Phase-locked Loop

Phase-locked Loop is a system used to provide electronic feedback. This loop consists of a phase comparator or phase detector, a voltage controlled oscillator (VCO), a low-pass filter and a dc amplifier. Input and output blocks constitutes forward path of the loop and VC)-phase detector block constitutes feedback path. A phase detector is a nonlinear circuit with two input signals. One input signal is an external signal and the other one output signal from VCO. Phase detector output is a dc signal which is proportional to the phase difference between the input signals. The phase difference is usually referred as error phase. The output of phase detector- dc voltage signal is fed to a low-pass filter and then to dc amplifier. The output of the dc amplifier is fed to the input of VCO. VCO is voltage controlled oscillator which is a free running oscillator whose frequency of oscillation is controlled by an external dc bias voltage. Thus in a VCO, for a dc signal input, we obtain frequency output. Either a single pole or multiple pole filter is used here as a low-pass filter.