Autodyne
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Originally an "Autodyne" circuit referred to a weakly oscillating receiver tube that made Interrupted Carrier Wave (ICW) Morse Code transmissions audible. By tuning the oscillator to a frequency a few hundred Hertz away from the incoming signal frequency, the Morse Code dots and dashes would be clearly heard as a series of beeps, instead of a series of thumps. The name is a contraction of "Auto" (self) and "Heterodyne" (generated by a difference), and refers to the audible beat note generated by the difference between the oscillator and signal frequencies.
However more recently the term "Autodyne" has come to refer to the self-oscillating mixer of a Superheterodyne radio receiver, which combines the functions of mixer and oscillator in one active device. Although the superheterodyne principle was devised by Edwin Armstrong in 1918, commercial acceptance was slow for a number of reasons, one being the requirement for a separate power-hungry tube for the local oscillator. By the early 1930s, improvements in AC vacuum tube design allowed a single Pentode tube to combine the functions of oscillator and mixer, but the original "autodyne" designs tended to be somewhat unreliable as it was heavily dependent of the characteristics of the particular tube used, which could vary considerably over its operational life, and from tube to tube.
By the mid-30s this technique had been made obsolete by purpose-designed multi-electrode tubes which combined separate oscillator and mixer functions in the same glass envelope. However it re-appeared with the emergence of transistor radios in the 1950s, much more successfully this time as transistors tend to have more stable characteristics. Virtually the same design has been used right from the start, and is still used in inexpensive radios today. In the early 1960s a version was develped for portable FM receivers, and again, virtually the same design has been used since then, although modern receivers tend to use integrated circuit designs where the circuit function is not visible.
