Phase Frequency Detector
Encyclopedia
A phase frequency detector (PFD), in electronics, is a device which compares the phase of two input signals. It has two inputs which correspond to two different input signals, usually one from a voltage-controlled oscillator
(VCO) and another from some external source. It has two outputs which instruct subsequent circuitry on how to adjust to lock onto the phase.
To form a Phase-locked loop
(PLL) the PFD phase error output is fed to a loop filter which integrates the signal to smooth it. This smoothed signal is fed to a voltage-controlled oscillator
which generates an output signal with a frequency that is proportional to the input voltage. The VCO output is also fed back to the PFD to form the PLL circuit.
The PFD is an improvement over the phase comparators of early PLLs in that it also provides a frequency error output as well as a phase error signal.
Voltage-controlled oscillator
A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation or phase...
(VCO) and another from some external source. It has two outputs which instruct subsequent circuitry on how to adjust to lock onto the phase.
To form a Phase-locked loop
Phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
(PLL) the PFD phase error output is fed to a loop filter which integrates the signal to smooth it. This smoothed signal is fed to a voltage-controlled oscillator
Voltage-controlled oscillator
A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation or phase...
which generates an output signal with a frequency that is proportional to the input voltage. The VCO output is also fed back to the PFD to form the PLL circuit.
The PFD is an improvement over the phase comparators of early PLLs in that it also provides a frequency error output as well as a phase error signal.