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A parametric oscillator is a driven harmonic oscillator in which the oscillations are driven by varying some parameter of the system at some frequency, typically different from the natural frequency of the oscillator. A simple example of a parametric oscillator is a child pumping a swing by periodically standing and squatting to increase the size of the swing's oscillations.〔 Note: In real-life playgrounds, swings are predominantly driven, not parametric, oscillators.〕〔 〕 The child's motions vary the moment of inertia of the swing as a pendulum. The "pump" motions of the child must be at twice the frequency of the swing's oscillations. Examples of parameters that may be varied are its resonance frequency and damping . Parametric oscillators are used in several areas of physics. The classical varactor parametric oscillator consists of a semiconductor varactor diode connected to a resonant circuit or cavity resonator. It is driven by varying the diode's capacitance by applying a varying bias voltage. The circuit that varies the diode's capacitance is called the "pump" or "driver". In microwave electronics, waveguide/YAG based parametric oscillators operate in the same fashion. Another important example is the optical parametric oscillator, which converts an input laser light wave into two output waves of lower frequency () When operated at pump levels below oscillation, the parametric oscillator can amplify a signal, becoming a parametric amplifier (paramp). Varactor parametric amplifiers have been developed as low-noise amplifiers in the radio and microwave frequency range. The advantage of a parametric amplifier is that it has much lower noise than an ordinary amplifier based on a gain device like a transistor or vacuum tube. This is because in the parametric amplifier a reactance is varied instead of a (noise-producing) resistance. They have been used in very low noise radio receivers in radio telescopes and spacecraft communication antennas. Parametric resonance occurs in a mechanical system when a system is parametrically excited and oscillates at one of its resonant frequencies. Parametric excitation differs from forcing since the action appears as a time varying modification on a system parameter. ==History== Michael Faraday (1831) was the first to notice oscillations of one frequency being excited by forces of double the frequency, in the crispations (ruffled surface waves) observed in a wine glass excited to "sing".〔Faraday, M. (1831) "On a peculiar class of acoustical figures; and on certain forms assumed by a group of particles upon vibrating elastic surfaces", ''Philosophical Transactions of the Royal Society (London)'', vol. 121, pages 299-318.〕 Melde (1859) generated parametric oscillations in a string by employing a tuning fork to periodically vary the tension at twice the resonance frequency of the string.〔Melde, F. (1859) "Über Erregung stehender Wellen eines fadenförmigen Körpers" (the excitation of standing waves on a string ), ''Annalen der Physik und Chemie'' (Ser. 2), vol. 109, pages 193-215.〕 Parametric oscillation was first treated as a general phenomenon by Rayleigh (1883,1887).〔Strutt, J.W. (Lord Rayleigh) (1883) "On maintained vibrations", ''Philosophical Magazine'', vol. 15, pages 229-235.〕〔Strutt, J.W. (Lord Rayleigh) (1887) "On the maintenance of vibrations by forces of double frequency, and on the propagation of waves through a medium endowed with periodic structure", ''Philosophical Magazine'', vol.24, pages 145-159.〕〔Strutt, J.W. (Lord Rayleigh) ''The Theory of Sound'', 2nd. ed. (N.Y., N.Y.: Dover, 1945), vol. 1, pages 81-85.〕 One of the first to apply the concept to electric circuits was George Francis FitzGerald, who in 1892 tried to excite oscillations in an LC circuit by pumping it with a varying inductance provided by a dynamo. Parametric amplifiers (paramps) were first used in 1913-1915 for radio telephony from Berlin to Vienna and Moscow, and were predicted to have a useful future (Ernst Alexanderson, 1916).〔Alexanderson, Ernst F.W. (April 1916) "A magnetic amplifier for audio telephony" ''Proceedings of the Institute of Radio Engineers'', vol. 4, pages 101-149.〕 The early paramps varied inductances, but other methods have been developed since, e.g., the varactor diodes, klystron tubes, Josephson junctions and optical methods. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Parametric oscillator」の詳細全文を読む スポンサード リンク
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