Editing Tuned radio frequency receiver (section)

== 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 (signal processing)|bandwidth]] and more [[selectivity (radio)|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 tube]]s and [[tuned circuit]]s.  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 is the triode's interelectrode capacitance.  The interelectrode capacitance allows energy in the output circuit to [[feedback]] into the input.  That feedback can cause instability and [[oscillation]] that 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.<ref name="Lee">{{cite book   
  | last = Lee
  | first = Thomas H. |author-link=Thomas H. Lee (electronic engineer)
  | title = The Design of CMOS Radio-Frequency Integrated Circuits |edition=2nd
  | publisher = Cambridge University Press
  | date = 2004
  | location = UK
  | pages = 16
  | url = https://books.google.com/books?id=io1hL48OqBsC&pg=PA16
  | doi = 
  | id = 
  | isbn = 0521835399}}</ref> 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."<ref>{{Citation |last= Terman |first= Frederick E. |author-link= Frederick Terman |title= Radio Engineers' Handbook |publisher= McGraw-Hill |year= 1943 |page=469}}</ref> "Perfect neutralization cannot be maintained in practice over a wide band of frequencies because leakage inductances and stray capacities" are not completely canceled.<ref>{{citation |first=Frederick Emmons |last=Terman |authorlink=Frederick Terman |title=Radio Engineering |edition=second |year=1937 |publisher=McGraw-Hill |location=New York |page=236}}</ref>  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.<ref>{{Harvtxt|Terman|1937|p=238}} 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."</ref>

[[Image:TFR Tube Layout.jpg|200px|Typical tube layout of a TRF radio]]  [[Image:TRF Dials.jpg|200px|Typical Dial Layout of Tuned Radio Frequency Receiver]]  [[Image:TRF Component Layout.jpg|200px|Typical Tuned Radio Frequency receiver component layout]]