The NE561 Phase Locked Loop (PLL) is a monolithic signal conditioner and demodulator system comprising a VCO, Phase Comparator, Amplifier, and Low Pass Filter, interconnected as shown in the accompanying block diagram. The Centre frequency of the PLL is determined by the free running frequency (Fo) of the VCO. This VCO frequency is set by an external capacitor and can be fine-tuned by an optional potentiometer. The low pass filter, which determines the capture characteristics of the loop, is formed by the two capacitors and two resistors at the Phase Comparator output.
