Editing Tuned radio frequency receiver (section)

== How it works ==
[[File:Tuned radio frequency (TRF) receiver block diagram 2.svg|thumb|upright=1.5|Block diagram of TRF receiver]]

The classic TRF receivers of the 1920s and 30s usually consisted of three sections: 
*one or more tuned RF amplifier stages.  These amplify the signal of the desired station to a level sufficient to drive the detector, while rejecting all other signals picked up by the antenna.
*a [[detector (radio)|detector]], which extracts the [[audio signal|audio]] ([[modulation]]) signal from the radio [[carrier signal]] by [[rectifier|rectifying]] it. 
*''optionally'', but almost always included, one or more [[audio amplifier]] stages which increase the power of the audio signal.

[[Image:Leutz Phantom 9 tube TRF receiver.jpg|thumb|upright=1.4|Leutz 9-tube receiver from 1927 clearly shows the component parts of a TRF set. Each RF stage is in a separate compartment. Within each compartment can be seen ''(from top)'': the triode tube, the interstage coupling coil, and the capacitor attached to its front panel tuning dial. The compartments contain ''(from left)'': the 4 RF stages, the detector stage, and the 4 tube audio amplifier.  The capacitors could be linked to a common shaft and tuned together, or tuned separately.]]

Each tuned RF stage consists of an amplifying device, a [[triode]] (or in later sets a [[tetrode]]) [[vacuum tube]], and a [[tuned circuit]] which performs the filtering function.  The tuned circuit consisted of an air-core RF coupling [[transformer]] which also served to couple the signal from the [[plate electrode|plate]] circuit of one tube to the input [[control grid|grid]] circuit of the next tube. One of the windings of the transformer had a variable [[capacitor]] connected across it to make a [[tuned circuit]]. A variable capacitor (or sometimes a variable coupling coil called a ''variometer'')  was used, with a knob on the front panel to tune the receiver.  The RF stages usually had identical circuits to simplify design.

Each RF stage had to be tuned to the same frequency,  so the capacitors had to be tuned in tandem when bringing in a new station.  In some later sets the capacitors were "ganged", mounted on the same shaft or otherwise linked mechanically so that the radio could be tuned with a single knob, but in most sets the [[resonant frequency|resonant frequencies]] of the tuned circuits could not be made to "track" well enough to allow this, and each stage had its own tuning knob.<ref name="Felix">{{cite journal
  | last1  = Felix
  | first1 = Edgar H.
  | title = Something about single control
  | journal = Radio Broadcast
  | volume = 11
  | issue = 3
  | pages = 151–152
  | publisher = Doubleday, Page and Co.
  | location = New York
  | date = July 1927
  | url = http://www.americanradiohistory.com/Archive-Radio-Broadcast/Radio-Broadcast-1927-07.pdf
  | issn = 
  | doi = 
  | id = 
  | accessdate = January 10, 2015}}</ref>

The detector was usually a [[grid-leak detector]]. Some sets used a [[crystal detector]] ([[semiconductor diode]]) instead.  Occasionally, a [[regenerative circuit|regenerative detector]] was used, to increase selectivity.

Some TRF sets that were listened to with [[earphone]]s didn't need an audio amplifier, but most sets had one to three transformer-coupled or RC-coupled audio amplifier stages to provide enough power to drive a [[loudspeaker]].

[[Image:TRF Schematic.jpg|700px|Schematic of Six Tube Design using Triode Tubes – Two Radio Frequency Amplifiers, One Grid-Leak Detector, Three Class ‘A’ Audio Amplifiers]]

The schematic diagram shows a typical TRF receiver.  This particular  example uses six triodes. It has two radio frequency amplifier stages, one grid-leak detector/amplifier and three class ‘A’ audio amplifier stages.  There are 3 tuned circuits ''T1-C1, T2-C2, and T3-C3''.  The second and third tuning capacitors, ''C2'' and ''C3'', are ganged together ''(indicated by line linking them)'' and controlled by a single knob, to simplify tuning.  Generally, two or three RF amplifiers were required to filter and amplify the received signal enough for good reception.