Design Of Removal Of Power-line Harmomics Basing On Substraction In MRS Measuring

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.As the signal is so weak(10nV-4000nV) and the place we carried out our exploration is so near to the power lines, the method is easily disturbed by the electromagnetic noise.It's necessary to get a method that how to filter out power-line harmonics from proton magnetic resonance measurements ,which will influence the application scope of MRS measurements. In order to improve performance, two existing filtering techniques were adapted to processing data of MRS:Block Subtraction and Sinusoid Subtraction.In our resaerch,we will realize Sinusoid Subtraction through hardware.The design is given out based on the Sinusoid Subtraction,then focusing on the theory of phase-tracking method and theory of frequency measurement accuracy.We confirm the feasibility of Sinusoid Subtraction through formula calculation and circuit simulation,which also provides reference of the program.Then basing on power-line harmonics characteristics, as well as JLMRS-I ,we design the overall program to eliminate power-line harmonics. The program is funded by the arbitrary signal generator, a high precision frequency measurement, high-precision amplitude measurement circuit, phase tracking circuit, and so on.Arbitrary signal generator is used to generate sinusoidal signal,which have the same parameters with power-line harmonics. It includes AD9852, Constant-temperature Crystal Oscillator, low-pass filter. Through communication with the host computer, we can made sinusoidal signal in the range of the Larmor frequency(800-2800Hz), which can be fine-tuning of 0.1Hz. Phase can change in the scope of 0-360 degrees, which can be fine-tuning of 1 degree. Amplitude can be controlled in the range of V-1mV,which can be fine-tuning of 1mV. Various parameters can be manipulated easily and completed quickly.High precision frequency measurement circuit is used to measure the frequency of power-line harmonics. It is composed of CPLD and conditioning circuit. Conditioning circuit can convert a sinusoidal signal into a square wave signal with same frequency, and the CPLD can measure the frequency. Along with the increasing of frequency of reference clock, the accuracy of measurement is increasing. Using Constant-temperature Crystal Oscillator of 8MHz ,measurement accuracy can meet our requirements of 0.1Hz.High-precision amplitude measurement circuit is used to measure the amplitude of power-line harmonics.It consists of rapid detection circuit and high-speed acquisition circuit.Rapid detection circuit is used to convert power-line harmonics to envelope ,through which we can measure the magnitude of the signal. It consists of high-speed comparators, reversed-phase amplifiers, as well as high-speed analog switches. It can quickly realize the convertion that change full-wave signals into envelope.High-speed acquisition circuit is used to measure the amplifiers of power-line harmonics, and providing the basis for the phase track. It is composed of ADS7815 and IDT7208. Acquisition circuit can achieve 16-bit, sampling frequency programmable from 10KHz to 250KHz, sampling points programmable from 1K to 8K.Phase tracking circuit is used to measure the phase of power-line harmonics.It consists of subtraction circuit and high-speed acquisition circuit.Subtraction circuit is used to realize two subtraction signals synthesis. Two OP37 are used to structure subtraction and follower, through which two-way signals will be superimposed. Acquisition circuit is same with circuit in high-precision amplitude measurement circuit.Then we debug various parts and make the overall system test. Preliminarily we certificate the method feasibility, then make some suggestions for the next step of work.