PLL-Phase Locked Loops

Introduction to PLL or Phase Locked Loops

Phase-locked loop is a feedback loop consisting of a phase detector, a low-pass filter, amplifier (optional) and a voltage-controlled oscillator (VCO), as illustrated in figure. It plays the same role in the frequency or phase world as the op-amp does in the voltage world. The op-amp has two voltage inputs, non-inverting and inverting (normally used for feedback from the output). Similarly, the PLL has two inputs; the PLL’s feedback input is normally connected to the circuits’ output. Digital frequencies are usually applied. The op-amp changes its output voltage to whatever values is necessary to drive the difference in voltage between its two inputs to zero. The PLL changes its output phase and frequency to whatever frequency or phase is necessary to make the two input frequencies and phase track. Placing a voltage divider in the feedback loop of an op-amp causes the output voltage to be increased by the amount of the feedback voltage division (amplification). Placing a frequency divider in the feedback of a PLL causes the output frequency to be increased by the amount of the feedback divider. A firm grasp on similarities between the PLL and the op-amp simplifies our analysis and design of circuits containing PLLs.

PLL Block

Phase Locked Loop

With the rapid development of IC technology, the phase-locked loop (PLL) has emerged as one of the fundamental building blocks in electronic technology.

Common applications of a PLL include

(i) frequency synthesizers that provide multiples of a reference signal frequency;

(ii) FM demodulation networks for FM operation with excellent linearity between the input signal frequency and the PLL output voltage;

(iii) demodulation of the two data transmission or carrier frequencies in digital-data transmission employed in frequency-shift keying (FSK) operation;

(iv) a wide variety of areas including telemetry receivers and transmitters, tone decoders, AM detectors, tracking filters and motor speed controls.

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