The Design Of Echo Signal Processing Circuit For Downhold NMR Formation Fluid Analyzer

In view of the requirement of real-time evaluation of oil and gas reservoir of downhold NMR formation fluid analyzer under the condition of downhole, a echo signal processing circuit with high gain and low noise is developed in this thesis. The echo signal processing circuit is the main component of the down-hole fluid NMR analyzer, and its performance has a direct impact on the evaluation results of the reservoir. This thesis introduces the echo signal processing circuit from three aspects:Firstly, a impedance matching method using Smith circle chart is proposed based on the analysis of the antenna impedance in this thesis. Combined with the simulation results of ADS software, the impedance matching of the antenna is completed by using the network analyzer.Secondly, Aiming at the characteristics of high frequency, low intensity, low SNR of the echo signal and special logging work environmenta, low noise preamplifier is designed and implemented. Circuit with low noise bipolar transistor MAT02 for core of the negative feedback amplifying circuit is the first level amplification. And it uses the low noise integrated operational amplifier AD8130 as the second and three level amplification, so as to improve the noise performance of the preamplifier.Finally, Aiming at the enhancement algorithm of real-time processing performance, an method of implementing DPSD algotithm based on FPGA is designed and realized. Distributed artithmetic is adopted in DPSD algotithm, which can convert the operation of tedious multiply accumulate into the operation of looking up table and shift accumulator, saving the logic resource of FPGA and improve the data processing, promoting data processing speed.The key part and the whole performance of the instrument are tested by using the diesel oil sample as the experimental object. The results show that the pre-amplifier has good linearity, flat bandwidth and excellent low noise performance at 150?. DPSD based FPGA algorithm can accurately restore the echo signal in high noise background. The inversion results of T2 spectrum and the Magritek lithology analysis instrument have high conformity. From two aspects of the function of the single board circuit and the whole system, the feasibility of the receiver system of the downhole formation fluid NMR analyzer is verified.