Editing Superheterodyne receiver (section)

== History ==

=== Conceptualisation ===
[[File:Prototype Armstrong superheterodyne receiver 1920.jpg|thumb|upright=1.5|One of the prototype superheterodyne receivers built at Armstrong's Signal Corps laboratory in Paris during World War I. It is constructed in two sections, the [[frequency mixer|mixer]] and [[local oscillator]] ''(left)'' and three IF amplification stages and a detector stage ''(right)''. The intermediate frequency was 75&nbsp;kHz.]]

Although a number of researchers discovered the superheterodyne concept, filing patents only months apart, American engineer [[Edwin Armstrong]] is often credited with the concept. He came across it while considering better ways to produce RDF receivers. He had concluded that moving to higher "short wave" frequencies would make RDF more useful and was looking for practical means to build a linear amplifier for these signals. At the time, short wave was anything above about 500&nbsp;kHz, beyond any existing amplifier's capabilities.

It had been noticed that when a regenerative receiver went into oscillation, other nearby receivers would start picking up other stations as well. Armstrong (and others) eventually deduced that this was caused by a "supersonic heterodyne" between the station's carrier frequency and the regenerative receiver's oscillation frequency. When the first receiver began to oscillate at high outputs, its signal would flow back out through the antenna to be received on any nearby receiver. On that receiver, the two signals mixed just as they did in the original heterodyne concept, producing an output that is the difference in frequency between the two signals.

For instance, consider a lone receiver that was tuned to a station at 300&nbsp;kHz. If a second receiver is set up nearby and set to 400&nbsp;kHz with high gain, it will begin to give off a 400&nbsp;kHz signal that will be received in the first receiver. In that receiver, the two signals will mix to produce four outputs, one at the original 300&nbsp;kHz, another at the received 400&nbsp;kHz, and two more, the difference at 100&nbsp;kHz and the sum at 700&nbsp;kHz. This is the same effect that Fessenden had proposed, but in his system the two frequencies were deliberately chosen so the beat frequency was audible. In this case, all of the frequencies are well beyond the audible range, and thus "supersonic", giving rise to the name superheterodyne.

Armstrong realized that this effect was a potential solution to the "short wave" amplification problem, as the "difference" output still retained its original modulation, but on a lower carrier frequency. In the example above, one can amplify the 100&nbsp;kHz beat signal and retrieve the original information from that, the receiver does not have to tune in the higher 300&nbsp;kHz original carrier. By selecting an appropriate set of frequencies, even very high-frequency signals could be "reduced" to a frequency that could be amplified by existing systems.

For instance, to receive a signal at 1500&nbsp;kHz, far beyond the range of efficient amplification at the time, one could set up an oscillator at, for example, 1560&nbsp;kHz. Armstrong referred to this as the "[[local oscillator]]" or LO. As its signal was being fed into a second receiver in the same device, it did not have to be powerful, generating only enough signal to be roughly similar in strength to that of the received station, although in practice LOs tend to be relatively strong signals.{{cn|date=August 2024}} When the signal from the LO mixes with the station's, one of the outputs will be the heterodyne difference frequency, in this case, 60&nbsp;kHz. He termed this resulting difference the "[[intermediate frequency]]" often abbreviated to "IF".

<blockquote>
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, for example 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.<ref name="Leutz_1922"/>
</blockquote>

The "trick" to the superheterodyne is that by changing the LO frequency you can tune in different stations. For instance, to receive a signal at 1300&nbsp;kHz, one could tune the LO to 1360&nbsp;kHz, resulting in the same 60&nbsp;kHz IF. This means the amplifier section can be tuned to operate at a single frequency, the design IF, which is much easier to do efficiently.

=== Development ===
[[File:Radiola AR-812 superheterodyne ad.jpg|thumb|upright=1.3|The first commercial superheterodyne receiver,<ref name="Malanowski_2011"/> the RCA Radiola AR-812, released on March 4, 1924, priced at $286 ({{Inflation|US|286|1924|fmt=eq|r=-1}}). It used 6 triodes: a mixer, local oscillator, two IF and two audio amplifier stages, with an IF of 45&nbsp;kHz. It was a commercial success, with better performance than competing receivers.]]

