Simple radio detectors filtered out the high-frequency carrier, leaving the modulation, which was passed on to the user's headphones as an audible signal of dots and dashes. In 1904, Ernst Alexanderson introduced the Alexanderson alternator, a device that directly produced radio frequency output with higher power and much higher efficiency than the older spark gap systems.
Due to the filtering effects of the receiver, these signals generally produced a click or thump, which were audible but made determining dot or dash difficult. In 1905, Canadian inventor Reginald Fessenden came up with the idea of using two Alexanderson alternators operating at closely spaced frequencies to broadcast the signals, instead of one.
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 1913, Edwin Howard Armstrong described a receiver system that used this effect to produce audible Morse code output using a single triode.
From this time, the superheterodyne design was used for virtually all commercial radio and TV receivers. ===Patent battles=== French engineer Lucien Lévy filed a patent application for the superheterodyne principle in August 1917 with brevet n° 493660.
Armstrong also filed his patent in 1917.
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.
He termed this resulting difference the "intermediate frequency" often abbreviated to "IF". In December 1919, Major E.
It was less popular when commercial radio broadcasting began in the 1920s, mostly due to the need for an extra tube (for the oscillator), the generally higher cost of the receiver, and the level of skill required to operate it.
Using this technique, a small number of triodes could do the work that formerly required dozens of triodes. In the 1920s, commercial IF filters looked very similar to 1920s audio interstage coupling transformers, had similar construction and were wired up in an almost identical manner, and so they were referred to as "IF transformers".
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.
Armstrong eventually sold his superheterodyne patent to Westinghouse, who then sold it to Radio Corporation of America (RCA), the latter monopolizing the market for superheterodyne receivers until 1930. Early superheterodyne receivers used IFs as low as 20 kHz, often based on the self-resonance of iron-core transformers.
By the 1940s the vacuum-tube superheterodyne AM broadcast receiver was refined into a cheap-to-manufacture design called the "All American Five", because it uses five vacuum tubes: usually a converter (mixer/local oscillator), an IF amplifier, a detector/audio amp, audio power amp, and a rectifier.
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