| With the continuous acceleration of the industrialization of 5G mobile communications,the accuracy and stability of the clock reference source are required to be higher in the fields of measurement and control,communications,satellite navigation,and power networks are becoming higher and higher.As a commonly used clock source,the Oven Controlled Crystal Oscillator(OCXO)has good short-term stability.However,due to the influence of aging,environmental temperature and other factors,the frequency of the OCXO will gradually drift,and the long-term stability is poor,so it cannot be independently applied to the fields of high-precision time service,time keeping,navigation and positioning.The main way to solve this problem is to use the Global Navigation Satellite Systems(GNSS)signal to tame and hold OCXO.After the GNSS signal is lost,the effective holding technology for a short time is a hot and difficult point at home and abroad.This thesis studies the improvement of clock holding technology under the tame of OCXO based on Beidou signal.The specific research contents and results are as follows:(1)Aiming at the technical research on the improvement of the output frequency retention ability of OCXO after losing the external reference.By analyzing the aging trend of OCXO output frequency,a holding model based on BP neural network algorithm is studied and a power exponential function model which is closer to the trend of most crystal oscillators is established.Based on the power exponential function model and BP neural network model,OCXO with different aging trends are fitted and predicted,and the prediction results are compared by using the sum of squares of residuals,chi-square coefficients and correlation coefficients to verify the correctness and applicability of the BP neural network holding model.(2)Research on the technology to improve the fitting ability and prediction accuracy of BP algorithm.Two filtering algorithms,Savitzky-Golay filter and Vondrak filter,are designed,and the frequency difference data of OCXO are preprocessed based on the two algorithms.Vondrak filtering algorithm can reduce the error between the predicted output and the expected output from4×10-4 to 2×10-4.Savitzky-Golay filtering algorithm is more flexible,the window size can be freely selected,the window size is set to 9,and the error can be reduced to1.5×10-4.(3)This thesis establishes the taming and holding system of the Oven Controlled Crystal Oscillator.The overall scheme of the system is designed,the function of each module is determined and the related software and hardware design is completed.Taking the Beidou1PPS signal as the reference,the time difference between the Beidou 1PPS signal and the OCXO frequency division signal is measured by the time interval measurement module GP21.According to the relationship between the time difference,frequency difference and voltage control sensitivity,it is converted into the voltage-controlled voltage for controlling the OCXO,so as to realize the taming of the OCXO.After the loss of 1PPS reference signal,switch to hold mode,and BP algorithm combined with filter processing is used to establish a hold model to realize the hold of OCXO.(4)The taming ability of OCXO in the system was tested.The frequency accuracy is better than1×10-1 1 within three days after taming,and the daily aging rate is increased from0.5ppb/d to 0.06ppb/d,which improves the long-term stability of OCXO.The simulation test is carried out on the holding ability of OCXO after the reference signal was lost.The frequency accuracy of the OCXO reached1.5×10-11in the first 6 hours,and the accuracy reached2×10-11 within 24 hours after the loss of lock,the time keeping accuracy can reach the index of close to 1.75?s. |
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