Phase-​locked loop

A PLL is an electronic circuit used to produce a signal that maintains the phase in synchronization to the phase reference clock. This device (PLL) consists of a variable frequency generation setup, namely voltage-​controlled oscillator, a phase locator, charge pump and clock divider in the feedback path. The phase locator produces a comparison between the input signal and the phase of the reference signal received from the oscillator. The phase locator output is fed into the charge pump circuit, which steps down the charging direction. The production of charge pump goes up for high input and low for low information (Weste and Harris 2011; Prakash and Hiremath 2017; Pahlevan et al. 2019). The analog output of the charge pump provides an initial voltage to generate frequency. Frequency of VCO oscillator automatically adjusts the frequency of the oscillator to keep both in phase.

Mathematically, frequency is a derivation of phase. If the input and output phases lock to each other, the input and output frequencies also lock with each other. Hence, a PLL can easily be identified as input frequency and also belongs to the properties to generate any frequency that is a real factor of the input frequency. These features are utilized for modulation–​demodulation and synthesis of frequency. The implementation of PLL is done with analog or digital components. The model for the PLL remains the same in both the cases: consisting three components in the forward path and one in feedback path, namely phase-​frequency locator (PFD), low-​pass filter (LPF), voltage-​controlled oscillator (VCO) and charge pump. The accessible structure of PLL is analog PLL (APLL), digital PLL (DPLL), all-​digital PLL (ADPLL), software PLL (SPLL), neuronal PLL (NPLL) etc. PLL finds its common applications in radio station, telemetry, FM radio band, telecommunications etc. (Ross et al. 2017). PLL is responsible to generate stable clock frequency or clock distribution networks in complex digital designs such as processors. PLL is utilized to produce frequencies from a few hertz to several gigahertz.