摘要
地面核磁共振找水方法是目前世界上唯一直接找水的地球物理方法。在与地磁场垂直的方向上加一个交变磁场去激发地下的氢质子,氢核的磁矩就会偏离地磁场的方向形成宏观磁矩。当激发场停止后,宏观磁矩又会恢复到沿着地磁场的方向并产生微弱的信号,利用该信号就能探测地下水情况。针对这一原理,提出接收这个微弱信号的核磁共振找水仪弱信号放大器的设计方案,同时研究了相关的关键技术。
首先论述国内外核磁仪器的发展现状,介绍核磁共振找水的原理,并对核磁共振找水仪整套仪器的工作原理进行阐述,结合核磁共振信号的特点提出核磁共振找水仪弱信号放大器的整体设计方案。其次在放大器的设计过程中完成干扰及噪声的抑制、超低噪声运放的选取、阻抗匹配及噪声匹配、放大器的高增益等的设计与实现。同时解决了微弱信号放大关键技术,开关电容选频滤波技术,程控增益放大技术等关键技术。在放大器测试部分,进行性能指标测试及野外实验,通过对实验结果的详细分析,验证核磁共振弱信号放大器的有效性及可靠性,并给出进一步的改进建议。
Since the Nuclear Magnetic Resonance phenomenon is discovered in 1946, along with the science and technology development, the Nuclear Magnetic Resonance phenomenon entered the application and the phase of exploitation by the fundamental research experiment. It is widely applied to such domain as physics, chemistry, biology, medicine, also obtained the widespread application in the geophysics (proton precession magnetometer, NMR wave spectrometer, NMR oil well logging meter). It is the new applicant that using the Nuclear Magnetic Resonance technology to detect the underground water. The first NMR underground water investigation instrument is developed successfully by former Soviet Union in 80 years. It is the present only one of geophysics method direct surveys underground water. Later the France company bought the patent and made the first instrument for commercial use. It uses the proton magnetic moment which display under the earth magnetic field. Along the earth magnetic field vertical direction, we add a alternating magnetic field in to stimulate the underground hydrogen proton, the proton magnetic moment to is able to deviate the earth magnetic field direction to form the macroscopic magnetic moment. After the excitation field stops, the macroscopic magnetic moment can restore to and produces the weak signal along the earth magnetic field direction. With the signal, we can gather the information about the water content, the depth and the porosity of the underground water. As the signal is so weak and the place we carried out our exploration is so near to the power lines, the method is easily disturbed by the electromagnetic noise. The amplifier is the key unit of the NMR instrument to improve the anti-disturb ability and weak signal picking up ability. Its performance decides the whole instrument’s exploration ability. The paper is based on the designing and realization of the NMR amplifier.
The design is given out based on the characteristics of the NMR signal (amplitude, bandwidth and the signal length) and the result of the electromagnetic noise and disturbances in the open air. The amplifier is made up of the pre-amp, broadband filter, procedure-gain-controlled amp, narrow band filter and the procedure-gain-controlled amp. The design is to suppress the noise and amplify the weak NMR signal. The whole amplifier is placed in the shielding box made up of mild steel whose surface is plated with the cuprum and nickel.
The pre-amp is made up of the impedance matching circuit and the low noise instrumentation amp. The amplifier achieves 85nV noise through reasonable board circuit design, aborative selection of the component and the design of the low noise power.
The broadband filter is composed of the 4th order double quadratic high pass filter and low pass filter. The main function of the filter is to amplify the NMR signal and suppress the 50Hz and its low frequency harmonious disturbance and high frequency noise.
The procedure-controlled amp is to adjust the amplitude of the signal before the narrow band filter. The narrow band filter is made up of four pieces of MAX260 switching capacitor filters. The clock of the MAX260 is supplied by the DDS chip. The center frequency, bandwidth and the gain of the narrow band filter can be changed neatly by the MCU. The signal comes out from the narrow band filter comes into the last procedure-controlled amp and output.
The whole amplifier is detailedly tested in the lab. The test matter contains the background noise test, gain test, bandwidth test, delay time test and the aberrance test. The test result shows that the amplifier achieves the index that needed. The amplifier then explored with the whole NMR instrument in different place and performed well.
Through whole year’s scrabble, design, and experiment, the NMR amplifier matured gradually through excessive debugging, testing and improvement. The amplifier works unfailing.
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