Editing Superheterodyne receiver (section)

=== 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
| align = right
| direction = horizontal
| image1   = Philco radio model PT44 front.jpg
| width1   = 150
| image2   = Philco radio model PT44 chassis back.jpg
| width2   = 150
| 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.