As already noted, an oscillatory system will, in the absence of a driving force to maintain the oscillation, eventually come to rest. In order to keep a constant level of oscillation, it is necessary to inject energy into the system, an action most efficiently performed by a periodic force at a resonant frequency. It is not necessary that the external source of energy itself be periodic, since the oscillating system can be made to draw energy automatically at its own frequency; it is then called a self-sustained oscillator. In essence, this is accomplished by driving the oscillator from a power source from which the transfer of energy to the oscillator is regulated by the oscillator itself. This amounts to using a power amplifier to drive the oscillator with an amplified version of its own oscillation. To sustain the oscillation, the amplified power must be fed back to the oscillating system in the proper phase to reinforce the oscillation already present. This is called positive feedback and is generally associated with a rapid buildup of energy, which in every practical situation, however, is always limited by the onset of some degradation of the conditions that led to the buildup, or some imposed limit. Self-sustaining motion (not perpetual motion) occurs in many kinds of systems. For example, in a very broad sense a steam engine is a self-sustaining rotator, in the sense that valves controlling the flow of steam into the cylinders to drive the pistons are in fact acting like power amplifiers, whose output, the force on the pistons, in addition to driving the train, also actuates the valves, providing positive feedback.
KeywordsQuartz Microwave Steam Radar Coherence
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