Research On Key Technologies Of Switching Between Main And Backup Atomic Clocks In Timekeeping System

Since the first cesium beam atomic clock was successfully developed in 1955,the timekeeping method based on atomic frequency standard has been widely used in satellite navigation,deep space exploration,modern communications and many other fields requiring high precision time and frequency.As a high precision time and frequency source,atomic clock can provide high performance time and frequency signal.Maintaining a continuous,stable and reliable physical time and frequency signal is the basis of the time system.To improve the reliability and continuity of physical time and frequency signal,the construction of the time system generally adopts the redundancy of atomic clocks.Even though atomic clocks are ultra-stable oscillators,they are subject to temperature,magnetic fields,radiation and aging,which can cause atomic clocks to become abnormal,such as frequency jump and phase jump.When the main atomic clock is abnormal,it needs to be switched to the backup atomic clock seamlessly in time to maintain the continuity and stability of the output signal of the whole system.At present,there are some relevant equipments for seamless switch between main and backup atomic clocks at home and abroad.Due to the blockage of foreign technologies,the lack of domestic research data,and the accuracy of product needs to be improved in domestic,it is very necessary to study the key technologies and the implementation of relevant algorithms for the seamless switching of main and backup atomic clocks.To realize seamless switch between main and backup atomic clocks,high-precision time difference measurement,atomic clock clean-up,abnormal detection and loop bandwidth adaptive are studied in this dissertation.The research content of this dissertationcan provide theoretical support for the development of seamless switching equipment of main and backup atomic clocks,enrich the theoretical research on seamless switching of main and backup atomic clocks in domestic,and realize the seamless and smooth switching of main and backup atomic clocks in the timekeeping system by simulation.The main contents of this dissertationare as follows:(1)In this dissertation,the principle and advantages of all-digital dual mixer time difference measurement are compared and analyzed based on the principle of analog dual mixer time difference measurement.The all-digital dual mixer time difference measurement method uses an analog-to-digital converter to digitize the analog signal,and obtains the phase difference between the signals of the multichannel atomic clock through a series of work such as digital mixing,digital low pass filtering and digital detector.The all-digital dual mixer time difference measurement method effectively avoids the influence of operational amplifier,zero-crossing detector and other devices on the background noise of the system,and adopts all-digital processing method to improve the measurement accuracy and reduce the background noise of the system.The simulation results show that when measuring 10 MHz frequency signal,the theoretical background noise of the all-digital dual mixer time difference measurement module can reach 2.36e-14@1s.(2)In this dissertation,the structure and principle of the seamless switching between main and backup atomic clocks are analyzed.Aiming at the loop bandwidth,which is the factor affecting the clean-up effect of atomic clock,an optimal searching algorithm for the loop bandwidth of atomic clock clean-up is proposed with the minimum phase jitter of system output signal as the objective function.Simulation experiments are carried out for MHM2010 hydrogen clock,HP5071 A cesium clock and PRS10 rubidium clock respectively.The results show that the optimal loop bandwidth obtained by this method can achieve good clean-up effect of atomic clock,and the shortterm stability of the output signal is close to the voltage controlled crystal oscillator,and the long-term stability is close to the reference source.(3)By improving the innovation extrapolation method based on the Kalman filter,the detection statistics of the innovation extrapolation method in several epochs are accumulated.It increases the noncentrality parameter of chi-square distribution and improve the detection probability.The simulation results show that the improved innovation extrapolation-accumulation method can obviously improve the detection probability and reliability,which increases by 81.29% on average.At the same time,in view of the Kalman algorithm relies on the initialization of noise matrix problem.In this dissertation,the prediction algorithm of p-step unbiased finite impulse response filter(UFIR)is used to avoid the influence of outliers effectively,and the detection statistics are constructed for the residual errors generated in the iterative Kalman-like recursive process.By adding the statistical detections of multiple epoch,the non-central parameters of statistical detections are increased,and the small phase and frequency jump can be detected more effectively.The correctness and effectiveness of this method are verified by simulating the phase and frequency anomaly data of hydrogen and cesium atomic clocks and collecting the real anomaly data of cesium atomic clocks in the laboratory,and the detection reliability is 60% higher than the method of the innovation extrapolation method based on the Kalman filter.(4)In this dissertation,unbiased finite impulse response filter is used to compensate the phase deviation caused by phase and frequency deviation at the switching moment of the different models of the same type of main and backup atomic clocks with small frequency deviation,so as to ensure that the phase of the output signal of the system does not produce phase jump before and after switching.Due to the large difference of loop bandwidths before and after switching between different types of atomic clocks,directly changing the loop bandwidths can cause nanosecond phase jump.By using the adaptive adjustment method of loop bandwidth,the loop bandwidth is adjusted slowly before the steady state of the loop,and converges quickly to the optimal bandwidth after the steady state of the loop,and the smooth and seamless switching of different types of atomic clocks is simulated.