Research On Self-calibration Technology Of Frequency Drift Of Crystal Oscillator

With the rapid development of technologies such as satellite navigation and mobile communications,higher requirements have been placed on the performance of clock frequency sources.At present,crystal oscillators stand out among many frequency sources because of their high cost performance,but because they are susceptible to internal aging,environmental factors,and changes in load characteristics during continuous operation,the output frequency value will drift in one direction,which limits its independent application in the field of high precision.In order to improve the performance of the crystal oscillator,a highly stable crystal oscillator that meets the standards of the 5G era has been developed.In this paper,a self-calibration technique for the frequency drift of the crystal oscillator is studied.This article firstly analyzes the working principle,frequency drift characteristics and existing frequency drift compensation technology of the crystal oscillator.for the crystal oscillator in the case of frequency drift,its resonance parameters will also change correspondingly,which will cause the change of transient phase shift of output at both ends of the resonator,the analysis proves that the crystal resonance parameters have a certain relationship with their output frequency and the transient phase shift at both ends.the output frequency of the crystal oscillator and the output transient phase shift at both ends are measured synchronously,the mathematical model of the output frequency of the crystal oscillator and the transient phase shift of the output at both ends is established by processing and fitting the data.aiming at the problem that the output of crystal oscillator will be doped with noise due to external unstable factors,the collected data is filtered by the mean value,and all the data points in the neighborhood are sorted according to the sorting statistics theory,and then the middle value is taken.To remove abnormal points to complete the filtering.in order to solve the problem that the direct output signal power at both ends of crystal resonator is too small,a double common emitter amplifier circuit with higher power gain is designed.Combined with L-type matching network,the power amplifier can obtain enough gain in the working frequency band and realize the maximum power matching.According to the voltage control characteristics of crystal oscillator,the A/D converter is controlled by a single chip microcomputer to output a voltage increasing from zero to the voltage control terminal,record the frequency change data,and uses least square method to obtain the corresponding relationship between the output frequency and the voltage controlled voltage.Finally,the functional relationship among the phase shift,frequency and voltage control voltage is obtained.Based on this theory,a self calibration technology of crystal oscillator frequency drift is studied.The system realizes high-precision measurement and control by digital technology.Firstly,the ADC is used to collect the output signals at both ends of the crystal resonator in real time,and the collected data is converted into the transient phase shift of the output at both ends of the resonator by using the principle of phase measurement at the same frequency.The single chip microcomputer calculates the cumulative change of the crystal oscillator output frequency through the function relationship between phase shift and frequency.Finally,the self calibration system controls the DAC output reverse compensation voltage to the crystal oscillator voltage control terminal to compensate the frequency drift of the crystal oscillator and improve its frequency accuracy.Experimental results prove that the system can compensate the frequency drift of the crystal oscillator,and the frequency accuracy of the compensated crystal oscillator is improved by an order of magnitude compared with the previous one.The system has the advantage of simple structure and flexible application and does not require the introduction of high-performance external reference sources,which provides a new method for the compensation technology of crystal oscillators,and has high application value and significance.