A phase-locked loop is made up of a phase comparator, a loop filter, a voltage controlled oscillator and a feedback loop from the output of the VCO to the phase comparator.
The phase comparator may be something as simple as an XOR operator.
Loop filters remove noise from the output of the phase comparator, and may be passive or active. A passive loop filter consists of capacitors and resistors, while an active loop filer involves opamps as well.
A VCO's frequency is controlled by the input voltage, and can be constructed based on anything from a 555 timer to an LC oscillator. In a PLL, the VCO takes the difference between two signals and corrects the input signal to lock with the reference signal.

A PLL can determine and fix error in an supervised-learning oscillatory neural network.

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