PLL Part 1. Phase-Locked Loops (PLL) Explained – How Phase Detectors, Filters & VCOs Work!
Phase Locked Loop
• PLL Part 1. Phase-Locked Loops (PLL) ...
A Phase-Locked Loop (PLL) is a control system that compares the phase of the input signal with that of a reference signal and adjusts the phase of the output signal to match that of the reference signal. Keeping the input and output phases 'in lock' also implies maintaining the input and output frequencies as equal, thus a phase-locked loop can also track frequency. By incorporating a frequency divider, a PLL can generate a stable frequency that is a multiple of the input frequency.
A PLL is a closed loop system used for frequency control. Several building blocks are common to most PLL designs:
The phase detector
The loop filter
The voltage-controlled oscillator
Phase Detector (PD): This component compares the phase of the input signal (feedback signal) with that of the reference signal (usually a stable oscillator signal) and produces an output signal proportional to the phase difference between the two.
Loop Filter: Usually, a lowpass filter. The output of the phase detector usually contains high-frequency noise. The lowpass filter removes this noise and extracts the low frequency component, which represents the phase error.
Voltage-Controlled Oscillator (VCO): Generates an output signal whose frequency is determined by the control voltage applied to it. The control voltage is provided by the loop filter, which adjusts it based on the error signal.
Feedback Loop: The output signal from the VCO is fed back to the phase detector, closing the loop. The phase difference between the input and feedback signals is minimized by adjusting the VCO's frequency via the control voltage.
A clear understanding of the concept of feedback control is illustrated by an everyday situation: the simple action of controlling the speed of a car. If the desired speed is 60 km/h, then this becomes the reference point. Any deviation from this speed is considered an error. The accelerator pedal serves as the control element. On level terrain, maintaining a constant pressure on the pedal will sustain a constant speed.
However, as the car ascends a hill, it will naturally decelerate. The variance between the actual speed and the reference value generates an error signal. This error then triggers a command to adjust the accelerator pedal accordingly. While pushing the pedal increases the speed, a slight error may persist. Subsequently, as the car crests the hill and begins to descend, its speed will accelerate. Releasing pressure on the pedal slows down this acceleration, yet an error may persist until a steady-state condition is reestablished.
In this example, the driver's brain acts as the feedback loop. By discerning when to apply pressure and when to release it, the driver regulates the feedback's effectiveness. Additionally, their reaction time influences how closely the car's speed aligns with the desired reference point. The driver can opt for rapid corrections to closely match the desired speed or choose a more gradual approach to ensure the average speed aligns with the target value. His actions coupled with the car’s controls form a system closely analogous to a phase lock loop. Replace the human with an electrical circuit that senses the speed error, include another circuit that tempers the response time, and couple it to the accelerator controls. This is the typical cruise control system.
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