| According to the properties of detection targets,that the targets can be divided into linear targets and nonlinear targets.When the linear target is irradiated by the electromagnetic wave,it will only scatter the fundamental wave signal.When the nonlinear target is irradiated by the electromagnetic wave,it will scatter not only the fundamental wave signal but also the harmonic signals.Nonlinear targets are entirely artificially synthesized,and semiconductor PN junctions are typical nonlinear targets.When the semiconductor PN junction is irradiated with electromagnetic waves,it will scatter the fundamental and harmonic signals.Among the scattered harmonic signals,the strongest power is the second harmonic signal.Based on the research background,the paper studies the detection of second harmonic signals generated by single target and cluster target,and mainly studies the modulation mode of harmonic radar,ranging method,selection of code type,detection and tracking of receiver signals,and the key technologies of baseband FPGA implementation.The main contributions of the article are as follows:(1)In terms of signal modulation,if the traditional two-phase coding method is used,the second harmonic signal scattered by the semiconductor PN junction cannot be effectively demodulated at the receiving end of the harmonic radar.Changing the initial phase of the two-phase coded signal from 0 degrees and 180 degrees to 0 degrees and 90 degrees can complete the effective demodulation of the second harmonic signal.(2)In order to solve the problem that most of the current harmonic radars have low detection accuracy and some cannot even measure the distance,the paper proposes a carrier phase distance measurement method.In the paper,the pseudo-code and carrier phase ranging accuracy are simulated and analyzed respectively.The results show that the accuracy of using carrier phase ranging is much higher than that using pseudo-code ranging.In order to be able to detect weak power signals below the system noise level,the processing gain of the pseudo-random code is used to improve the system sensitivity.(3)At the receiving end of the harmonic radar,in order to realize the effective detection of the target,the matched filtering method is used.The second harmonic signal scattered by the semiconductor PN junction is very weak.In order to obtain accurate phase information,Costas ring technology is used.The output of the Costas loop is the output signal of the voltage controlled oscillator.The signal retains the phase information of the received second harmonic signal,and its amplitude and signal-to-noise ratio are superior to the received second harmonic signal.In order to improve the stability of the Costas ring and solve the problem of full-round ambiguity of the carrier phase ranging,the design method of the code ring auxiliary carrier ring is adopted.The noise of the environment and the system will introduce errors into the phase information of the loop output.The Kalman filter will be used to denoise the received second harmonic signal to effectively reduce the phase error of the loop output.(4)In terms of hardware design,the paper proposes a complete hardware architecture.For the key technology of baseband signal processing part,FPGA design is completed on ISE,and RTL level simulation verification diagram is given.In the design process,in order to save system resources,a numerically controlled oscillator based on CORDIC algorithm is used.Finally,through simulation verification,it can be known that the average detection error of this scheme is only 1.76 cm when the signal-to-noise ratio is0 d B,which meets the design requirements. |
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