A phase locked loop, (PLL), is basically a closed loop feedback system, the action of which is to lock or synchronise the frequency of a controlled oscillator to that of an incoming signal. Phase lock principles are by no means new, synchronous reception of radio signals using PLL techniques was described as early as 1932. However, when implemented with discrete components, phase lock techniques involve circuits of considerable cost and complexity and for this reason their use in the past has been limited to specialised measurements requiring a high degree of noise immunity and very narrow bandwidth. The development of integrated circuit PLL’s now makes it economically possible to employ the sophisticated phase lock techniques in a wide variety of signal detection and processing applications.
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