Research On A Clock Disciplining And Holding System Based On Beidou Time Service

Clock synchronization is one of the key technologies for the reliable operation of a communication network.To avoid the impact or even interruption of wireless communication services caused by cross-slot interference,a unified time-frequency reference is required by each node of the network.As the most widely used frequency source,the oven controlled crystal oscillator(OCXO)has excellent short-term stability,but its temperature and aging characteristics lead to poor frequency accuracy and long-term stability,which can not be applied to high-accuracy timing and timekeeping.This thesis has researched a high-precision clock disciplining and holding system.The standard reference signal provided by the Beidou satellite navigation system is used to discipline the OCXO,and the self-adaptive holdover is realized after the reference signal is interrupted,which can effectively improve the frequency accuracy and long-term frequency stability of the OCXO.It can be applied to 5G communication base stations and other scenes that need highprecision time-frequency signals and are sensitive to volume and cost.The author’s major contributions are outlined as follows:(1)The clock disciplining and holding system is designed,and the system-level printed circuit board is realized.In the disciplining mode,Beidou 1PPS is used as the reference to discipline the OCXO,and the frequency state parameters are obtained to train the aging model.When the system loses the reference source,the frequency drift of the OCXO is estimated and corrected in time according to the aging model established in the disciplining mode and temperature model to improve the holdover capacity of the OCXO.(2)The data processing algorithm is researched for the error of the observation data caused by factors such as the phase jitter of 1PPS,the characteristics of the OCXO and the noise in the measurement process.The real-time filtering algorithm based on Pauta criterion in dynamic measurement environment is used to eliminate outliers;By analyzing the state space model of phase difference and the parameter extraction method of Kalman filter,the Kalman filter suitable for this system is researched;The output of Kalman filter is processed by arithmetic average filter,and the phase information of local clock and 1PPS is accurately obtained.According to the linear phase comparison frequency measurement method,the frequency state parameters of local OCXO are obtained,and the OCXO is disciplined by the frequency adjustment algorithm of voltage step.(3)Aiming at the interruption of Beidou satellite reference signal,the research on selfholdover technology of the OCXO has been carried out.The Savitzky-Golay filter is used to smooth and denoise the data of the temperature variation experimental.The temperature compensation formula under the condition of temperature rise and drop is extracted by the general linear parameter least square method,and its repeatability is researched,and then the temperature compensation model of the OCXO is established;According to the relationship between temperature and aging drift,the effects of the two on the output frequency of the OCXO are effectively separated,and the aging parameters of the OCXO in the free running state are obtained.According to the stored historical data,the aging model is adaptively trained by the linear regression analysis;Meanwhile,the key factors such as residual frequency offset and step residual error that affect the holding performance are eliminated,and the high-precision holdover of the OCXO is realized through the established temperature and aging model.(4)The test of the disciplining and holding performance of the system is carried out by the built hardware platform.The results show that the short-term frequency stability of the OCXO disciplined by this system has maintained its original index,while its frequency accuracy and long-term frequency stability have been significantly improved.The frequency accuracy has increased from 9×10-11 to 2.475×10-13,and the 10000s-level frequency stability has reached 7.80×10-13.The maximum phase deviation of the OCXO within 24 hours does not exceed 1.5μs when the 1PPS is lost.