Editing Regenerative circuit (section)

==Regenerative receiver==
[[File:Armstrong regenerative receiver circuit.svg|thumb|upright=1.3|Vacuum tube regenerative receiver schematic. Most regenerative receivers used this [[Armstrong oscillator|Armstrong circuit]], in which the feedback was applied to the input (grid) of the tube with a "tickler coil" winding on the tuning inductor.]]

The [[Gain (electronics)|gain]] of any amplifying device, such as a [[vacuum tube]], [[transistor]], or [[op amp]], can be increased by feeding some of the energy from its output back into its input in phase with the original input signal. This is called [[positive feedback]] or ''regeneration''.<ref>{{Cite web |url=http://www.nj7p.org/Manuals/PDFs/Books/Sturley_1.pdf |title=K. R. Sturley, ''Radio Receiver Design'' (Part I), New York: John Wiley and Sons, 1943, p. 392 |access-date=2018-07-04 |archive-date=2017-06-27 |archive-url=https://web.archive.org/web/20170627063935/http://nj7p.org/Manuals/PDFs/Books/Sturley_1.pdf |url-status=dead }}</ref><ref name=Everitt01/>  Because of the large amplification possible with regeneration, regenerative receivers often use only a single amplifying element (tube or transistor).<ref>[https://archive.org/stream/ThermionicValveCircuits/Williams-ThermionicValveCircuits#page/n165/mode/2up  E. Williams, 1961, pp. 156-158]</ref> In a regenerative receiver the output of the tube or transistor is connected back to its own input through a [[tuned circuit]] (LC circuit).<ref>[https://archive.org/stream/Electronic_Circuits_and_Tubes_Cruft_Laboratory_at_Harvard_University_1947#page/n763 Cruft Electronics Staff, ''Electronic Circuits and Tubes'', New York: McGraw-Hill, 1947, pp. 741-744]</ref><ref name="robinson">H. A. Robinson, "Regenerative Detectors", ''[[QST]]'', vol. XVII, no. 2, p. 26, Feb. 1933</ref>  The tuned circuit allows positive feedback only at its [[resonant frequency]]. In regenerative receivers using only one active device, the same tuned circuit is coupled to the antenna and also serves to select the radio frequency to be received, usually by means of variable capacitance. In the regenerative circuit discussed here, the active device also functions as a [[grid-leak detector|detector]]; this circuit is also known as a ''regenerative detector''.<ref name="robinson"/> A regeneration control is usually provided for adjusting the amount of feedback (the [[loop gain]]). It is desirable for the circuit design to provide regeneration control that can gradually increase feedback to the point of oscillation and that provides control of the oscillation from small to larger amplitude and back to no oscillation without jumps of amplitude or hysteresis in control.<ref>{{Cite web |url=http://www.nj7p.org/Manuals/PDFs/Books/Sturley_1.pdf |title=K. R. Sturley, 1943, pp. 394-395 |access-date=2018-07-04 |archive-date=2017-06-27 |archive-url=https://web.archive.org/web/20170627063935/http://nj7p.org/Manuals/PDFs/Books/Sturley_1.pdf |url-status=dead }}</ref><ref>[https://www.americanradiohistory.com/Archive-Experimental%20Wireless/40s/Wireless-Engineer-1946-08.pdf E. E. Zepler, "Oscillation Hysteresis in Grid Detectors", ''Wireless Engineer'', vol. XXIII, no. 275, Aug. 1946, p. 222]</ref><ref name="cruft283"/><ref>E. E. Zepler, ''The Technique of Radio Design'', 2nd ed., New York: John Wiley and Sons, 1951, p. 168</ref>

