Overhauser磁力仪探头激发频率测试仪研制

摘要

Overhauser磁力仪探头中自由基溶液激发频率的精度直接决定探头输出拉莫尔旋进信号的质量.为准确获得激发频率,本文设计了一种Overhauser磁力仪探头激发频率测试仪.针对目前测试设备存在的分辨率较低、操作复杂等问题,采用高分辨率DDS作为可控射频信号源;运用ADC同步采集技术,结合归一化数据处理方法有效解决50MHz~100MHz范围内幅频特性曲线不平坦的问题;采用射频功放、小信号放大作为辅助测量通道,实现仪器的标准、辅助测量一体化.通过对探头谐振腔的等效电路进行仿真,得到微调电容对探头谐振频率和品质因数的影响程度,并用商用探头进行测试,进一步验证仿真结果;同时得到自由基溶液的极化度与射频激发功率、激发时间的关系.最终实验结果表明,利用本文研制的Overhauser磁力仪探头激发频率测试仪,能够准确测量自由基溶液的激发频率、频率带宽以及品质因数,还可用于在磁力仪设计过程中对激发功率和激发时间的确定,为进一步设计Overhauser磁力仪奠定了基础.%The excitation frequency of radical solution in Overhauser magnetometer probe directly determines the quality of larmor precession signal,which is generated by the probe.In order to obtain the excitation frequency accurately,a test apparatus for measuring the excitation frequency of Overhauser magnetometer probe was designed.In view of the existing problems of present test apparatuses,such as lower resolution,complex operation and so on.This paper adopts a high-resolution DDS as the controllable RF signal source;using the ADC synchronization acquisition technology,combined with the data processing method of normalization,and the uneven problem of the amplitude-frequency characteristic curve in the range of 50MHz~100MHz has been solved effectively;moreover,by adopting the RF power amplifier,weak signal amplification as an auxiliary measurement channel,the apparatus''s integration of standard measurement and aided measurement has been achieved.The equivalent circuit of the probe''s resonant cavity is simulated,and the effect degree of the adjustable capacitor to the resonant frequency and quality factor of the Overhauser magnetometer probe is obtained.The simulated results are further verified by testing the curve of cavity resonance of the commercial probe under different parameters'' conditions.Meanwhile,the relationship between the polarization degree of the radical solution and RF excitation power & time are also obtained.The test method and results are given and the experimental data are analyzed.Finally,the experiment results demonstrate that the excitation frequency of the radical solution,excitation frequency bandwidth and quality factor can be found accurately by using this apparatus.And the apparatus can be also used for the determination of excitation power and excitation time in the process of magnetometer design,which lay the foundation for the design of Overhauser magnetometer in the future.