Phase locked loops (PLL)

Operational amplifier is a linear integrated circuit. It finds application in adder, subtractor, integrator, differentiator, instrumentation amplifier, log/antilog amplifier, multiplier, divider, analog computation, clipper, clamper and sample and hold circuits. The phase-locked loop is an important building block of linear systems. During its initial days, PLL was used extensively in radar synchronization and communication applications. Nowadays inexpensive PLLs made of monolithic ICs are widely used for electronic frequency control in satellite communication systems, computers, navigation systems and FM communication systems. A phase-locked loop is consists of a phase detector (or phase comparator), low pass filter, error amplifier and a voltage controlled oscillator.  A multivibrator can be used as a voltage controlled oscillator. Frequency of the multivibrator is determined by an external timing capacitor and an external resistor. When an input signal reaches PLL, the phase detector (or phase comparator) compares the phase and frequency of the signal with the output of the voltage controlled oscillator. The difference between these signals is known as error signal. High frequency components of error signal are removed by low pass filter and remaining components are amplified by the error amplifier. This signal is fed to voltage controlled oscillator which tracks the signal.