Research And Development Of The Receiver In Transient Pulse Radar Imaging Logging System

Transient pulse radar imaging logging system, as a member of the big family of GPR, is the extension of the borehole radar technology. By utilizing the single hole reflection pattern, the system could be used to detect the pore crack on borehole. High resolution and far detection distance compared to the traditional logging method outstand the system, making it a research hotspot. Receiver of a radar system plays an important role in the signal processing unit, and greatly affects the performance of the whole system. Different radar systems have different requirements on the receiver. Therefore, it is necessary to research on the receiver which belongs to a transient pulse radar imaging logging system.The receiver designed in this paper captures a time domain wave with nanosecond pulse width from receiving antennas, and finally outputs a microsecond signal with stable power. In order to achieve directional system, multiple directional receiving antennas are required to connect with the receiver. High temperature and high pressure in the boreholes require the receiver to work in a high temperature environment. Tremendous loss for the electromagnetic wave in the underground medium asks for a large dynamic range to complete on near field large signal and far field small signal receptions. As radar imaging system extracts target information from the time domain waveform amplitude,the gain of the receiver should be adjustable, in order to get the best echo signal in time domain.This paper introduces the structure of the transient pulse radar imaging logging system, principle of the receiver theory, and analysis of the signal characteristics of the system. Combined with laboratory and field site data, the design is developed and continuously optimized. The receiver designed includes three modules: channel conditioning unit, gain controlling unit, and equivalent sampling unit. The channel conditioning unit is responsible for receiving antenna matching, limiting the large amplitude of excessive signal, and also switching among the four channels. Gain controlling unit is the main part of the whole receiver circuit, mainly affects on amplifying or attenuating the different signal in different degree, meeting the requirements of the sampling module. Equivalent sampling unit, based on sequential equivalent sampling theory, samples the transient pulse signal of nanosecond width in equivalent sampling rate of 1GHz/2GHz optionally, and extends the signal to a microsecond width level, lowering the frequency of the signal and reducing the sampling pressure of the AD chip followed.In the designing process, part of the unit circuits, especially the sequential equivalent sampling unit, is simulated in ADS. While circuit diagram and PCB drawing are carried out by using Altium Designer, a metal shielding cavity is designed with AutoCAD. The existing achievements include three sets of processed receivers, which meet different experimental needs respectively. The receivers above have similar system structure, so this paper mainly introduces and tests the latest type.Measurements show that the newest version of receiver achieves a frequency range of 50 MHz ～ 500 MHz, sensitivity of-80 dBm, dynamic range of 100 dB, noise figure of4.6dB(could be reduced to 3dB), adjustable gain range of-35 dB ～ 30 dB with a gain step of 5dB, and could work in a temperature of as high as 115?C. The final design of the receiver has been applied in the transient pulse radar imaging logging system prototype,and related experiments show that the system has a good performance, and the design properly achieves its goal.