| Since 1946 discovered the Nuclear Magnetic Resonance phenomenon,along with the science and technology development, the NuclearMagnetic Resonance phenomenon entered the application and the phaseof exploitation by the fundamental research experiment. It widely appliesto domain and so on physics, chemistry, biology, medicine, also obtainedthe widespread application in the geophysics (proton precessionmagnetometer, NMR wave spectrometer, NMR oil well logging meter). Itis the new domain that using the Nuclear Magnetic Resonancetechnology survey underground water. The first NMR underground waterinvestigation instrument developed successfully in the world by formerSoviet Union in 80 years. It is the present only one of geophysics methoddirect surveys underground water. It use the proton magnetic momentwhich display under the earth magnetic field , With the earth magneticfield vertical direction adds a alternating magnetic field in to stimulate theunderground hydrogen proton, the proton magnetic moment to be able todeviate the earth magnetic field direction to form the macroscopicmagnetic moment. After the excitation field stops, the macroscopicmagnetic moment can restore to and produces the weak signal along theearth magnetic field direction, use this signal can be able to survey theunderground water situation. The present paper is studies the overall planand the essential technology of Nuclear Magnetic Resonanceunderground water Investigation prototype.This paper has used based on the powerful resonant principle, thehigh accuracy launching mode. Use DDS to generate the local Larmorfrequency, produces the high efficiency square-wave signal through IGBT,finally realization high efficiency launch by the resonance of thetransmitting coil and the resonant capacity. In order to solve the problemwhich in the nuclear magnetic resonance the transmitter fast shuts off,ingenious has used the high efficiency diode and in this design which twopros and cons connects in parallel fashion and used the high voltagevacuum relay success realization launch fast to shut off and the receivecut and the isolation. When the instrument launches, vacuum relayseparation protection accepting circuit, after launches stops waiting for 40-75ms the discharge time, after treats in the capacity and the coil surplusenergy consumption, closed relay, receiver circuit work. As a result of twodiodes existences, the receive signal also is the nV level signal is smallerthan forward voltage of diode, therefore the transmission circuit and thereceiver circuit very good isolation, has realized the launch and thereceive cut.In the transmitter launch power source used the DC_DC to carry onthe charge for the 0.22F capacity, and then the capacity carried on thelaunch for the launch circuitry power supply. Select this method to beallowed greatly to reduce the DC_DC power. DC_DC power itself hasused the entire bridge soft switch technology, has utilized AmericanUnitrode Corporation UC series chip, reduced the periphery electric circuit.Actuates the electric circuit to use the transformer to make the isolationactuation, but the power switch has used international rectificationcompany's IRFP37N50A, its breakdown voltage reaches 500V, rDS only0.038 ohm. In receiver because surface NMR signal extremely weak, thesignal to noise ratio is very low, this set the very high request to the NMRamplifier. In the this paper used the discrete component to make the inputcircuit, in addition input impedance matching and the latter level choosesthe frequency amplifier, finally reasonable PCB technology and so onfurther reduce the amplifier noise. On the other hand, uses the methodwhich superimposes many times to increase the signal to noise ratio,therefore the amplifier arrived our requirements.Through more than year of open country experiments, tried to find outand the study and unceasingly improves the instrument, finally wesucceeded receive the surface NMR signal, had proven this planfeasibility and arrived our requirements.
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