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A Phase–locked loop (PLL) is an electronic circuit used to attain synchronization between an external reference signal and an internal signal. With the use of a reference frequency generated by the application of an external clock, the PLL is able to lock onto the frequency of the reference and modify a program’s internal frequency to match the reference frequency. The mechanism of the PLL is widely used in Clock Generators, Frequency Synthesizers, and many more in modern electronics.

In order to accommodate different operating frequencies, applications of phase-locked loops vary from one field to another. In the field of digital communication, they are used to synthesize stable low-noise clock signals. In the realm of broadcasting systems, they are employed to develop tunable, high precision signals. In communication equipment and air-traffic control systems, they are used to track frequency variations or to develop local oscillators from outside radio frequency signals. In the field of computing and software engineering, phase-locked-loops are used to acquire thermal synchronization.

Phase-locked loops are made up of two components: a phase detector, and a voltage-controlled oscillator (VCO). The phase detector monitor the output of the voltage-controlled oscillator and its reference signal, then it produces an error signal in order to adjust the oscillator. This signal is proportional to the phase difference between the signals, and is also responsible for establishing synchronization. The voltage-controlled oscillator is a linear oscillator whose output frequency is adjusted according to a voltage input. While building the loop, the output signal of the oscillator is combined with the reference signal before the phase detector.

Phase-locked-loops basically work by using negative feedback to reduce the frequency difference between the two locked frequencies. The mechanism which allows for the two signals to become synchronized is depicted in Figure 1 below.

In Figure 1, the Phase Detector (PD), which is also known as a phase comparator, calculates the phase and frequency difference between the reference signal and the feedback signal and then produces an error signal. The Voltage-controllers oscillator (VCO) tries to reduce the difference between the two signals by interpreting the error signal. The loop filter on the other hand, is responsible for producing the control voltage which is used by the oscillator to adjust its frequency.

In conclusion, this article has explored the application and working principle of the phase-locked loop mechanism. PLLs are employed in different industries to help attain synchronization between the reference and internal signals and can be found in various applications such as communication equipment, broadcast systems, computing, and air-traffic control systems. The mechanism of the PLL is composed of a phase detector, a voltage-controlled oscillator, and a loop filter. The phase detector observes the reference signal and the output of the voltage-controlled oscillator, then produces a corresponding error signal which is used by the VCO in order to adjust its frequency.

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