Two important attributes of a radio receiver are ''sensitivity'' and ''selectivity''.<ref>[https://archive.org/stream/Electronic_Circuits_and_Tubes_Cruft_Laboratory_at_Harvard_University_1947#page/n763 Cruft Electronics Staff, 1947, p. 741]</ref> The regenerative detector provides sensitivity and selectivity due to voltage amplification and the characteristics of a resonant circuit consisting of inductance and capacitance. The regenerative voltage amplification <math>u_{\mathrm{o}}</math> is <math>u_{\mathrm{o}} = u / (1-ua)</math> where <math>u</math> is the non-regenerative amplification and <math>a</math> is the portion of the output signal fed back to the L2 C2 circuit. As <math>1-ua</math> becomes smaller the amplification increases.<ref>[https://archive.org/details/communicationeng00ever W. L. Everitt, 1937, p. 464]</ref> The <math>Q</math> of the tuned circuit (L2 C2) without regeneration is <math>Q=X_{\mathrm{L}}/R</math> where <math>X_{\mathrm{L}}</math> is the reactance of the coil and <math>R</math> represents the total dissipative loss of the tuned circuit. The positive feedback compensates the energy loss caused by <math>R</math>, so it may be viewed as introducing a negative resistance <math>R_{\mathrm{r}}</math> to the tuned circuit.<ref name="cruft283">[https://archive.org/stream/Electronic_Circuits_and_Tubes_Cruft_Laboratory_at_Harvard_University_1947#page/n765 Cruft Electronics Staff, 1947, p. 743]</ref> The <math>Q</math> of the tuned circuit with regeneration is <math>Q_{\mathrm{reg}} = X_{\mathrm{L}}/(R-|R_{\mathrm{r}}|)</math>.<ref name="cruft283"/> The regeneration increases the <math>Q</math>. Oscillation begins when <math>|R_{\mathrm{r}}|=R</math>.<ref name="cruft283"/>

Regeneration can increase the detection gain of a detector by a factor of 1,700 or more. This is quite an improvement, especially for the low-gain vacuum tubes of the 1920s and early 1930s.  The type 36 screen-grid tube (obsolete since the mid-1930s) had a non-regenerative detection gain (audio frequency plate voltage divided by radio frequency input voltage) of only 9.2 at 7.2&nbsp;MHz, but in a regenerative detector, had detection gain as high as 7,900 at critical regeneration (non-oscillating) and as high as 15,800 with regeneration just above critical.<ref name="robinson"/> The "... non-oscillating regenerative amplification is limited by the stability of the circuit elements, tube [or device] characteristics and [stability of] supply voltages which determine the maximum value of regeneration obtainable without self-oscillation".<ref name="robinson"/>  Intrinsically, there is little or no difference in the gain and stability available from vacuum tubes, JFETs, MOSFETs or bipolar junction transistors (BJTs).

A major improvement in stability and a small improvement in available gain for reception of CW radiotelegraphy is provided by the use of a separate oscillator, known as a ''heterodyne oscillator'' or ''beat oscillator''.<ref name="robinson"/><ref name="talbert">R. J. Talbert, "The Simple Regenerative Receiver with Separate Beat Oscillator", ''[[QST]]'',  vol. XX, no. 2, p. 15, Feb. 1936</ref> Providing the oscillation separately from the detector allows the regenerative detector to be set for maximum gain and selectivity - which is always in the non-oscillating condition.<ref name="robinson"/><ref name="decola"/> Interaction between the detector and the beat oscillator can be minimized by operating the beat oscillator at half of the receiver operating frequency, using the second harmonic of the beat oscillator in the detector.<ref name="talbert"/>

===AM reception===
For [[amplitude modulation|AM]] reception, the gain of the loop is adjusted so it is just below the level required for [[oscillation]] (a loop gain of just less than one).  The result of this is to greatly increase the gain of the amplifier at the bandpass frequency (resonant frequency), while not increasing it at other frequencies.   So the incoming radio signal is amplified by a large factor, 10<sup>3</sup> - 10<sup>5</sup>, increasing the receiver's sensitivity to weak signals.  The high gain also has the effect of reducing the circuit's [[Bandwidth (signal processing)|bandwidth]] (increasing the [[Q factor|Q]]) by an equal factor, increasing the [[selectivity (radio)|selectivity]] of the receiver.<ref>{{cite book|title=The Radio Amateur's Handbook|year=1978|publisher=[[American Radio Relay League]]|pages=241–242}}</ref>

===CW reception (autodyne mode)===
{{main|Autodyne}}
For the reception of [[Continuous wave|CW]] [[radiotelegraphy]] ([[Morse code]]), the feedback is increased just to the point of oscillation. The tuned circuit is adjusted to provide typically 400 to 1000 Hertz difference between the receiver oscillation frequency and the desired transmitting station's signal frequency. The two frequencies [[beat frequency|''beat'']] in the nonlinear amplifier, generating [[heterodyne]] or ''beat'' frequencies.<ref>[https://archive.org/stream/principlesunderl00unitrich#page/502/mode/2up Signal Corps U.S. Army, ''The Principles Underlying Radio Communication'', 2nd ed. Washington, DC: U.S.G.P.O., 1922, p. 501]</ref> The difference frequency, typically 400 to 1000 Hertz, is in the audio range; so it is heard as a tone in the receiver's speaker whenever the station's signal is present.

