Research And Implementation Of Disciplined Crystal Oscillator Holding Technology

The increasing demand for frequency stability in modern electronics makes it difficult for frequency scales such as highly stable crystal oscillators to meet the demand,while primary frequency standard such as atomic clocks are too expensive for large-scale applications.Disciplined crystal oscillator corrects the crystal oscillator by satellite 1PPS signal,thus making a substantial improvement in the frequency stability of the system within the limited cost.The proper operation of the disciplined oscillator is dependent on the stability of the satellite signal.When the satellite signal is lost,the stability of the disciplined oscillator will deteriorate dramatically and will not work properly.To solve this problem,a set of disciplined crystal oscillator holding algorithm has been designed and implemented in this thesis,which works as follows.(1)The principles of crystal oscillator,satellite timing system and crystal disciplined system are studied,and several common holding algorithms are investigated.(2)The feasibility of using frequency tunning word instead of the actual output frequency of crystal oscillator for simulation analysis has been studied,then a set of hold-up algorithms based on the frequency control word for fitting and prediction is designed based on the simulation results.(3)The algorithm has been implemented based on AD9548,which is responsible for the discipline of the oscillator,and microcontroller,which is responsible for the process control and the implementation of the holding algorithm.When the system is in the disciplined state,the microcontroller trains the model based on the frequency tunning word;after the satellite signal is lost,the microcontroller calculates the predicted value of the frequency tunning word based on the trained model and writes it back to the direct digital frequency synthesizer for hold.At the same time,the problem that it is difficult for the microcontroller to perform high-precision operations is solved by data truncation and customizing floating-point numbers.(4)The implemented system has been tested.The test is divided into three parts:first,the test of frequency accuracy when the satellite signal is valid The test results showed that the output frequency accuracy of the tame crystal in the tame state reached10^-12.Subsequently,the program was debugged to ensure that the model could be generated smoothly and the predicted values could be generated smoothly.Finally,the holding effect of the algorithm was tested.The test results shows that the hold-up system with the algorithm maintained the frequency accuracy in the order of 10^-10 after5 hours of satellite signal loss,and in the order of 10^-9 after 24 hours of loss.