Armstrong put his ideas into practice, and the technique was soon adopted by the military. 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. For early domestic radios, [[tuned radio frequency receiver]]s (TRF) were more popular because they were cheaper, easier for a non-technical owner to use, and less costly to operate. Armstrong eventually sold his superheterodyne patent to [[Westinghouse Electric Corporation|Westinghouse]], which then sold it to [[RCA|Radio Corporation of America (RCA)]], the latter monopolizing the market for superheterodyne receivers until 1930.<ref name="Katz"/>

Because the original motivation for the superhet was the difficulty of using the triode amplifier at high frequencies, there was an advantage in using a lower intermediate frequency. During this era, many receivers used an IF frequency of only 30&nbsp;kHz.<ref name="Bussey_1990"/> These low IF frequencies, often using IF transformers based on the self-resonance of iron-core [[transformer]]s, had poor [[#Image frequency (fIMAGE)|image frequency]] rejection, but overcame the difficulty in using triodes at radio frequencies in a manner that competed favorably with the less robust [[neutrodyne]] TRF receiver. Higher IF frequencies (455&nbsp;kHz was a common standard) came into use in later years, after the invention of the [[tetrode]] and [[pentode]] as amplifying tubes, largely solving the problem of image rejection. Even later, however, low IF frequencies (typically 60&nbsp;kHz) were again used in the ''second'' (or third) IF stage of [[#Multiple conversion|double or triple-conversion]] communications receivers to take advantage of the [[selectivity (radio)|selectivity]] more easily achieved at lower IF frequencies, with image-rejection accomplished in the earlier IF stage(s) which were at a higher IF frequency.

In the 1920s, at these low frequencies, commercial IF filters looked very similar to 1920s audio interstage coupling transformers, had similar construction, and were wired up in an almost identical manner, so they were referred to as "IF transformers". By the mid-1930s, superheterodynes using much higher intermediate frequencies (typically around 440–470&nbsp;kHz) used tuned transformers more similar to other RF applications. The name "IF transformer" was retained, however, now meaning "intermediate frequency". Modern receivers typically use a mixture of [[ceramic resonator]]s or [[surface acoustic wave]] resonators and traditional tuned-inductor IF transformers.

{{multiple image
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| image1   = Philco radio model PT44 front.jpg
| width1   = 150
| image2   = Philco radio model PT44 chassis back.jpg
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| footer   = "[[All American Five]]" vacuum-tube superheterodyne AM broadcast receiver from 1940s was cheap to manufacture because it only required five tubes.
}}

By the 1930s, improvements in vacuum tube technology rapidly eroded the TRF receiver's cost advantages, and the explosion in the number of broadcasting stations created a demand for cheaper, higher-performance receivers.

The introduction of an additional grid in a vacuum tube, but before the more modern screen-grid tetrode, included the [[Bi-grid valve|tetrode with two control grids]]; this tube combined the mixer and oscillator functions, first used in the so-called [[autodyne]] mixer. This was rapidly followed by the introduction of tubes specifically designed for superheterodyne operation, most notably the [[pentagrid converter]]. By reducing the tube count (with each tube stage being the main factor affecting cost in this era), this further reduced the advantage of TRF and regenerative receiver designs.

By the mid-1930s, commercial production of TRF receivers was largely replaced by superheterodyne receivers.  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 used five vacuum tubes: usually a converter (mixer/local oscillator), an IF amplifier, a detector/audio amplifier, audio power amplifier, and a rectifier.  Since this time, the superheterodyne design was used for almost 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.<ref name="Koster_2016"/> Armstrong also filed his patent in 1917.<ref name="Howarth_2017"/><ref name="luxor"/><ref name="Sarkar_2006"/>  Levy filed his original disclosure about seven months before Armstrong's.<ref name="Klooster_2009"/>
German inventor [[Walter H. Schottky]] also filed a patent in 1918.<ref name="Koster_2016"/>

At first the US recognised Armstrong as the inventor, and his US Patent 1,342,885 was issued on 8 June 1920.<ref name="Klooster_2009"/> After various changes and court hearings Lévy was awarded 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.<ref name="Klooster_2009"/>
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