Phase detector

From Wikipedia, the free encyclopedia

A phase detector is a frequency mixer or analog multiplier circuit that generates a voltage signal which represents the difference in phase between two signal inputs. It is an essential element of the phase-locked loop (PLL).

Detecting phase differences is very important in many applications, such as motor control, servo mechanisms, and demodulators.

Phase detectors range from very simple to complex in design. An XOR logic gate makes a passable phase detector. When the two compared signals are completely in phase, the two equal inputs to the XOR gate will output a constant level of zero. With a 1° phase difference, the XOR gate will output a "1" for the duration of the signals being different (1/360th of the cycle). When the signals are 180° apart, (one is high when the other is low, and vice versa) the XOR gate puts out a steady "1" signal. Integration of the output signal results in an analog voltage proportional to the phase difference.

A phase detector can also be made from an analog multiplier, sample and hold circuit, charge pump or a logic circuit consisting of flip-flops. These phase detectors have more desirable properties such as better accuracy at small phase differences or ability to phase lock to signals with large frequency mismatches.

False lock is a phenomenon that can occurs when a phase-locked loop synchronizes to a harmonic of the intended signal rather than the signal itself. Limiting the frequency at which the PLL can operate can prevent false lock, but it may not be desirable to limit the PLL oscillator this way.

A phase-frequency detector is an asynchronous sequential logic circuit made of two flip-flops which has the function of detecting when frequency as well as phase does not match between the input signals. A phase-frequency detector prevents the false lock phenomenon.