Heterodyne - History

History

In 1901, Reginald Fessenden demonstrated the heterodyne detector as a method of making continuous wave radiotelegraphy signals audible. The detector did not see much application because stable local oscillators would not be available until the invention of the triode vacuum tube oscillator. In a 1905 patent, Fessenden states the frequency stability of his oscillator was one part per thousand.

Early radio transmitters sent information by radiotelegraphy. The characters of a text messages were translated into the short duration dots and long duration dashes of Morse code that were sent as bursts of radio waves. The receiving radiotelegraph operators would hear those different duration bursts as buzzing in their headphones and transcribe the sounds back into characters.

Fessenden's detector was not needed to receive the damped wave signals produced by the spark gap transmitters that were common at the time. Damped wave signals were amplitude modulated at an audio frequency by the sparks, and a simple detector would produce an audible signal in the headphones.

With the advent of the arc converter, continuous wave (CW) transmitters were adopted. CW signals did not have audio amplitude modulation, so a different detector was needed. The heterodyne detector was invented to make continuous wave Morse code signals audible.

Fessendon's "heterodyne" or "beat" receiver had a local oscillator (LO) that produced a radio signal adjusted to be close in frequency to the incoming signal being received. When the two signals are mixed, a "beat" frequency equal to the difference between the two frequencies is created. By adjusting the local oscillator frequency correctly, the beat frequency is in the audio range, and can be heard as a tone in the receiver's earphones whenever the transmitter signal is present. Thus the Morse code "dots" and "dashes" are audible as beeping sounds. This technique is still used in radio telegraphy, the local oscillator now being called the beat frequency oscillator or BFO. Fessenden coined the word heterodyne from the Greek roots hetero- "different", and dyn- "power" (cf. δύναμις or dunamis).