What is the relationship between phase noise and the performance of active oscillators?
Phase noise, abbreviated as PN, is a highly significant performance characteristic of active oscillators. It has a significant impact on the output frequency stability of active oscillators functioning as oscillators. Generally, lower phase noise indicates better performance of active oscillators and higher output signal stability.
Impact of Phase Noise on Oscillator Circuits
Q Value
The Q value of active oscillators is particularly crucial for improving phase noise near the carrier frequency. Regardless of the offset frequency, higher signal levels result in lower phase noise levels. Lowering phase noise can be achieved by setting the signal level as high as possible within the system, but there is an upper limit to the excitation level applied to active oscillators. Excessive excitation levels may lead to unnecessary vibration modes and potentially abnormal oscillation states.
Harmonics have higher Q values and are effective near offsets. However, caution is needed, as operating at high excitation levels increases the resistance loss of quartz crystals and oscillator circuits, leading to greater frequency fluctuations and inevitable signal purity degradation.
Frequency Fluctuations
Furthermore, with increasing power, frequency fluctuations caused by the nonlinearity of crystal transducers also increase. Therefore, operation at excessively high excitation levels can lead to the deterioration of phase noise. It is crucial to select semiconductor devices with excellent noise performance. Flicker noise affects offset frequencies around 10 kHz from the carrier frequency, and thermal noise uniformly affects the entire offset band, similar to the impact of signal levels.
How to Reduce Phase Noise?
Firstly, ensuring the Q value of the quartz crystal oscillator's oscillation circuit is essential.
The higher the Q value of the active oscillator, the smaller the resistance loss of the oscillation circuit and the higher the Q value of the oscillation loop. Noise reduction of the device is imperative. Choose devices with a low noise figure (NF) and flicker corner frequency to reduce semiconductor noise, such as thermal noise, granular noise, and flicker noise.
In addition, multiplication by PLL circuits can lead to the degradation of phase noise.
This point should also be noted. It is also advisable to increase the excitation level of the oscillation circuit as much as possible. Since noise characteristics are relative values between signal levels and noise power, higher signal levels are more favorable. However, using the excitation level characteristics of active oscillators in the flat region is a condition. Harmonics have higher Q values and are highly useful near offsets. However, caution is necessary when operating at high excitation levels, as the resistance loss of crystals and oscillator circuits increases, and due to frequency fluctuations, phase noise deteriorates, hence the need for careful handling.
Finally, appropriately placing a bypass capacitor at the shortest distance from the power source and GND terminals can suppress power source noise.
All of these methods effectively optimize the phase noise performance of active oscillators and greatly contribute to enhancing their performance.