Mills, Thomas B. National Semiconductor Corporation, Santa Clara, California.
Last reviewed:January 2020
- Basic operation
- Voltage-controlled oscillators
- Phase detectors
- Loop filter
- Uses of phase-locked loops
- Related Primary Literature
- Additional Reading
Electronic circuits for locking an oscillator in phase with an arbitrary input signal. A phase-locked loop (PLL) is used in three fundamentally different ways: (1) as a demodulator, where it is employed to follow (and demodulate) frequency or phase modulation; (2) as a tracker of a carrier or synchronizing signal which may vary in frequency with time; and (3) as a frequency synthesizer, where an oscillator is locked to a multiple of an accurate reference frequency. When operating as a demodulator, the phase-locked loop may be thought of as a matched filter operating as a coherent detector. When used to track a carrier, it may be thought of as a narrow-band filter for removing noise from the signal and regenerating a clean replica of the signal. When used as a frequency synthesizer, a voltage-controlled oscillator (VCO) is divided down to a reference frequency that is locked to a frequency derived from an accurate source such as a crystal oscillator. See also: Demodulator; Electric filter; Frequency-modulation detector; Frequency multiplier; Oscillator; Phase-modulation detector
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