The Detection Circuit Design Of OVERHAUSER Magnetometer

OVERHAUSER magnetometer is developed on the base of component protonprecession magnetometer,the probe is made with the method of dynamic nuclearpolarization, which is an electronic-nucleus double resonance technology. If asubstance contains electron spin and nuclear spin two systems at the same time, whenat the high frequency field in which the frequency is equal to the free electronparamagnetic resonance frequency, there will occur energy level transition of freeelectron.Through the interaction between electronic and nuclear spin system, changesthe related nuclear spin level distribution, and greatly increases the particle numberdifference Δn during nuclear spin levels, and also significantly increases the nuclearpolarization. If nuclear magnetic resonance signal has been observed at this time,signal strength will increase hundreds even thousands times when compares withstatic polarization mode, OVERHAUSER magnetometer is developed based on thiseffect. In addition，it also have advantages of large range of magnetic survey,continuous measurement and low power consumption, etc.This paper firstly introduces the research background and significance, aiming atthe research status at home and abroad of the topic, illustrates the meaning of thistopic research. Expounds the related theory of dynamic nuclear polarization, throughthe formula, clarifies the mechanism OVERHAUSER effect how to enhance nuclearmagnetic resonance’s signal strength. According to the working principle andcharacteristics of the magnetic signal which is under test，puts forward to theinstrument system design scheme. OVERHAUSER magnetometer’s realization circuitof signal detection system is given, mainly includes the high frequency excitationdevice, DC polarization circuit, preamplifier, wide-band amplifier, narrowband filter amplifier, frequency circuit, program-controlled amplifier circuit, peak detector,shaping circuit, A/D sampling circuit, FPGA and main controller,and expounds thedesign and implementation of each component in detail. Compile OVERHAUSERmagnetometer’s upper computer control software, sends command control to lowercomputer’s master controller to make the circuit module harmonious working formagnetic field measurement.When under the high frequency signal excitation of the frequency60.7MHz，thefree radicals solution in the probe occur OVERHAUSER effect, the host controller letDC polarization circuit input DC pulse timely to the probe, the probe will output thelarmor signal. When connect the measurement switch, signal go into the detectioncircuit, after frequency selective amplification, begin to acquire signal by A/Dconverter, and do spectrum analysis in FPGA, the host controller set narrow-bandfilter center frequency and the initial order capacitance value according to thefrequency value of spectrum analysis, at this point, A/D stop working. After signalgoing through a narrow-band filter and a program-controlled amplifier, transformsdamped sine wave to square wave by shaping circuit. Accomplish frequencymeasurement for square wave signal by frequency measurement module within theFPGA, then sent data to the host controller. When there is mutations in the signalfrequency result in magnetic field anomalies, narrow-band filter will filter magneticfield signal because of center frequency hysteresis, at this time do spectrum analysisagain by A/D, to update the filter center frequency, harmonic distribution capacitancevalue is also automatically updated. Program-controlled amplifier set reasonablemagnification according to the signal amplitude, in order to meet the needs of theshaping circuit. Upper computer correspond with the host controller through a serialport, to control the process of signal collection and measurement.Indoor function test shows that the circuit function module works normal, theeffect is good. Indoor frequency measurement precision satisfies the requirement ofthis kind of magnetometer magnetic’s survey precision. Under the condition of thisarticle，pick up outdoor larmor signal with high SNR. Complete the outdoor field test,the test system can basically consistent with GEM company’s OVERHAUSER magnetometer in the test results of geomagnetic field’s changing scope and trends.Anomalous field test shows that there is a good response when measure mutation ofthe anomalous field. How does the DC polarization influence the sensor output signalis studied, provide a basis for the sampling rate increase.