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A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and output frequencies the same, thus a phase-locked loop can also track an input frequency.
Following Gardner's results, by analogy with the Egan conjecture on the pull-in range of type 2 APLL, Amr M. Fahim conjectured in his book [8]: 6 that in order to have an infinite pull-in(capture) range, an active filter must be used for the loop filter in CP-PLL (Fahim-Egan's conjecture on the pull-in range of type II CP-PLL).
In the 1st edition of his well-known work, Phaselock Techniques, Floyd M. Gardner introduced a lock-in concept: [8] If, for some reason, the frequency difference between input and VCO is less than the loop bandwidth, the loop will lock up almost instantaneously without slipping cycles.
Costas loop working in the locked state. In the classical implementation of a Costas loop, [4] a local voltage-controlled oscillator (VCO) provides quadrature outputs, one to each of two phase detectors, e.g., product detectors.
A frequency synthesizer may use the techniques of frequency multiplication, frequency division, direct digital synthesis, frequency mixing, and phase-locked loops to generate its frequencies. The stability and accuracy of the frequency synthesizer's output are related to the stability and accuracy of its reference frequency input.
The term charge pump is also commonly used in phase-locked loop (PLL) circuits even though there is no pumping action involved unlike in the circuit discussed above. A PLL charge pump is merely a bipolar switched current source. This means that it can output positive and negative current pulses into the loop filter of the PLL.
Clock recovery is very closely related to the problem of carrier recovery, which is the process of re-creating a phase-locked version of the carrier when a suppressed carrier modulation scheme is used. These problems were first addressed in a 1956 paper, which introduced a clock-recovery method now known as the Costas loop. [3]
A PLL multibit or multibit PLL is a phase-locked loop (PLL) which achieves improved performance compared to a unibit PLL by using more bits. Unibit PLLs use only the most significant bit (MSB) of each counter's output bus to measure the phase, while multibit PLLs use more bits. [1] PLLs are an essential component in telecommunications.