This process is called frequency pull in. So at first, the VCO frequency has to be tuned to the input frequency by the feedback loop. Also, the VCO center frequency ω 0 is usually different from the frequency ω i of the input signal. Phase Locked Loop Stage: It is the stage when the frequency comparison process stops and the input and output frequencies are adjusted.įor a PLL to know the phase of the input signal, first it has to be in the phase locked state. Generally, we can divide the operation of the PLL into 3 stages:įree-Running Stage: It is the stage at which no voltage input is applied to the PLL.Ĭapture Stage: In this stage, an input frequency is applied VCO output begins changing and produces an output for comparison purpose.
It has the advantages of giving better performance and sensitivity.Įxample for Digital Phase Detector - Exclusive OR Phase Detector This dc voltage will be directly proportional to the phase difference between the input and VCO frequencies.Įxample for Analog Phase Detector - MixerBased Detectorĭouble balanced four diode ring mixer is one of the commonly used Analog Phase Detector. It compares the input and VCO output frequency, thereby producing a DC voltage. The main function of the phase detector inside a PLL is to act as a comparator circuit. And discrete type phase detectors are of digital type. Monolithic PLL circuits use analog phase detectors. Phase detectors used inside a PLL can be of digital or analog type.