Fessenden's heterodyne radio receiver circuit. The incoming radio frequency and local oscillator frequency mix in the crystal diode detector.
Reginald Fessenden demonstrated a
direct-conversion heterodyne receiver or beat receiver as a method of making
radiotelegraphy signals audible. Fessenden's receiver did not see much application because of its local oscillator's stability problem. While complex
isochronous electromechanical oscillators existed,
 a stable yet inexpensive local oscillator was not available until
Lee de Forest invented the
triode vacuum tube oscillator.
 In a 1905 patent, Fessenden stated that the frequency stability of his local oscillator was one part per thousand.
In radio telegraphy, the characters of text messages are translated into the short duration dots and long duration dashes of
Morse code that are broadcast as radio signals.
Radio telegraphy was much like ordinary
telegraphy. One of the problems was building high power transmitters with the technology of the day. Early transmitters were
spark gap transmitters. A mechanical device would make sparks at a fixed but audible rate; the sparks would put energy into a resonant circuit that would then ring at the desired transmission frequency (which might be 100 kHz). This ringing would quickly decay, so the output of the transmitter would be a succession of
damped waves. When these damped waves were received by a simple detector, the operator would hear an audible buzzing sound that he could transcribe back into alpha-numeric characters.
With the development of the
arc converter radio transmitter in 1904,
continuous wave (CW) modulation began to be used for radiotelegraphy. CW Morse code signals are not amplitude modulated, but rather consist of bursts of sinusoidal carrier frequency. When CW signals are received by an AM receiver, the operator does not hear a sound. The direct-conversion (heterodyne) detector was invented to make continuous wave radio-frequency signals audible.
The "heterodyne" or "beat" receiver has a
local oscillator that produces 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).
Block diagram of a typical superheterodyne receiver. Red
parts are those that handle the incoming radio frequency (RF) signal; green
are parts that operate at the intermediate frequency (IF), while blue
parts operate at the modulation (audio) frequency.
An important and widely used application of the heterodyne technique is in the
superheterodyne receiver (superhet), which was invented by U.S. engineer
Edwin Howard Armstrong in 1918. In the typical superhet, the incoming
radio frequency signal from the antenna is mixed (heterodyned) with a signal from a local oscillator (LO) to produce a lower fixed frequency signal called the
intermediate frequency (IF) signal. The IF signal is amplified and filtered and then applied to a
detector that extracts the audio signal; the audio is ultimately sent to the receiver's loudspeaker.
The superheterodyne receiver has several advantages over previous receiver designs. One advantage is easier tuning; only the RF and LO are tuned by the operator; the fixed-frequency IF is tuned ("aligned") at the factory and is not adjusted. In older designs such as the
tuned radio frequency receiver (TRF), all of the receiver stages had to be simultaneously tuned. In addition, since the IF filters are fixed-tuned, the receiver's selectivity is the same across the receiver's entire frequency band. Another advantage is that the IF signal can be at a much lower frequency than the incoming radio signal, and that allows each stage of the IF amplifier to provide more gain. To first order, an amplifying device has a fixed
gain-bandwidth product. If the device has a gain-bandwidth product of 60 MHz, then it can provide a voltage gain of 3 at an RF of 20 MHz or a voltage gain of 30 at an IF of 2 MHz. At a lower IF, it would take fewer gain devices to achieve the same gain. The
regenerative radio receiver obtained more gain out of one gain device by using positive feedback, but it required careful adjustment by the operator; that adjustment also changed the selectivity of the regenerative receiver. The superheterodyne provides a large, stable gain and constant selectivity without troublesome adjustment.
The superior superheterodyne system replaced the earlier TRF and regenerative receiver designs, and since the 1930s most commercial radio receivers have been superheterodynes.