The PLLs—Phased Locked Loops—constitute a basic building block of modern electronic circuits. PLLs are used in a host of applications, including communication and multimedia. These come in two variants: linear and non-linear. While linear PLLs are suitable for most applications, the non-linear PLLS are relatively complex and difficult to design.
Basic Blocks Of Phase Locked Loops
The PLL is made up of a voltage controlled oscillator and a phase detector. These devices are placed in a loop so that the phase detector’s output becomes the input of the VCO, while the VCO’s output is fed back to the phase detector.
Here’s a block diagram of a basic PLL.
Working Of Phase Detectors In Phase Locked Loops
The phase detector compares phases of two different signals and produces an output error voltage accordingly. In phase locked loops, the phase detector detects the differences in the input frequency and the feedback frequency from the voltage controlled oscillator. It then generates DC voltage proportional to this difference in frequencies.
The DC voltage from the phase detector becomes the input of a low-pass filter. Low-pass filters allow lower frequency components to pass through while the higher frequency components are blocked. This results in a noise-free, steady-level DC voltage.
Types Of Phase Detectors
The phase detector can be classified in different ways, however, here’s one classification:
- Phase sensitive detectors
- Phase-frequency detectors
Phase Sensitive Detectors
These are pretty straightforward in their working. They produce an output signal based on the frequency difference between two incoming signals. The output is constant if the phase difference is steady. However, a varying output is generated if there’s a difference in frequencies in the signal. This output’s frequency corresponds to the frequency difference between the two signals.
Capture Range
The loop filter in a PLL has a limited frequency range over which it can function—its pass-band. If the difference of frequencies in the two incoming signals falls outside this range, the error signal won’t pass through from the filter to the voltage controlled oscillator to go into lock state. Therefore, the phase locked loop can only lock in a limited range—the capture range.
Phase-Frequency Detector
These produce a voltage corresponding to the difference in phases if they are in the range of ±180 degrees. This ensures that the detector’s output falls in the pass-band of the filter, locking the PLL. The JK flip-flop is one example of a phase-frequency detector.
Applications Of PLLs
PLLs are used in a number of applications, as:
- They are used to control motor speeds.
- They are used to clock recovery and jitter reduction.
- They are used in FM operations.
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