The Development Of Low Noise Transient Electromagnetic Methods Receiver Based On Equivalent Sampling Technology

Time Domain electromagnetic Methods, or which calls Transient Electromagnetic methods (TEM), which utilizes re-line untouched to ground (big re-line or magnetism even source) or electricity power touched to ground (electricity-even-source), it sends the first field and measures the induced electromagnetic field (the second field) changed with time which is created by underground substance. So it can measure the position of underground inhomogeneous substance and estimate its scale and electricity capability. The TEM instrument system includes receiver and transmitter, the receiver of high capability can acquire valid signal and reflect the geology condition. The title of this dissertation is the Development of low Noise Transient Electromagnetic methods Receiver based on Equivalent Sampling Technology, The instrumental system of this receiver is well designed and developed. The equivalent sampling technology is a new technology which can achieve ultra high speed data acquisition by using normal AD. This new technology is used at the cycle signal's data acquisition of radar ,sonar and so on. The wave received by TEM receiver is in the beginning period of transmitter cut-off, the reason that the acquisition second field (TEM signal) quickly decreases in the beginning period and has a large breadth (from dozens of uS to hundreds of uS, the breadth decreases from about 10V to mV or even uV), in middle and late period, the TEM signal slowly decreases and has a small breadth (from hundreds of uS to several mS or even several second, the breadth decreases from about mV or uV to uV or nV). It needs enhance the acquisition rate to several uS if acquiring second filed signal in cut-off beginning period. For these features, we can used the equivalent sampling technology to acquire the early stage of the signal. The sampling rate can attain 0.5us. In terms of the basic principle of TEM, we designs the large dynamic range, fast speed data acquisition module based on floating point amplification. This module includes two main sections, namely the front acquisition—fast speed 8bits A/D and the rear acquisition—16bits A/D. The front 8bits A/D outputs suitable amplification times by decoding circuit to control amplification circuit. Then the amplification signal inputs into 16bits A/D. So the result consists of exponent-code (the output of decode circuit) and tail-code (the output of 16bits A/D). The rate of the data acquisition module used floating-point amplification can attain 5uS, and the dynamic range can attain 156dB. Many measures and technology are used in the study and design of the low noise circuit to reduce the noise and to amplify the useful TEM signal. Particularly, the technology of low noise anti-saturation subsection are used to restrain the noise and improve the SNR .The noise lever of this system is reduced a degree and the noise index lever can attain uV lever. In software design the data that is acquired in different period is stacked in step and the stack times can amount to 9999, the noise is cleaned efficiently. Thinking of using VGA 320*240 LCD (Liquid Crystal Display), we adopt the design style of UCDOS CSDK. After the design of every window is completed, different data files has formed and saved in 8M disk, this made the design concise, it also saves room. The total system of this receiver is collectively designed with the idea of modularization, which imitates the connection of the construction of the...