Editing Tetrode (section)

==The superheterodyne receiver==
{{main|Superheterodyne receiver}}
The principle of the modern [[superheterodyne]] (or ''superhet'') receiver (originally named the ''super-sonic heterodyne'' receiver, because the [[intermediate frequency]] was at an [[ultrasonic]] frequency) was invented in France by Lucien Levy in 1917<ref><Thrower></ref> (p 66), though credit is usually also given to [[Edwin Armstrong]]. The original reason for the invention of the superhet was that before the appearance of the screen-grid valve, amplifying valves, then [[triode]]s, had difficulty amplifying radio frequencies (i.e. frequencies much above 100&nbsp;kHz) due to the [[Miller effect]]. In the superheterodyne design, rather than amplifying the incoming radio signal, it was first [[Frequency mixer|mixed]] with a constant RF oscillator (the so-called [[local oscillator]]) to produce a [[heterodyne]] of typically 30&nbsp;kHz.  This intermediate frequency (IF) signal had an identical [[Envelope (waves)|envelope]] as the incoming signal but a much lower [[Carrier signal|carrier]] frequency, so it could be efficiently amplified using triodes. When [[Detector (radio)|detected]], the original [[modulation]] of the higher frequency radio signal is obtained.<ref name="Admiralty">{{cite book   
  | last = Murray
  | first = O.
  | title = Admiralty Handbook of Wireless Telegraphy 1931
  | publisher = HMSO
  | year = 1931
  | location = London
  | pages = 723
  }}</ref>  A somewhat complicated technique, it went out of favor  when screen-grid tetrodes made [[tuned radio frequency]] (TRF) receivers practical.{{cn|date=December 2021}}  However the superheterodyne principle resurfaced in the early 1930s when their other advantages, such as greater [[selectivity (radio)|selectivity]] became appreciated, and almost all modern receivers operate on this principle but with a higher IF frequency (sometimes higher than the original RF) with amplifiers (such as the tetrode) having surpassed the triode's limitation in amplifying high (radio) frequency signals.

The superheterodyne concept could be implemented using a valve as the local oscillator and a separate valve as the mixer which takes the antenna signal and the local oscillator as input signals. But for economy, those two functions could also be combined in a single bi-grid tetrode which would both oscillate and frequency-mix the RF signal from the antenna.<ref name="Williams"></ref> In later years this was similarly accomplished by the [[pentagrid converter]] tube, a similar two-input amplifying/oscillating valve, but which (like [[pentode]] tubes) incorporated a [[suppressor grid]] and in this case two screen grids in order to electrostatically isolate the plate and both signal grids from each other. In today's receivers, based on inexpensive semiconductor technology ([[transistor]]s), there is no cost benefit in combining the two functions in one active device.