The principle of phase-locked loop for programmable crystal oscillator

Phase Locked Loop (PLL) is a feedback system that can compare the phase difference between output and input. It uses an external reference signal to control the frequency and phase of the oscillation signal inside the loop, synchronizing the oscillation signal with the reference signal. The Phase Locked Loop (PLL) technology plays a key role in programmable crystal oscillators. The following is a detailed explanation of PLL technology in programmable crystal oscillators:

一、 The basic principle of phase-locked loop technology

1. A phase-locked loop is a negative feedback control system that uses the voltage generated by phase synchronization to tune a voltage controlled oscillator (VCO) to produce a target frequency. It uses the principle of automatic control to control the frequency and phase of the oscillation signal inside the loop using an external reference signal, achieving automatic tracking of the output signal frequency to the input signal frequency.

2. A phase-locked loop typically consists of a phase detector (PD), a loop filter (LF), and a voltage controlled oscillator to form a forward path, and a frequency divider to form a frequency phase feedback path. The working principle of a phase-locked loop is to detect the phase difference between the input signal and the output signal, and convert the detected phase difference signal into a voltage signal output through a phase detector. After filtering through a low-pass filter, it forms the control voltage of a voltage controlled oscillator, which controls the frequency of the oscillator output signal. Then, the frequency and phase of the oscillator output signal are fed back to the phase detector through a feedback path.

3. During the operation of a phase-locked loop, when the frequency of the output signal proportionally reflects the frequency of the input signal, the output voltage and input voltage maintain a fixed phase difference, thus locking the phase of the output voltage and input voltage.

二、 Application of Phase Locked Loop Technology in Programmable Crystal Oscillators

1. Implement frequency synthesis and conversion

The phase-locked loop technology can synthesize various target frequency signals by doubling and dividing the original fixed frequency signal generated by the crystal oscillator. It uses the phase difference between a reference clock signal and the output signal of a voltage controlled oscillator (VCO) to adjust the frequency of the VCO. When the phase difference is zero, the output frequency of the VCO is locked to an integer or fractional multiple of the frequency of the reference clock signal. Enabling programmable crystal oscillators to provide multiple frequency options according to the needs of different devices and systems greatly enhances the versatility and flexibility of the crystal oscillator.

2. Frequency accuracy and stability

The phase-locked loop continuously monitors and adjusts the output frequency of the VCO to maintain precise phase and frequency synchronization with the reference clock signal. Even if the VCO frequency deviates due to external factors such as temperature, power supply voltage fluctuations, etc., the phase-locked loop can quickly detect and adjust it, controlling the frequency deviation within a very small range. Ensure high precision and stability of the output frequency of the programmable crystal oscillator.

3. Implement low jitter clock output

The short-term instability of the clock signal in the cycle is suppressed and filtered by the phase-locked loop technology through its feedback control mechanism, which suppresses and filters the phase noise of the VCO output signal. It can transfer the low jitter characteristics of the input reference signal to the output signal, while effectively attenuating the jitter generated by the VCO itself.

4. Adjustable frequency

By adjusting the control input of the phase-locked loop, precise control of the output frequency of the programmable crystal oscillator can be achieved to adapt to different working modes or communication standards.

5. Phase synchronization

In communication systems, phase-locked loops are used to ensure that the clock signals at the transmitting and receiving ends are phase synchronized, thereby ensuring the accuracy of data transmission. This is particularly important for the application of programmable crystal oscillators in the field of communication.

6. Suppress noise

The phase-locked loop helps to suppress the phase noise and frequency noise of programmable crystal oscillators through feedback mechanism, and improve the overall performance of the system.