Research On Self Calibration Method Of Oven Controlled Crystal Oscillator

Oven controlled crystal oscillators have the advantages of small size,low power consumption and good short-term stability,which are widely used in many fields such as satellite navigation,communication,electronic instruments and measurement.With the increase of working time or the change of temperature,the output frequency of the oven controlled crystal oscillator will drift slowly,and it cannot be independently used in high-precision fields such as time service and time keeping.In this thesis,a new crystal oscillator aging compensation technology is proposed by studying the characteristics of the phase shift change at both ends of the crystal resonator.In the case of no external calibration source,according to the characteristics of the change of the phase shift at both ends of the resonator,the aging compensation of the oven controlled crystal oscillator is realized.The main contents of this thesis are as follows:(1)The working principle of the oven controlled crystal oscillator and the equivalent circuit of its internal crystal resonator are deeply studied.The increase of working time or the change of the external temperature will cause nonlinear change in the equivalent resonance parameters of the resonator,which will lead to the change of the crystal oscillator output frequency.The theoretical formulas of the load frequency and impedance angle of the resonator are derived.It is proved that the change of the equivalent resonance parameters will cause the change of the impedance angle of the crystal resonator,and the change of the impedance angle will eventually lead to the change of the load frequency of the resonator.When the change of impedance angle is less than 1E-12,the relationship between the change of crystal oscillator output frequency and the change of phase shift at both ends of the resonator can be approximately equivalent to a linear relationship.(2)The phase shift at both ends of the resonator and the frequency of the output signal of the oven controlled crystal oscillator are measured synchronously for many times and the phase shift and frequency data are extracted.Using the equal-depth binning method,after multiple processing to determine the depth parameter to change the binning interval,and combine the mean filtering to smooth the data,and fit the mathematical relationship between the phase shift change at both ends of the resonator and the output frequency change of the oven controlled crystal oscillator.After using Kalman algorithm to smoothly correct the coefficients several times,the phase shift is used as the independent variable to establish a mathematical model between the phase shift and the output frequency.(3)The self-calibration compensation control system is established by using the method of digital linear phase comparison,and the phase shift of the signals at both ends of the resonator is collected.According to the established mathematical model and the voltage control sensitivity of the sample crystal oscillator,the adjusted voltage value is obtained and fed back to the voltage control terminal of the crystal oscillator.The time resolution of the acquisition selected by the system is 0.76 ps,and the phase shift of the sample crystal oscillator changes approximately 4～5ps within 10 s,so the oven controlled crystal oscillator is controlled at least every 10 s to improve the frequency drift of the oven controlled crystal oscillator.The experimental results show that the self-calibration method can effectively improve the aging drift of the oven controlled crystal oscillator.After compensation by self-calibration system,the daily aging rate of oven controlled crystal oscillator is increased from 4.49E-9to 2.52E-10.