Research On Self-calibration Technology Of Crystal Oscillator

With the rapid development of science and technology,more and more high quality frequency sources are needed in many systems as carriers of increasingly complex baseband information,such as communication,navigation and so on.And higher and higher requirements are put forward for the performance indicators of frequency sources.Crystal oscillator is widely applied in satellite navigation,national defense,military industry,communications,astronomy and other high-tech fields,as well as in consumer products,due to low cost,the high short-term,etc.However,the aging drift of crystal oscillator has always been an important defect.The research on improving the aging drift of crystal oscillator is of great significance and application value.In this paper,the physical characteristics and working principle of crystal oscillator are thoroughly analyzed,and various factors affecting the aging drift of crystal oscillator are studied.It is found that the change of the equivalent resonant parameters of crystal oscillator will cause the change of the output frequency,while the equivalent resonant parameters will change with the working time of crystal oscillator due to the factors of "stress relaxation" and "mass transfer".At the same time,there is a functional relationship between the voltage-controlled terminal voltage and the output frequency.Based on this theory and combining with the traditional method of improving the aging drift of crystal oscillator,this paper presents a new method of designing digital self-calibration system to improve the aging characteristics of crystal oscillator.The digital self-calibration system does not need to introduce a frequency source with higher frequency stability as a reference,but uses one resonant signal of its internal crystal resonator as a clock to sample another resonant signal A/D.According to the principle of phase measurement at the same frequency,the phase difference of two signals at the same frequency is also called phase shift,which can be obtained by transforming the amplitude data collected into trigonometric function relations.The change of phase shift can reflect the change of output frequency.At the same time,there is a fixed functional relationship between output frequency and voltage-controlled terminal voltage.In this paper,the frequency counter is used to synchronously acquire the output frequency of the crystal oscillator and the phase shift data of the signal at the two ends of the resonator,and the single chip computer is used to control the output of the analog-to-digital converter from zero to the voltage control terminal,recording the frequency change data.The function relation between phase shift change and output frequency change,and between output frequency and voltage-controlled voltage obtained by least square fitting are used as the basis of conversion between parameters in the algorithm.The digital self-calibration system detects the phase shift change of the signals at the two ends of the resonator.According to the functional relationship among phase shift,frequency and voltage-controlled voltage,the voltage control end of the crystal oscillator is controlled by feedback,and the output frequency is pulled back to the nominal frequency to improve the aging drift of crystal oscillator.The experimental results show that the digital self-calibration system can effectively improve the aging drift of crystal oscillator.The daily aging rate of crystal oscillator decreases from 10-9 to 10-10.After observing the aging drift for ten days,the aging rate of the system after control is obviously improved compared with that before control.