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((more citations needed|date=August 2011))
[[File:Jan 1916 QST Oscillating Audion.png|thumb|Schematic of an early autodyne receiver.]]

The '''autodyne''' circuit was an improvement to radio signal amplification using the [[Lee De Forest|De Forest]] [[Audion]] vacuum tube amplifier. By allowing the tube to oscillate at a frequency slightly different from the desired signal, the sensitivity over other receivers was greatly improved.<ref>((Harvnb|Bureau of Standards|1922|p=503))</ref> The autodyne circuit was invented by [[Edwin Howard Armstrong]] of [[Columbia University]], New York, NY.((citation needed|date=May 2012))  He inserted a tuned circuit in the output circuit of the Audion [[vacuum tube]] amplifier.((citation needed|date=May 2012))<!-- why use Audion when triode was available? July 2018 - Huh? The Audion IS a triode! -->  By adjusting the tuning of this tuned circuit, Armstrong was able to dramatically increase the gain of the Audion amplifier.  Further increase in tuning resulted in the Audion amplifier reaching self-oscillation.

This [[oscillating]] receiver circuit meant that the then latest technology continuous wave (''CW'') transmissions could be demodulated.  Previously only spark, interrupted continuous wave (''ICW'', signals which were produced by a motor chopping or turning the signal on and off at an audio rate), or modulated continuous wave (MCW), could produce intelligible output from a receiver.

When the autodyne oscillator was advanced to self-oscillation, continuous wave [[Morse code]] dots and dashes would be clearly heard from the headphones as short or long periods of sound of a particular tone, instead of an all but impossible to decode series of thumps. Spark and chopped CW (ICW) were amplitude modulated signals which didn't require an oscillating detector.
Such a [[regenerative circuit]] is capable of receiving weak signals, if carefully coupled to an antenna. Antenna coupling interacts with tuning, making optimum adjustments difficult.

==Heterodyne detection==
((Main|Heterodyne))

===Damped wave transmission===
Early transmitters emitted ''damped waves'', which were radio frequency sine wave bursts of a number of cycles duration, of decreasing amplitude with each cycle. These bursts recurred at an audio frequency rate, producing an amplitude modulated transmission.<ref>((Harvnb|Bureau of Standards|1922|p=353))</ref> The damped waves were a result of the available technologies to generate radio frequencies.  ''See'' [[spark gap transmitter]]. The transmitters could be keyed on and off to send Morse code.

Receivers could be made with a tuned circuit, a [[crystal detector]], and a headphone. The headphone would respond to the detected bursts, and the operator could copy the Morse code. The received signal was not a sinewave. Instead of a crystal detector, a Fleming valve (tube diode) could be used; it was a stable detector, but not very sensitive. Even better was a using a vacuum triode because it provided some amplification.<ref>((Harvnb|Bureau of Standards|1922|p=426))</ref>  The [[regenerative receiver]] supplied even more gain, but required careful adjustment.<ref>((Harvnb|Bureau of Standards|1922|p=427))</ref>

===Undamped wave transmission===
Damped wave transmission had drawbacks, and the focus shifted to ''undamped waves'' or ''continuous wave'' (CW) transmission. The [[arc converter]] could produce high power CW transmissions.

The typical damped wave receiver was ineffective for receiving CW because CW had, ideally, no modulation of the radio frequency during the period of the dot or dash. Several methods were employed to generate an audible tone at the receiver: (1) a chopper, (2) a variable condensor with rotating plates (slope demodulation), (3) a [[tikker]], (4) a separate heterodyne, and (5) the autodyne.<ref>((harvnb|Bureau of Standards|1922|p=430))</ref>

Fessenden researched the heterodyne detector.<!-- and need more... -->

==Application==
The autodyne was widely used in both commercially produced and amateur receiver designs from shortly after the time of its invention until the middle 1930s. It became popular at the beginning of the [[Great Depression|Depression]] (ca early 1930s) for [[Frequency_mixer|''first detector'']] applications in superheterodyne receivers.<ref>[https://archive.org/stream/bitsavers_malloryMYE_73827616/MYE_Technical_Manual_1942#page/n31/mode/2up  P. R. Mallory & Co., ''MYE Technical Manual'', Indianapolis: P. R. Mallory & Co., 1942, p. 32]</ref>

More recently, ''autodyne converters'' are employed in radio receivers for the AM and FM broadcast band. A single transistor combines the functions of amplifier, mixer and local oscillator of an otherwise conventional [[superheterodyne receiver]]. Such a stage accepts as input the antenna signal, and provides an output to the intermediate frequency amplifier. In this application, the transistor is made to self-oscillate at the local oscillator frequency.

