Phase-locked loop
Simple example
A simple analog PLL is an electronic circuit consisting of a variable frequency oscillator and a phase detector in a feedback loop (Figure 1). The oscillator generates a periodic signal Vo with frequency proportional to an applied voltage, hence the term voltage-controlled oscillator (VCO). The phase detector compares the phase of the VCO's output signal with the phase of periodic input reference signal Vi and outputs a voltage (stabilized by the filter) to adjust the oscillator's frequency to match the phase of Vo to the phase of Vi.
Clock analogy
Phase can be proportional to time, so a phase difference can correspond to a time difference.
Clock analogy
An early electromechanical version of a phase-locked loop was used in 1921 in the Shortt-Synchronome clock.
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