Overview: The article explores the importance of phase lock loops in synchronizing and managing frequency variations. It highlights the fundamental components, their working principles, and the different modes of operation.

Phase lock loops (PLL) are extensively employed in many practical applications related to power electronic device control and signal synchronization. PLL technology has substantially contributed to technological developments in numerous areas.

What is a PLL?

A PLL basically consists of a feedback loop that regulates the phase error. It generates an output signal whose phase is in relation to the input signal. Hence, PLLs are widely used in numerous electrical applications, ranging from communication to computation.

The PLL control system keeps the input and output signals at the same phase and frequency. The loop is in the lock condition when the output frequency equals the input frequency, and there is either no phase difference or a constant phase difference between the two signals. Three fundamental blocks make up the PLL. Fig. 1 shows a basic PLL diagram.

Fig. 1 Illustration of the PLL. Source: MDPI

Working of PLL

Phase Detector

A phase detector's primary function is to compare the phase and frequency of input reference signals to the feedback signal from the VCO. It generates the error signal in relation to the phase difference.

Low-Pass Filter

It smooths out the error signal from the phase detector. After going through a low pass filter, this error signal is converted into error voltage. This acts as the control voltage, which regulates the output frequency of VCO. A low pass filter is crucial since it reduces high frequencies and modifies the speed of the circuit. 

Voltage-Controlled Oscillator

In a VCO, as the control voltage from the low-pass filter changes, the frequency of the output signal also changes. It regulates the phase difference observed by the phase detector. The VCO either increases or decreases its frequency based on the error voltage and keeps doing so until it locks onto the input frequency. There may be no phase difference or a constant phase difference between the two signals in the lock state.

Stages of Operation

There are three modes in which the PLL operates: 

  • Free running mode
  • Capture mode
  • Phase lock mode

Free running mode

The free-running state is the condition in which there is no input voltage. When the loop is turned on, the VCO operates at the center frequency, also referred to as the free running frequency.

Capture mode

This phase, known as the capture state, begins as soon as the input frequency is applied, and the VCO switches on and generates an output frequency. The capture range is the range of input frequencies around the VCO center frequency to which the loop can lock when starting from an unlocked state.

Phase lock mode

When the output and input frequencies are equal, the frequency synchronization stops. It's called the phase-locked stage. The loop stays in lock mode within this range of input frequencies.

Applications

PLL is a compact circuit block that is extensively used in electronics and wireless applications, including radios, TVs, cell phones, Wi-Fi routers, and other devices like frequency synthesizers, AM and FM demodulators, recovery, etc.

  • They are utilized in frequency modulation and demodulation circuits in communication. They are commonly employed in frequency shift keying.
  • They are used as a frequency synthesizer, generating output frequencies based on the input reference frequency.
  • When used as a frequency multiplier, they provide an output frequency that is a multiple of the input frequency.
  • The PLL can recover clock timing from the incoming bitstream.
  • They are employed to produce a stable system clock.
  • They are employed in noise and jitter reduction.
  • They are even used to compare and synchronize motor rotation with the reference signal to manage the speed of motors.

Application of PLL in Power Grids

The grids are complicated, dynamic structures. Furthermore, due to the rapid growth of renewable energy sources like solar and wind, the percentage of power electronic equipment such as reactive compensators and grid-connected inverters has increased over time, creating several new issues for the grid. This would unavoidably lead to a stability issue with the grid. 

It is crucial to precisely identify variations in phase and frequency with grid voltage to manage current and voltage and achieve synchronization. PLL provides synchronization between the grid system and the control loop. It is the most popular method for measuring frequency in a power system and assisting in synchronizing. Its indirect measurement system precisely detects deviations and contributes to the control of power, voltage, and frequency, all necessary to preserve power system stability.

To ensure the stability of the electricity system, every signal is observed and monitored. Hence, PLLs are essential parts of electronic systems that provide accurate phase and frequency control and are used in a variety of applications.

PLL to consider

The ADF4155BCPZ, shown in Fig. 2, is a high-performance, phase-locked loop synthesizer from Analog Devices. It is designed for applications requiring precise frequency synthesis and is suitable for use in a variety of wireless communication systems.

It requires an external loop filter, a VCO, and a reference frequency. The device can operate with external VCO parts up to an 8 GHz operating frequency, and the high-resolution programmable modulus ensures the synthesis of exact frequencies with zero Hz error.

Fig. 2 ADF4155BCPZ. Source: oemsecrets

It can be used in various applications, including wireless infrastructure, point-to-point and point-to-multipoint microwave links, test equipment, wireless LANs and CATV equipment, clock generation, etc.

Summarizing the Key Points

  • PLLs are essential in electronics for maintaining phase and frequency synchronization between input and output signals.
  • PLLs operate in three modes: free running, capture, and phase lock mode, ensuring accurate signal synchronization.
  • PLL technology is widely used in communication devices, frequency modulation circuits, and noise reduction applications.
  • PLLs play a critical role in regulating grid systems, detecting frequency variations, and preserving stability in power grids.

Reference

Yuxia Jiang et al., “Phase-Locked Loop Research of Grid-Connected Inverter Based on Impedance Analysis,” Energies 11, no. 11 (November 8, 2018): 3077, https://doi.org/10.3390/en11113077.

Chandra Keerthi Pothina et al., “Design of Efficient Phase Locked Loop for Low Power Applications,” March 13, 2023, https://doi.org/10.3390/hmam2-14157.

Orlando David Guerrero-Bermúdez et al., “Comparison of Phase-Locked Loops Used for Frequency Measurements in a Low-Inertia Power Grid with Wind Generation,” Electronics 11, no. 19 (October 8, 2022): 3226, https://doi.org/10.3390/electronics11193226.