Demodulation of a signal in this manner, by use of a single amplifying device as oscillator and [[Frequency mixer|mixer]] simultaneously, is known as ''autodyne'' reception.<ref>[https://archive.org/stream/principlesunderl00unitrich#page/502/mode/2up Signal Corps U.S. Army, 1922, p. 503]</ref> The term ''autodyne'' predates multigrid tubes and is not applied to use of tubes specifically designed for frequency conversion.

===SSB reception===
For the reception of [[Single-sideband modulation|single-sideband]] (SSB) signals, the circuit is also adjusted to oscillate as in CW reception. The tuning is adjusted until the demodulated voice is intelligible.

=== Advantages and disadvantages ===
Regenerative receivers require fewer components than other types of receiver circuit, such as the [[Tuned radio frequency receiver|TRF]] and [[superheterodyne]].  The circuit's advantage was that it got much more amplification (gain) out of the expensive [[vacuum tube]]s, thus reducing the number of tubes required and therefore the cost of a receiver.  Early vacuum tubes had low gain and tended to oscillate at [[Radio frequency|radio frequencies]] (RF). TRF receivers often required 5 or 6 tubes; each stage requiring tuning and neutralization, making the receiver cumbersome, power hungry, and hard to adjust.  A regenerative receiver, by contrast, could often provide adequate reception with the use of only one tube.  In the 1930s the regenerative receiver was replaced by the superheterodyne circuit in commercial receivers due to the superheterodyne's superior performance and the falling cost of tubes.  Since the advent of the [[transistor]] in 1946, the low cost of active devices has removed most of the advantage of the circuit.  However, in recent years the regenerative circuit has seen a modest comeback in receivers for low cost [[digital radio]] applications such as [[garage door opener]]s, [[keyless entry|keyless locks]], [[RFID]] readers and some [[cell phone]] receivers.

A disadvantage of this receiver, especially in designs that couple the detector tuned circuit to the antenna, is that the regeneration (feedback) level must be adjusted when the receiver is tuned to a different frequency. The antenna impedance varies with frequency, changing the loading of the input tuned circuit by the antenna, requiring the regeneration to be adjusted. In addition, the Q of the detector tuned circuit components vary with frequency, requiring adjustment of the regeneration control.<ref name="TM11-665" />{{rp|p.189}}

A disadvantage of the single active device regenerative detector in autodyne operation is that the local oscillation causes the operating point to move significantly away from the ideal operating point, resulting in the detection gain being reduced.<ref name="decola">R. De Cola, "Increased Sensitivity With the Regenerative Detector", ''[[QST]]'', vol. XVIII, no. 12, p. 24, Dec. 1934</ref>

Another drawback is that when the circuit is adjusted to oscillate it can radiate a signal from its antenna, so it can cause [[Radio-frequency interference|interference]] to other nearby receivers. Adding an RF amplifier stage between the antenna and the regenerative detector can reduce unwanted radiation, but would add expense and complexity.

Other shortcomings of regenerative receivers are the sensitive and unstable tuning. These problems have the same cause: a regenerative receiver's gain is greatest when it operates on the verge of oscillation, and in that condition, the circuit behaves [[chaos theory|chaotically]].<ref>Domine M.W. Leenaerts and Wim M.G. van Bokhoven, “Amplification via chaos in regenerative detectors,” ''Proceedings of SPIE'' *, vol. 2612**, pages 136-145 (December 1995).
(* SPIE = Society of Photo-optical Instrumentation Engineers; renamed: International Society for Optical Engineering) 
(** Jaafar M.H. Elmirghani, ed., ''Chaotic Circuits for Communication'' -- a collection of papers presented at the SPIE conference of 23–24 October 1995 in Philadelphia, Pennsylvania.)</ref><ref>Domine M.W. Leenaerts, “Chaotic behavior in superregenerative detectors,” ''IEEE Transactions on Circuits and Systems Part 1: Fundamental Theory and Applications'', vol. 43, no. 3, pages 169-176 (March 1996).</ref><ref>In 1922, during his development of the superregenerative receiver, [[Edwin Howard Armstrong|Edwin Armstrong]] noted signs of chaotic behavior in his circuits.  See:  Edwin H. Armstrong (1922) [https://books.google.com/books?id=xUISAAAAIAAJ&pg=PA244 "Some recent developments of regenerative circuits,"] ''Proceedings of the Institute of Radio Engineers'', '''10''' (8) :  244-260.  From p. 252: " ... a free oscillation starts every time the resistance of the circuit becomes negative. ... The free oscillations produced in the system when no signaling emf. is impressed, must be initiated by some irregularity of operation of the vacuum tubes, ... ."</ref> Simple regenerative receivers electrically couple the antenna to the detector tuned circuit, resulting in the electrical characteristics of the antenna influencing the resonant frequency of the detector tuned circuit. Any movement of the antenna or large objects near the antenna can change the tuning of the detector.

