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The autodyne circuit was an improvement to radio signal amplification using the 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. The autodyne circuit was discovered by Edwin Howard Armstrong of Columbia University, New York, NY. He inserted a tuned circuit in the output circuit of the Audion vacuum tube amplifier. 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 newly produced CW or continuous wave transmissions could be aurally decoded. Previously only spark, ICW (Interrupted Continuous Wave) signals which were produced by a motor chopping or turning the signal quickly on and off at an audio rate, or modulated continuous wave (MCW), could be decoded.
When the autodyne oscillator was advanced to self-oscillation, continuous wave Morse code dots and dashes would be clearly heard as a series of beeps, instead of an all but impossible to decode series of thumps. Spark and chopped CW (ICW) had audio tones which didn't require an oscillating detector to extract from the radio signal. 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.
Damped wave transmission
Early transmitters emitted damped waves, which were decaying sinewaves at the transmission frequency. Bursts were made at an audible frequency. 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. The regenerative receiver supplied even more gain, but required careful adjustment.
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 no audible modulation. 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.
Fessenden researched the heterodyne detector.
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.
- Direct-conversion receiver (also called "homodyne" or "zero-IF" receiver)
- Frequency mixer
- Heterodyne has comments on Fessenden and theory
- Bureau of Standards 1922, p. 503
- Bureau of Standards 1922, p. 419
- Bureau of Standards 1922, p. 426
- Bureau of Standards 1922, p. 427
- Bureau of Standards 1922, p. 430
- P. R. Mallory Co. 1942, p. 32 Autodyne First Detector Combinations: "The Autodyne reached it greatest popularity and development during the beginning of the depression...."
- Bruin, F.; Van Soest, P. C. (September 1960), Transistorized Autodyne Detector for ESR and NSR, Review of Scientific Instruments 31 (8), doi:10.1063/1.1717092
- Brandwein, Leonard; Lipsicas, Max (September 1970), Application of Frequency Locking and Control to an Autodyne Oscillating NMR Detector, Review of Scientific Instruments 41 (9): 1293–1295, doi:10.1063/1.1684797
- Medvedev, Iu. V.; Raksina, F. P.; Popov, L. N. (April 1978), Autodyne Detector of Optical Signals, Radiotekhnika (in Russian) 33: 32–35
- Nowakowski, N; Gutkowicz-Krusin, E.; Lind, G. (20 April 1990), Innovative Techniques for High-Resolution Imaging and Precision Tracking, AD-A221380. Lidar target tracking with autodyne.
- Bureau of Standards (1922), The Principles Underlying Radio Communication (Second ed.), U.S. Army Signal Corps, Radio Communications Pamphlet No. 40 Revised to 24 May 1921. Google Books. Damped and undamped wave transmission. page 503 describes autodyne 0.015 pW sensitivity. p 505, heterodyne not so good for damped waves; mushy sounds.
- P. R. Mallory Co. (1942), "Section 2, Superheterodyne First Detectors and Oscillators", Mye Technical Manual, P. R. Mallory Co., OCLC 2452978
- Morse, A. H. (1925), Radio: Beam and Broadcast, Ernest Benn Limited. History of radio in 1925.
- Hogan, John V. L. (April 1921), The Heterodyne Receiver, Electric Journal 18: 116
- Ferris, Clifford D (1978), Introduction to Bioinstrumentation: With Biological, Environmental, and Medical Applications, Clifton, NJ: Humana Press, ISBN 0-89603-000-8
- http://ieee.cincinnati.fuse.net/reiman/01_2003.html reprint of portion of Brittain, James E. (July 1992), Scanning the Past, Proceedings of the IEEE 80 (7). About John V. L. Hogan
- Belrose, John S. (April 2002), Reginald Aubrey Fessenden and the Birth of Wireless Telephony, IEEE Antennas and Propagation Magazine 44 (2): 38–47, doi:10.1109/map.2002.1003633
- Poole, Ian (ed.), Captain H. J. Round, Adrio Communications Ltd, retrieved 26 April 2012
- Museum of Broadcasting, 2012, Mystery Radio, http://www.museumofbroadcasting.org/pearson.html early radio design goal was reducing the number of tubes
- Conklin, James W., Detector, Jan 28, 1933, Aug 11 1936, US Patent 2050963. Autodyne detector after RF amplifier and limiter
- Handy, Francis Edward; Hull, Ross Amos (1927), The Radio Amateur's Handbook, ARRL, p. 44, "... 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."