What does phase locked loop (PLL) mean

1. The concept of PLL

What we call PLL. In fact, it is a phase-locked loop, referred to as a phase-locked loop for short. For many electronic devices to work normally, the external input signal is usually synchronized with the internal oscillating signal. This can be achieved by using a phase-locked loop. Phase-locked loop is a feedback control circuit, referred to as phase-locked loop (PLL). The characteristic of the phase-locked loop is that the frequency and phase of the oscillating signal inside the loop are controlled by the reference signal input from the outside.

Because the phase-locked loop can automatically track the frequency of the output signal to the frequency of the input signal, the phase-locked loop is usually used in a closed-loop tracking circuit. During the operation of the phase-locked loop, when the frequency of the output signal is equal to the frequency of the input signal, the output voltage and the input voltage maintain a fixed phase difference, that is, the phase of the output voltage and the input voltage are locked, which is phase lock The origin of the ring name.

At present, phase-locked loops mainly include analog phase-locked loops, digital phase-locked loops, and phase-locked loops with memory capability (computer-controlled).

2. The composition of PLL

The phase-locked loop usually consists of three parts: a phase detector (PD), a loop filter (LF) and a voltage-controlled oscillator (VCO). The principle block diagram of the phase-locked loop is shown in Figure 8-4-1. The phase detector in the phase-locked loop is also called the phase comparator. Its function is to detect the phase difference between the input signal and the output signal, and convert the detected phase difference signal into a uD(t) voltage signal for output. After filtering by the low-pass filter, the control voltage uC(t) of the voltage-controlled oscillator is formed, and the frequency of the output signal of the oscillator is controlled.

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