| In morden well-logging tools, the precise measurement of the weak signal is very important. The performance of the weak-signal-detection system is vital to well-logging tools. This thesis has developed a weak signal detection system which can realize the reliable detection of the a hundred-n volt level array induction signal with the guidance of weak signal detection theory.The thesis first introduce the basic theory of noise , the principle and method of designing low-noise preamplifier and the basic structure of weak-signal-detection system. Then the weak signal detection platform including receiver section and DSP data acquire section of array induction logging tools is given based on array induction logging theory and the characteristic of array induction signal. The action of every circuit module on the whole is illuminated and in particular the circuit structrue of the preamplifier and band-pass filter is analyzed. The noise contribution of preamplifier circuit component is discussed and improved motheod is proposed. The noise performance of the preampflier circuit is qualitative calculated based on noise emulation analyse results. A kind of second-order infinite gain band-pass filter multi-feedback active filters is discussed and transfer function ,bandpass gain,center frequency are calculated.Results of calculation based on emulation analyse show that the smallest noise figure of the circuit is 1.5dB; the equivalent noise voltage is 2 .5nV /Hz,it can realize the reliable detection of the a hundred-n volt level array induction signal. |
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