Frequency Of OCXO Calibration System Base On GPS

With the development of the message exchange,the importance of the frequency source increases. Instability of frequency source will lead to less accuracy in synchronization, and even may cause system paralysis in severe cases.At present, the kind of frequency source with high precision is OCXO.However,the existence of long-term cumulative error makes it difficult to be applied in synchronous systems.American Global Positioning System(GPS) equipped with a atomic clock of high stability,which exports the 1PPS signal with the feature of good short-term stability. Therefore, this article uses GPS second signal for calibrating OCXO in order to get the frequency source with high precision( GPS frequency calibration system).With FPGA digital technology as the means of measurement and ARM7 processor as the algorithm control center, using PLL frequency synthesizer technology, GPS frequency calibration system tracks and locks the phases of crystal vibration division frequency second signal and GPS second signal,and changes the frequency according to the phase differences between them to correct the voltage. Based on improving its long-term stability of crystals without destroying its short-term stability of frequency,it aims to obtain the frequency source of high stability and reliability. The system consists of four modules,that is,a GPS signal receiver, a high-precision digital phase detector, data filtering and system control, as well as control of the crystal oscillator.the main function of GPS receiver module is to receive the second signal-1PPS from the GPS through the special receiving antenna placed outdoors.Adopting digital measurement technology,the high-precision digital phase detector module aims to achieve the high-resolution measurement of phase differences with counting method using a high-speed clock.The error analysis and smooth control are treated as design though in data filtering and system control module,with the achievement of moving average filter algorithm and adaptive PID control algorithm in the ARM.Using a low-noise DA control circuit with good electromagnetic compatibility,crystal oscillator control module converts a great number of bits exported by the digital processor into high-precision control voltage in DA circuit.The prolonged observation of the developed GPS frequency calibration system in laboratory shows that:The phase difference between crystal vibration division frequency second signal and GPS second signal does not exceed 80 ns,which helps to better achieve the phase-lock adjustment.And the frequency accuracy after being calibrated reaches the magnitude of 10-10. The system is also applied to a digital ionosonde-PDI-1 developed by the laboratory.Oblique side tests on two instruments are carried out successfully,which well meets the requirements of time synchronization needed in oblique side test.