English: Block diagram of a single conversion superheterodyne radio receiver. Invented by Edwin Armstrong in 1918 during World War 1, the superheterodyne is the design of almost all modern radio receivers. The incoming radio signal from the antenna is first amplified in a radio frequency (RF) amplifier, then mixed with an unmodulated sine wave from a local oscillator in a nonlinear transistor called a mixer. This produces a "beat" frequency or heterodyne at the difference between these two frequencies, a lower frequency called the intermediate frequency. This is bandpass filtered in several stages of tuned IF amplification, and applied to a demodulator which extracts the audio modulation (sound) from the radio carrier wave. This is amplified by an audio amplifier and applied to the speaker.