Design Of MRS Signal Collector Based On Orthogonal Vector Lock-in Amplifier

Water is one of the most valuable natural resources for the survival of mankind. To solve the water crisis, high-tech investigation for groundwater has become an urgent task. It is the geophysical method for direct groundwater detection, which has been developed for two decades. It makes hydrogen proton in the groundwater form macroscopic magnetic moment by artificial electromagnetic field of Lamor frequency. The macroscopic magnetic moment spins in the Geomagnetic field and its frequency is special for hydrogen proton. We can know whether the groundwater exists by picking up electromagnetic signal whose frequency is equal to local Lamor frequency generated by macroscopic magnetic moment spinning using coil.Underground nuclear magnetic resonance signal contains information on key parameters of underground water. We can invert underground water information, determine the underground water and ground structure for engineering guidance by extracting the initial signal amplitude, relaxation time, the initial phase of MRS signal. MRS signal is weak and vulnerable to all kinds of noise interference when using 100 m Tx / Rx coil, MRS signal is about dozens of nV ~ 3000nV. In this paper, MRS signal acquisition module is the key for weak signal detection and extracting weak MRS signal and its performance has a direct impact on the performance of the entire instrument.The paper extracted key parameters of MRS signal by envelope acquisition according to the characteristics of MRS signal. As general diode envelope detector circuit can not extract the initial phase and improve SNR, the paper acquired the envelope of MRS signal by orthogonal vector amplification method which extracted key parameters of MRS signal and improved SNR according to single frequency characteristic of MRS signal.Orthogonal vector amplification method is implemented by lock-in amplifier. Phase-sensitive detector of lock-in amplifier detects input signal x(t) taking reference signal r(t) as the standard. Make sure the frequency of reference signal is equal to the frequency of input signal and move the spectrum of x(t) fromω=ω0 toω= 0. After LPF, the output is not only related to the amplitude of input signal, but also to the phase difference of input signal and reference signal. As the band can be very small, large SNR can be achieved by using lock-in amplifier. Orthogonal vector lock-in amplifier has in phase channel and out-phase channel, so detects input signal by two reference signal of 90°phase difference. Then output in phase and out of phase and do the vector analysis for input signal.Store two by two sinusoidal reference signal of 90°phase difference in CPLD and put them into D/A converter in specific frequency under the control of MCU. The MCU controls the frequency of reference signal to make sure reference signal and MRS signal are of the same frequency. The amplified MRS signal transferred into the reference voltage input of D/A converter. Thus, we get product of MRS signal and reference signal from the output of D/A converter, phase-sensitive detecting MRS signal. MCU controlled acquisition circuit samples in phase and out of phase and stores the data in the memory. LPF consists of switched-capacitor filter MAX260 and its cut-off frequency can be adjusted by PC according to different detection condition. Acquisition circuit consists of AD7656, which has 6 synchronous acquisition channel and meets the need of acquiring in phase and out of phase synchronously.PC software can control the timing of acquisition module and adjust the sampling frequency from 250Hz to 1000Hz according to different Lamor frequency in different place. Transmits the data in the memory through 485 communication circuit, displays the data in waveform after data processing and extracts the key parameters of MRS signal.After testing the designed MRS signal acquisition module indoor, the module had been verified by MRS signal simulating source. Plenty of field tests were done combined with NMR system which acquired effective MRS signal. The module had been proved its stability and practicability.