Electronic Oscillator - History

History

One of the first electronic oscillators was an oscillating arc built by Elihu Thomson in 1892. Thomson's oscillator placed an LC tuned circuit in parallel with the arc, used metal electrodes, and included a magnetic blowout. Another "singing arc" was described by William Duddell in 1900; Duddell used carbon electrodes but did not use a magnetic blowout. Electric arcs were used to provide illumination in the 19th century, but the arc current was unstable and they often produced hissing, humming or howling sounds. Duddell, a student at London Technical College, investigated this effect. He attached an LC circuit to the electrodes of an arc lamp, and the negative resistance of the arc excited audio frequency oscillations in the tuned circuit at its resonant frequency. Some of the energy was radiated as sound waves by the arc, producing a musical tone. To demonstrate his oscillator before the London Institute of Electrical Engineers, Duddell wired a series of tuned circuits to the arc and played a tune, "God Save The Queen". Duddell didn't further develop his invention, but in 1902 Danish physicists Valdemar Poulsen and P. O. Pederson were able to increase the frequency produced into the radio range, inventing the Poulsen arc radio transmitter, the first continuous wave radio transmitter, which was used through the 1920s.

The vacuum tube feedback oscillator was invented around 1912, when it was discovered that feedback in the recently-invented audion vacuum tube could produce oscillations. At least six researchers independently made this discovery and can be said to have some role in the invention. In the summer of 1912, Edwin Armstrong observed oscillations in audion radio receiver circuits and went on to use positive feedback in his invention of the regenerative receiver. German Alexander Meissner independently discovered positive feedback and invented oscillators in March 1913. Irving Langmuir at General Electric observed feedback in 1913. Fritz Lowenstein may have preceded the others with a crude oscillator in late 1911. In Britain, H. J. Round patented amplifying and oscillating circuits in 1913. In August 1912, Lee De Forest, the inventor of the audion, had also observed oscillations in his amplifiers, but he didn't understand its significance and tried to eliminate it until he read Armstrong's patents in 1914, which he promptly challenged. Armstrong and De Forest fought a protracted legal battle over the rights to the "regenerative" oscillator circuit which has been called "the most complicated patent litigation in the history of radio". De Forest ultimately won before the Supreme Court in 1934 on technical grounds, but most sources regard Armstrong's claim as the stronger one.

Feedback oscillators became the basis of radio transmission by 1920. Mathematical conditions for feedback oscillations, now called the Barkhausen criterion, were derived by Heinrich Georg Barkhausen in 1921. The first stable mathematical model of an electronic oscillator, the Van der Pol oscillator, was derived by Balthasar van der Pol in 1927. He showed that the stability of the oscillations (limit cycles) in actual oscillators was due to the nonlinearity of the amplifying device.