The autodyne detector has appeared in specialized fields in the 1960s through the 1990s.<ref>((Citation |last1=Bruin |first1=F. |last2=Van Soest |first2=P. C. |title=Transistorized Autodyne Detector for ESR and NSR |date=September 1960 |journal=Review of Scientific Instruments |volume=31 |issue=8 |doi=10.1063/1.1717092 |pages=909|bibcode=1960RScI...31..909B ))</ref><ref>((Citation |last1=Brandwein |first1=Leonard |last2=Lipsicas |first2=Max |title=Application of Frequency Locking and Control to an Autodyne Oscillating NMR Detector |journal=Review of Scientific Instruments |date= September 1970 |volume=41 |issue=9 |pages=1293–1295 |doi= 10.1063/1.1684797|bibcode=1970RScI...41.1293B ))</ref><ref>((Citation |last1=Medvedev |first1=Iu. V. |last2=Raksina |first2=F. P. |last3=Popov |first3=L. N. |title=Autodyne Detector of Optical Signals |journal=Radiotekhnika |volume=33 |date=April 1978 |pages=32–35 |language=ru |bibcode=1978RaT....33...32M))</ref><ref>((Citation |last1=Nowakowski |first1=N |last2=Gutkowicz-Krusin |first2=E. |last3=Lind |first3=G. |date=20 April 1990 |title=Innovative Techniques for High-Resolution Imaging and Precision Tracking |journal=Final Technical Report Riverside Research Inst |bibcode=1990rri..rept.....N |id=AD-A221380 |url=https://apps.dtic.mil/sti/pdfs/ADA221380.pdf |archive-url=https://web.archive.org/web/20130408130659/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA221380 |url-status=live |archive-date=April 8, 2013 )). Lidar target tracking with autodyne.</ref>

==See also==
*[[Direct-conversion receiver]] (also called "homodyne" or "zero-IF" receiver)
*[[Frequency mixer]]
*[[Heterodyne]] has comments on Fessenden and theory

==References==
((reflist|30em))

*((Citation |author=Bureau of Standards |author-link=NIST |title=The Principles Underlying Radio Communication |edition=Second |year=1922 |id=Radio Communications Pamphlet No. 40 |publisher=U.S. Army Signal Corps |url=https://archive.org/details/principlesunderl00unitrich )) Revised to 24 May 1921.  [https://books.google.com/books?id=TsTZCjhSG2EC Google Books.] Damped and undamped wave transmission. page 503 describes autodyne 0.015&nbsp;pW sensitivity. p 505, heterodyne not so good for damped waves; mushy sounds.
* ((Citation |author=P. R. Mallory Co.|title=Mye Technical Manual |chapter= Section 2, Superheterodyne First Detectors and Oscillators |publisher=P. R. Mallory Co. |year=1942 |oclc=2452978 |chapter-url=https://books.google.com/books?id=_0FUAAAAYAAJ ))

==External links==
*((Citation |last=Morse |first=A. H. |title=Radio: Beam and Broadcast |year=1925 |publisher=Ernest Benn Limited |url=https://archive.org/details/radiobeamandbroa029214mbp )). History of radio in 1925.
*((Citation |first=John V. L. |last=Hogan |journal=Electric Journal |title=The Heterodyne Receiver |date=April 1921 |volume=18 |page=116 |url=https://archive.org/details/electricjournal18elecuoft ))
* ((Citation |last=Ferris |first=Clifford D |title=Introduction to Bioinstrumentation: With Biological, Environmental, and Medical Applications |year=1978 |publisher=Humana Press |location=Clifton, NJ |url=https://books.google.com/books?id=0RR6M66EwDIC&pg=PA203 |isbn=978-0-89603-000-8 ))
* https://web.archive.org/web/20120207233031/http://ieee.cincinnati.fuse.net/reiman/01_2003.html reprint of portion of ((Citation |first=James E. |last=Brittain |title=Scanning the Past |journal=Proceedings of the IEEE |volume=80 |issue=7 |date=July 1992 )). About John V. L. Hogan
* ((Citation |first=John S. |last=Belrose |title=Reginald Aubrey Fessenden and the Birth of Wireless Telephony |journal=IEEE Antennas and Propagation Magazine |volume=44 |issue=2 |date=April 2002 |pages=38–47 |doi= 10.1109/map.2002.1003633|bibcode=2002IAPM...44...38B |citeseerx=10.1.1.205.7804 |s2cid=771931 ))
* [https://doi.org/10.1007%2F0-306-47545-6_3 DOI.org]
* ((Citation |editor-first=Ian |editor-last=Poole |title=Captain H. J. Round |publisher=Adrio Communications Ltd |url=https://www.electronics-notes.com/articles/history/pioneers/captain-h-j-round.php |access-date=26 April 2012 ))
* Museum of Broadcasting, 2012, Mystery Radio, https://web.archive.org/web/20150504041938/http://www.museumofbroadcasting.org/pearson.html early radio design goal was reducing the number of tubes
* ((cite patent |inventor-last=Conklin |inventor-first=James W. |title=Detector |fdate=1933-01-28 |pubdate=1936-08-11 |country=US |number=2050963)). Autodyne detector after RF amplifier and limiter
* ((citation |first1=Francis Edward |last1=Handy |first2=Ross Amos |last2=Hull |author2-link=Ross A. Hull |publisher=ARRL |title=The Radio Amateur's Handbook |year=1927 |page=44 |quote=... the autodyne method of reception. One vacuum tube generates oscillations. Incoming signals are coupled into the grid circuit of this same tube. A single tube thus acts as oscillator, detector, and amplifier. |url=https://books.google.com/books?id=7DXWAAAAMAAJ&q=Autodyne ))
*((Citation |title=VT Fuzes For Projectiles and Spin-Stabilized Rockets |author=Bureau of Ordnance |date=15 May 1946 |url=http://www.maritime.org/doc/vtfuze/index.htm |publisher=U. S. Navy Bureau of Ordnance |series=Ordnance Pamphlet |volume=OP 1480 )); single tube in VT fuze used as oscillator, transmitter, and autodyne detector.

[[Category:Electronic engineering]]

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