English: A heterodyne radio receiver circuit, invented in 1901 by Canadian engineer Reginald Fessenden to receive continuous wave (CW) radiotelegraphy transmissions. This was the first use of the "heterodyne" principle which Fessenden discovered. The CW radiotelegraph signal from the antenna consisted of pulses ("dots" and "dashes") of carrier wave at a radio frequency f_C, which spelled out text messages in Morse code. The problem was that since the carrier had no modulation, it simply sounded like silence in the earphones of an ordinary receiver. In the heterodyne receiver, an electronic oscillator generates an unmodulated sine wave signal at a frequency f_O offset from the carrier, which is added to the incoming signal from the tuned circuit. These two frequencies mix in the nonlinear crystal detector, generating a heterodyne (beat) frequency at the difference between these frequencies: f_H = f_C − f_O whenever the carrier is present. If the local oscillator frequency is chosen correctly this heterodyne frequency will be in the audio range. Therefore the "dots" and "dashes" of Morse code will be audible as musical "beeps" in the earphones.

When Fessenden invented it, there was no practical source of continuous RF sine waves to use for the oscillator. Fessenden used his enormous prototype radio alternator, but this wasn't practical. The heterodyne receiver remained a laboratory curiosity until 1913, when a cheap compact source of sine waves came along, the triode vacuum tube feedback oscillator invented by Edwin Armstrong, after which it became the standard way of receiving CW radiotelegraphy signals. Fessenden's heterodyne oscillator is the ancestor of the beat frequency oscillator used in modern superheterodyne receivers to receive CW signals.

Alterations to image: changed "alternator" schematic symbol in original image to general AC source symbol, added labels