Digital Realization Of A VHF Phase Interferometer Direction Determination System For Thunder Lightning Signal

Lightning detection is of great significance in the national life, in which the VHF-band detection plays an important role. In this paper, the methods of digital realization for VHF lightning radiation interferometer system were studied, a signal processing platform based on software defined radio were established. The system consists of FPGA and DSP, in which digital complex demodulation techniques were adopted, instead of traditional analog realization. Digital realization greatly improved the system's operation accuracy and anti-noise performance and the receive frequency can be changed real time. Digital intermediate frequency (IF) demodulation based on FPGA show good coordination to deal with the contradiction between the capacity and flexibility. This paper discussed the principles of digital IF and the realization on FPGA, and the test results were given to verify the performance of the system.The main work were as follows:1. Discussed the basic theory of software defined radio and digital IF, such as multi-rate signal processing theory, efficient digital filter theory, etc., and discussed the CORDIC arithmetic, DA arithmetic, which are often used in FPGA implementation.2. Determined the system program depending on the parameters required, completed the digital realization of the whole system using FPGA+DSP architecture. The implementation of digital complex demodulation program was discussed in detail.3. Described the specific implementation structure of modules such as digital mixing module and decimation filter module in the FPGA part, which plays as signal preprocessing sub-system. In the digital mixer module, the LUT method and CORDIC method were discussed to achieve the resource optimized structure, and several advanced structures of traditional DDC were introduced. NCO part adopted CORDIC algorithm, FIR filter part adopted DA algorithm. In the decimation filter part, several filter structures and their scope of application were discussed, such as CIC and HB filter. CIC's spectral characteristics were introduced, and the multi-stage cascade implementation in FPGA was given. The main research on FIR filter focused on resources and speed improvements.4. Simulated the former module's structures and performance. Implemented the signal preprocessing unit on FPGA chip and tested to verify the correctness of the design.