Research On Satellite Timing And Frequency Calibration System

The development of the modern science and technology has put forward higher and higher requirement to time and frequency primary standard.How to get high accuracy time and how to keep accurate time synchronization,they are the key problem of realizing high accuracy frequency source.At present,the technology of adopting clock source of the satellite synchronous local clock has already become one of the major methods that produce high precision and high stability clock at home and abroad.The core idea of this technology is:when the satellite is normal,the local clock is calibrated by the satellite clock;When the satellite signal is unstable,the satellite clock is replaced by the local clock.This paper combines the concrete national conditions,utilizes BD system with independent property empoldered by our country,studies a kind of frequency calibration instrument of dual-mode satellite time service combining GPS with BD.At first,the principle of the satellite dual-mode time service technology was explained,the precision of time service of GPS and BD system has been tested.By analyzing the tested data,the feasibility that the second pulse signal of GPS and BD system is used as the timing reference of CDMA mobile communication system base station has been proved.Then,on the basis of analyzing external time reference,this article proposes a kind of scheme that utilizes time service function of the satellite system,draws the second pulse signal through the receiver and calibrates output frequency of local oscillator after carrying on performance selection of the two second pulse signal.Finally,with the programmable logic device FPGA as the core,the specific function modules is designed in detail.The concrete circuit principle and logic chart of time sequence are presented.Under EDA developing platform,the main function is emulated with VHDL language,code downloading and on-line debugging of software and hardware are finished.This system has realized a measurement method of the time interval based on digital phase-shifting technique and designed a scheme of correcting the measurement data.This paper has also proposed a method of measuring frequency based on digital phase-locked loop,realized the accurate measurement of frequency, corrected the measured frequency offset utilizing PI regulator,increased local oscillator frequency accuracy from 1E-10 to the 1E-11.The experimental result shows,the high stability crystal oscillator frequency accuracy after calibration is better than the accuracy of the pre-calibration.The system uses a practical and convenient way to achieve the purpose that high stability crystal oscillator will be calibrated to the expected.