Superheterodyne receiver

A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. It was invented by US engineer Edwin Armstrong in 1918 during World War I. Virtually all modern radio receivers use the superheterodyne principle.In December 1919, Major E. H. Armstrong gave publicity to an indirect method of obtaining short-wave amplification, called the super-heterodyne. The idea is to reduce the incoming frequency, which may be, say 1,500,000 cycles (200 meters), to some suitable super-audible frequency that can be amplified efficiently, then passing this current through an intermediate frequency amplifier, and finally rectifying and carrying on to one or two stages of audio frequency amplification. A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. It was invented by US engineer Edwin Armstrong in 1918 during World War I. Virtually all modern radio receivers use the superheterodyne principle. 'Superheterodyne' is a contraction of 'supersonic heterodyne', where 'supersonic' indicates frequencies above the range of human hearing. The word heterodyne is derived from the Greek roots hetero- 'different', and -dyne 'power'. In radio applications the term derives from the 'heterodyne detector' pioneered by Canadian inventor Reginald Fessenden in 1905, describing his proposed method of producing an audible signal from the Morse code transmissions of the new continuous wave transmitters. With the older spark gap transmitters then in use, the Morse code signal consisted of short bursts of a carrier wave. Since these bursts were derived from the output of an alternator, they modulated the carrier at a frequency within the audio range and thus could be heard as a chirp or a buzz in the receiver's headphones. However, the signal from a continuous wave transmitter is at a single frequency well above the audio range, and Morse Code from one of these would be heard only as a series of clicks or thumps. Fessenden's idea was to run two Alexanderson alternators, one producing a carrier frequency 3 kHz higher than the other. In the receiver's detector, the two carriers would beat together to produce a 3 kHz tone, thus in the headphones the Morse signals would then be heard as a series of 3 kHz beeps. For this he coined the term 'heterodyne,' meaning 'generated by a difference' (in frequency). The French engineer Lucien Lévy filed a patent application for the superheterodyne principle in August 1917 with brevet n° 493660.The American Edwin Howard Armstrong also filed a patent in 1917. Levy filed his original disclosure about seven months before Armstrong's.The German inventor Walter H. Schottky also filed a patent in 1918.At first the US recognised Armstrong as the inventor, and his US Patent 1,342,885 was issued on 8 June 1920.After various changes and court hearings Lévy was awarded a US patent No 1,734,938 that included seven of the nine claims in Armstrong's application, while the two remaining claims were granted to Alexanderson of GE and Kendall of AT&T. Armstrong invented his 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 (anode) 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.