Time Comparison Of Digital Real-time Processing

The measurement accuracy of the time comparison system is heavily dependent on the time-frequency characteristics.Time comparison is an important application of satellite in the field of time and frequency,which is of great significant for the realization of long-distance time transmission and synchronization services.Traditional time comparison system relies on the down converter and time interval counter of the physical environment,and the principle errors of the counting method are never overcome.At the same time,the technological level of the hardware circuit limits the further improvement of measurement accuracy and fast response,and the complicated circuits will greatly increase the cost and workload of time comparison system.With the rapid development of high performance analog-to-digital converters and digital signal processing devices in the semiconductor industry,digitalization has gradually become the mainstream development direction.The time comparison technology of digital real-time processing discussed in this paper,assisted by the border effect of ADC and the digital measurement principle of edge fitting,digitizes the local frequency standard signal and the time interval signal between satellite signal and local frequency standard signal directly.Firstly,it is revealed that the stability of the edge of digital quantization fuzzy area will directly determine the final accuracy of data conversion.And the concept of border effect is proposed to suppress the quantization error in digital conversion.In the digital measurement of periodic signals,when the analog input signal and the code clock signal satisfy the principle of clock cursor with a specific frequency relationship,thus the local second pulse signal can be obtained directly through digitalization.Secondly,the time interval signal obtained by the digital timing comparison is extended by the edge expansion circuit to achieve the stable expansion of the edge,and the high accuracy edge fitting function is reconstructed to complete the high precision time interval measurement.The digital processing of time comparison converts the work that needs to be done in the physical environment during the process of time transfer directly into the digital environment.Moreover,the high performance of analog-to-digital converter and the high-speed computing power of the digital signal processor greatly improve the response time of measurement system and ensure the real-time performance of time comparison.Based on the digital achievement of the above two aspects,many problems such as complex circuits and long-term drift of the whole equipment in signal processing under analog environment are solved.The experimental prototype of time comparison of digital real-time processing is developed on board,which achieves the conversion from analog to digital environment.And the resolution of time comparison can reach nanosecond order.In the system performance test experiment,OCXO 8607-BE oven controlled crystal oscillator and XHTF1003C-G rubidium atomic clock are used as local frequency standard sources respectively,and time comparison with satellite signals is carried out in turn.The stability and reliability of digital time comparison technology are tested and verified.In addition,the time comparison system of digital real-time processing can be used to directly control the voltage controlled crystal oscillator and the digital controlled oscillator by comparing the time between the second pulse of local frequency standard signal and the second pulse of satellite signal to achieve the time transfer and synchronization between local clock and satellite clock.Alternatively,the time comparison results can be sent to the same time comparison receiving device of digital real-time processing at any other place to achieve the time transfer and synchronization between two or more different places.