Superheterodyne receiver

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In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency. Virtually all modern radio and television receivers use the superheterodyne principle.

Contents

History

The word heterodyne is derived from the Greek roots hetero- "different", and -dyne "power". The original heterodyne technique was pioneered by Canadian inventor Reginald Fessenden, [1] but it was not pursued far because local oscillators available at the time were unstable in their frequency output, and vacuum tubes were not yet available.[2]

The superheterodyne principle was revisited in 1918 by U.S. Army Major Edwin Armstrong in France during World War I. [1] He invented this receiver as a means of overcoming the deficiencies of early vacuum tube triodes used as high-frequency amplifiers in radio direction finding equipment. Unlike simple radio communication, which only needs to make transmitted signals audible, direction-finders measure the received signal strength, which necessitates linear amplification of the actual carrier wave.

In a triode radio-frequency (RF) amplifier, if both the plate and grid are connected to resonant circuits tuned to the same frequency, stray capacitive coupling between the grid and the plate will cause the amplifier to go into oscillation if the stage gain is much more than unity. In early designs, dozens (in some cases over 100) low-gain triode stages had to be connected in cascade to make workable equipment, which drew enormous amounts of power in operation and required a team of maintenance engineers. The strategic value was so high, however, that the British Admiralty felt the high cost was justified.

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