Editing Superheterodyne receiver (section)

=== 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.