===History===
[[File:1915 Armstrong Tickler regen receiver.gif|thumb|1915 Armstrong regenerative receiver]]
The inventor of [[FM broadcasting|FM]] radio, [[Edwin Armstrong]], filed US patent 1113149 in 1913 about regenerative circuit while he was a junior in college.<ref>{{Citation |title=The Armstrong Patent |date=May 1922 |journal=Radio Broadcast |location=Garden City, NY |publisher=Doubleday, Page & Co. |volume=1 |issue=1 |pages=71&ndash;72 |url=https://books.google.com/books?id=VMcnAAAAYAAJ&pg=PA71 }}</ref> He patented the superregenerative circuit in 1922, and the [[superheterodyne]] receiver in 1918.

[[Lee De Forest]] filed US patent 1170881 in 1914 that became the cause of a contentious lawsuit with Armstrong, whose patent for the regenerative circuit had been issued in 1914. The lawsuit lasted until 1934, winding its way through the appeals process and ending up at the [[Supreme Court of the United States|Supreme Court]]. Armstrong won the first case, lost the second, stalemated at the third, and then lost the final round at the Supreme Court.<ref>{{Harvnb|Morse|1925|p=55}}</ref><ref>{{Harvnb|Lewis|1991}}</ref>

At the time the regenerative receiver was introduced, [[vacuum tube]]s were expensive and consumed much power, with the added expense and encumbrance of heavy batteries. So this design, getting most gain out of one tube, filled the needs of the growing radio community and immediately thrived. Although the superheterodyne receiver is the most common receiver in use today{{Citation needed|reason=direct conversion is the standard for the vast majority of single chip receivers used in bluetooth, wifi, LTE, SDRs etc, but which is most common?|date=January 2024}}, the regenerative radio made the most out of very few parts.

In World War II the regenerative circuit was used in some military equipment. An example is the German field radio "Torn.E.b".<ref>{{langx|de|Tornisterfunkgerät}} = [[Joint Tactical Radio System#JTRS Handheld, Manpack & Small Form Fit (HMS)|Manpack]] radio</ref> Regenerative receivers needed far fewer tubes and less power consumption for nearly equivalent performance.

A related circuit, the ''superregenerative detector'', found several highly important military uses in World War II in  [[Identification friend or foe|Friend or Foe]] identification equipment and in the top-secret [[proximity fuze]]. An example here is the miniature RK61 [[thyratron]] marketed in 1938, which was designed specifically to operate like a [[Triode|vacuum triode]] below its ignition voltage, allowing it to amplify analog signals as a self-quenching superregenerative detector in [[radio control]] receivers,<ref>{{cite web |url=http://www.mif.pg.gda.pl/homepages/frank/sheets/138/r/RK61.pdf |title=''Subminiature gas triode type RK61'' data sheet |publisher=[[Raytheon|Raytheon Company]] |access-date=20 March 2017 |archive-date=20 March 2017 |archive-url=https://web.archive.org/web/20170320233833/http://www.mif.pg.gda.pl/homepages/frank/sheets/138/r/RK61.pdf |url-status=dead }}</ref> and was the major technical development which led to the wartime development of radio-controlled weapons and the parallel development of [[radio-controlled models|radio controlled modelling]] as a hobby.<ref name=Honnest-Redlich>George Honnest-Redlich ''Radio Control for Models (1950)'' p. 7</ref>

In the 1930s, the [[superheterodyne]] design began to gradually supplant the regenerative receiver, as tubes became far less expensive. In Germany the design was still used in the millions of mass-produced German "peoples receivers" ([[Volksempfänger]]) and "German small receivers" (DKE, Deutscher Kleinempfänger). Even after WWII, the regenerative design was still present in early after-war German minimal designs along the lines of the "peoples receivers" and "small receivers", dictated by lack of materials. Frequently German military tubes like the "RV12P2000" were employed in such designs. There were even superheterodyne designs, which used the regenerative receiver as a combined IF and demodulator with fixed regeneration. The superregenerative design was also present in early FM broadcast receivers around 1950. Later it was almost completely phased out of mass production, remaining only in hobby kits, and some special applications, like gate openers.