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tuned radio frequency receiver
A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an audio frequency amplifier. This type of receiver was popular in the 1920s. Early examples could be tedious to operate because when tuning in a station each stage had to be individually adjusted to the station's frequency, but later models had ganged tuning, the tuning mechanisms of all stages being linked together, and operated by just one control knob. By the mid 1930s, it was replaced by the superheterodyne receiver invented by Edwin Armstrong.
== Background ==
The TRF receiver was patented in 1916 by Ernst Alexanderson. His concept was that each stage would amplify the desired signal while reducing the interfering ones. Multiple stages of RF amplification would make the radio more sensitive to weak stations, and the multiple tuned circuits would give it a narrower bandwidth and more selectivity than the single stage receivers common at that time. All tuned stages of the radio must track and tune to the desired reception frequency. This is in contrast to the modern superheterodyne receiver that must only tune the receiver's RF front end and local oscillator to the desired frequencies; all the following stages work at a fixed frequency and do not depend on the desired reception frequency.
Antique TRF receivers can often be identified by their cabinets. They typically have a long, low appearance, with a flip-up lid for access to the vacuum tubes and tuned circuits. On their front panels there are typically two or three large dials, each controlling the tuning for one stage. Inside, along with several vacuum tubes, there will be a series of large coils. These will usually be with their axes at right angles to each other to reduce magnetic coupling between them.
A problem with the TRF receiver built with triode vacuum tubes was the triode's interelectrode capacitance. The interelectrode capacitance allowed energy in the output circuit to feedback into the input. That feedback could cause instability and oscillation that would frustrate reception and produce squealing or howling noises in the speaker. In 1922, Louis Alan Hazeltine invented the technique of ''neutralization'' that uses additional circuitry to partially cancel the effect of the interelectrode capacitance. Neutralization was used in the popular Neutrodyne series of TRF receivers. Under certain conditions, "the neutralization is substantially independent of frequency over a wide frequency band." "Perfect neutralization cannot be maintained in practice over a wide band of frequencies because leakage inductances and stray capacities" are not completely canceled. The later development of the tetrode and pentode vacuum tubes minimized the effect of interelectrode capacitances and could make neutralization unnecessary; the additional electrodes in those tubes shield the plate and grid and minimize feedback.〔 states, "Neutralization is always necessary in tuned triode amplifiers for otherwise the input resistance will be so low that oscillations can be expected. It is not employed with pentode and screen-grid amplifiers, however, because the direct-capacity coupling between grid and plate in such tubes is very small."〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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