Phase locked loops are used in many RF systems: radios, as FM detectors as well as within frequency synthesizers that form the local oscillator in radio receivers. Here's a video introduction to their design and use.
PLLs are a key electronics building block for "locking" on incoming frequencies. They consist of a phase detector, voltage controlled oscillator and a loop filter as well as a reference signal source. Within the phase locked loop, the incoming reference hits the phase detector along with a signal from the PLL voltage controlled oscillator. A signal proportional to the phase difference between the two is generated and this is passed through a loop filter to remove unwanted signals. The resulting error signal is applied to the input of the voltage controlled oscillator with the effect that the phase between the reference and the VCO signals is reduced. Eventually a steady fixed phase difference is reached. At this point the phase lock loop is said to be in lock and the frequency of the reference and VCO are exactly the same.
Using the basic phase locked loop, it is possible to achieve a wide variety of functions, but possibly they are most widely known for their use in frequency synthesizers. Here they enable a single highly stable frequency source to be used to generate a host of other frequencies, all with the same accuracy as the reference.
This phase locked loop tutorial gives all the basics required for an understanding of PLL technology.
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