Phase locked loop composition and working mechanism
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Phase locked loop composition and working mechanism

Posted Date: 2024-02-02

What three parts does a phase locked loop consist of?

Phase Locked Loop (PLL) usually consists of the following three parts:

1. Phase Comparator/Phase Detector: The phase comparator is used to compare the phase difference between the input signal and the feedback signal, and output a control voltage or digital value that represents the phase difference.

2. Voltage Controlled Oscillator (VCO): The voltage controlled oscillator receives the control signal from the phase comparator and adjusts its own oscillation frequency according to the voltage or digital value of the control signal.

3. Feedback Circuit: The feedback circuit feeds back the VCO output signal to the phase comparator for phase comparison with the input signal, and then generates a control signal for controlling the VCO.

These three parts interact to form a closed-loop control system, so that the phase of the output signal and the phase of the input signal are locked or tracked with each other. Phase-locked loops are widely used in clock synchronization, frequency synthesis, digital-to-analog conversion and other fields.

How phase locked loop works

The most basic phase-locked loop system mainly contains three basic modules: phase detector (Phase Detector: PD), loop filter (L00P Filter: LF), which is actually a low-pass filter, and voltage controlled oscillator (Voltage Controlled Oscillator: VCO). With these three modules, the most basic phase-locked loop can be run. But in our actual use, the phase-locked loop system will also add some frequency dividers, frequency multipliers, mixers and other modules. (This can be compared to the minimum system of STM32 and the development board we actually use STM32)

Analyze from the moment the phase-locked system starts running. At this time, the phase detector has two input signals, one is the input reference signal Vin, and the other is the inherent oscillation signal Vout of the voltage-controlled oscillator.

At this time, since the frequencies of the two signals are different, a phase difference will occur due to the frequency difference. If no operation is performed on the voltage controlled oscillator, the phase difference will continue to accumulate, thus spanning the 2Π angle and restarting the phase measurement from zero. This It is the measurement dead zone. It is obvious that the phase is getting larger, but the phase detector can only measure the range of 0~2Π, and the maximum measured phase difference is 2Π. This results in the output voltage of the phase detector can only be in the range of 0 to 2Π. Fluctuate within a certain range.

The ideal state is to keep the phase difference between the two signals within the range of 2Π without entering the measurement dead zone. Then at the beginning of the system, the phase detector measures the phase difference between the two signals, and converts the phase difference time signal into an error voltage signal for output (see the explanation of the phase detector for the specific conversion process).

It is converted into a voltage-controlled voltage through the loop filter and added to the voltage-controlled oscillator, so that the output frequency Vout of the voltage-controlled oscillator is gradually synchronized with the input signal Vin until the frequencies of the two signals are gradually synchronized and the phase difference is within the measurement error range. within, then the entire system will stabilize.

The phase difference between the two signals will not increase cumulatively, but will remain relatively fixed. (Not fixed in the conventional sense, but small fluctuations within the allowable error range).

What are the reasons why phase locked loop loses lock?

Phase Locked Loop (PLL) loss of lock may be caused by the following reasons:

1. Input signal interference: When the input signal is interfered by noise, distortion, attenuation, etc., the phase comparator may not be able to correctly compare the phase difference between the input signal and the VCO feedback signal, resulting in loss of lock.

2. Excessive frequency deviation: If there is a large deviation between the frequency of the input signal and the oscillation frequency of the VCO output, which exceeds the tracking range of the PLL, the PLL may lose lock.

3. Improper loop bandwidth setting: The loop bandwidth of the PLL determines its tracking speed of the input signal. If the loop bandwidth is set too narrow or too wide, it may cause the PLL to lose lock.

4. Phase comparator failure: The phase comparator is one of the key components of the PLL. If the phase comparator fails or does not work properly, it may cause the PLL to lose lock.

5. Power supply noise and power supply problems: If the power supply of the PLL is noisy or unstable, it may have a negative impact on the various components of the PLL, leading to loss of lock.

6. Temperature changes and environmental changes: Temperature changes may cause changes in the parameters of electronic components inside the PLL, thereby affecting the performance and stability of the PLL, leading to loss of lock.

7. Other external interference: External factors such as electromagnetic interference, radiation interference, poor oscillator quality, etc. may also cause the PLL to lose lock.


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