| Ground penetrating radar launches electromagnetic wave by transmitting antenna in form of high frequency electromagnetic pulse, and then receives the backward electromagnetic wave by the receiving antenna. According to the analysis of the backward electromagnetic wave, GPR can find out the specific situations. Ground penetrating radar is widely used in the kinds of areas, for example detection of road, detection of underground pipeline, search of underground object, analysis of geology, archaeology, military affairs and so on. Therefore it is deeply significant for the research and improvement of GPR.The following works for the research of GPR are described in the thesis.The antenna system and the transmitting and receiving systems of GPR are analyzed in this thesis based on a deep understanding of the whole GPR system by the papers at home and abroad. The working theory and working process of GPR are studied, and a full explanation for the transmitting and receiving systems of GPR is made.Two kinds of avalanche circuits are designed through analysis the principle of avalanche transistor in this thesis. The trigger pulse is a kind of square wave which is produced by FPGA. The frequency of this pulse is 200 kHz, and its duty ratio is under 10%. Avalanche voltage in the avalanche circuit is provided by adjustable DC voltage. As the results of actual test and measurement, many kinds of transmitting pulses are gotten on a 51Ωload, the widths of which are from 1.337ns to 42.91ns, and the amplitudes are dozens of volts. Two kinds of RF transistors and one kind of avalanche transistor are used to make contrast in the experiments. The method of reducing the pulse width and improving the pulse amplitude is summarized in this thesis.According to the requirements of different probing depth by GPR, antennas used in the actual measurement are analyzed in this thesis. Different kinds of antennas should be used to fit the different conditions, and analysis and comparison are proceeded between commonly used GPR antennas. Detailed analysis and discusses are made for the horn antenna, and the parameters of the waveguide and horn antenna are calculated based on the transmitting pulse in this thesis.The receiving unit of GPR is designed on the basis of equivalent time sampling principle in this thesis, in which there mainly include amplifier circuit with time varying gain, stepping pulse generating circuit and sampling gate circuit. The high frequency signals from GPR antenna can be converted into low frequency signal by the receiving unit. |
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