Research On Direct Wave Cancellation Technology Of High Frequency Continuous Wave Radar

High-frequency ground wave radar uses electromagnetic wave diffraction to accurately detect surface warships and low-altitude flying targets,with features such as anti-stealth targets and all-weather operation.Its unique advantages have attracted the attention of many countries in offshore exploration.Unlike high-frequency ground-wave pulse radars,high-frequency continuous wave radars have advantages such as low transmission power,strong anti-interference ability,and high resolution.However,it has not been widely used.The reason is that there are direct wave interference problems in high-frequency continuous wave radar.This thesis proposes two solutions for direct wave suppression of high frequency continuous wave radars.Aiming at the demand of high-frequency continuous wave radar analog receivers,this thesis proposes a direct wave RF cancellation scheme based on direct digital frequency synthesis technology,and designs this cancellation system.Based on the idea of RF cancellation,this thesis uses DDS and digitally controlled attenuators to generate high-precision cancellation reference signals which cancel with direct-wave signals.According to the characteristics of the actual receiver's parameters,based on the theory that the system needs to reach the minimum level of elimination,the structure and function of each module of the system are studied.Using MATLAB simulation software to simulate the system scheme,the results achieved more than 20 dB cancellation depth,verifying the feasibility of the cancellation system scheme.Finally,the hardware design and assembly of each module form a direct wave cancellation device for the processor with FPGA as its core.Tested with chirp signals to verify the practicality of the direct wave suppression system.For digital RF receivers,this thesis proposes an algorithm based on fractional Fourier transform to suppress direct wave signals.Through analysis of the definition and properties of FRFT,the method of sampling FRFT discretization was studied.After that,the time delay estimation accuracy of discrete FRFT and the direct wave suppression of discrete FRFT algorithm are studied.The effect of discrete FRFT on direct wave suppression was simulated and RD spectrum extraction was performed on the signal after suppression.Because this method has side-lobe masking,the algorithm is further improved and direct-wave suppression based on NLMS-based FRFT is proposed.The results show that the suppression depth of the improved algorithm is improved by about 6dB,which satisfies the requirement of high-frequency continuous wave radar direct wave suppression.