| Nowadays,the advent of the“cloud”era and the 5G era have imposed higher requirements on the performance indicators of frequency sources.In a large number of high-level frequency sources,the crystal oscillator is still in a very important core position rather than a subordinate position.In order to cope with the rapid development of the market,it is an inevitable choice to develop new technologies and higher-level crystal oscillators.The frequency stability is the key technical index to evaluate the performance of the crystal oscillator.The traditional method of measuring frequency stability cannot measure the stability of transient frequency due to the limitation of frequency range,which leads to the fact that the law of the stability and physical nature of the frequency source cannot be fully reflected.Therefore,the traditional method of improving the frequency stability of the crystal oscillator is mostly improved by aging compensation,optimization of the oscillating circuit and the manufacturing process,instead of studying and improving it from the essence.As a result,its improvement effect is not significant or uncertain.With the development of digital full-response frequency stability measurement technology,the limitations of traditional measurement techniques have been overcome,the measurement of full response time frequency stability of the crystal oscillator has been achieved,which laid the foundation for the study and improvement of frequency sources from its nature.The main contributions of the paper are as follows:1.The full response time frequency stability of crystal oscillators and Rubidium Atomic clock is measured by the digital full-frequency stability measurement technique.At the same time,their phase noise is measured by the existing phase noise measurement system.Then through the comprehensive analysis of the corresponding characteristics between the frequency stability characteristics and phase noise of them,the one-to-one correspondence between the law of frequency stability change with time and the actual effect of phase noise is established for the first time,which reveals the nature of phase or frequency control of the frequency source.Then the stability characteristics of various time-frequency devices have been further explored.The stability of crystal oscillators has formed the stability background of time-frequency devices such as atomic clocks and DDS.The stability of phase-locked frequency source based on the crystal oscillator still keeps 1/?,and the stability of the frequency-locked frequency source coincides with1/?1/2.2.Based on the above analysis results,in order to improve the stability characteristics of the crystal oscillator.We propose a scheme of delayed feedback control which combines high stability of transmission delay and ADC?analog to digital converter?digital acquisition in this paper.After the output signal of the VCXO is delayed by the delay unit,it is used as the ADC clock signal to collect the crystal oscillator signal without delay.The phase fluctuation information of the signal can be detected through this self-standard comparison,and then the phase fluctuation information is fed back to the VCXO's voltage control terminal to improve the transient stability.Since the transient to short-term stability of the crystal oscillator is kept constant as 1/?,the full-response time frequency stability characteristics can be affected by the improvement of transient and short-term stability.Theoretical analysis and preliminary experimental results proved that this delayed feedback compensation method not only can truly reflect the working characteristics of the crystal oscillator,but also make the crystal oscillator frequency stability increase by 2-3times.This method with the advantages of self-comparison and homologous calibration opens up new ideas for improving the performance of frequency sources,it has very important reference and further research value. |
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