle that if two signals are combined in an electronic circuit known as a mixer, the mixer will output the two signals, their sum, and their difference. On reception, the sum or difference contains the information to be extracted. Assuming amplitude modulation, the information in the modulated signal came from the sum and difference between the beat frequency from the local oscillator, and the known unmodulated carrier frequency.

For transmission, the beat frequency is the carrier frequency to be modulated, and all or part of the mixer output is amplified and connected to the transmitting antenna.