Magnetic Field Locking System Based on Fluxgate and Time Domain Digital Frequency Discrimination

doi:10.11938/cjmr20222990

Abstract

Abstract:

The magnet of permanent magnet magnetic resonance instrument is susceptible to the interference of temperature and other environmental magnetic fields, resulting in fluctuations of the main magnetic field, which affects the repeatability and accuracy of the measurement. In this paper, two locking methods to solve the magnetic field fluctuation are discussed. On the one hand, the transient magnetic field caused by environmental fluctuation is detected by fluxgate sensor with high sensitivity, and then the field programmable gate array is used for real-time processing and calculation of the magnetic field compensation. On the other hand, the time-domain digital frequency discrimination locking method is employed for the slow magnetic field drift caused by changes in ambient temperature. After the locked sample being excited by radio frequency, nuclear magnetic resonance (NMR) signal is converted to a lower frequency range through frequency mixing, converted into a square wave signal, and directly sent to the field programmable gate array for periodic measurement. The magnetic field compensation amount is also obtained by calculation. The magnetic field compensation obtained by the two methods is superimposed, and then converted into current signal to drive B₀ compensation coil mounted on the magnet, thereby a magnetic field locking system is developed to realize the locking of the magnetic field. The test is carried out on a 0.5 T food rapid detection magnetic resonance analyzer. When subjected to transient interference, the magnetic field can be stabilized within the range of ±4 Hz (corresponding to ±0.093 9 μT), and the magnetic field drift caused by temperature can also be accurately measured, which verifies the feasibility of the magnetic field locking method presented in this paper.

Key words: nuclear magnetic resonance (NMR), magnetic field locking, fluxgate sensor, time domain digital frequency discrimination

CLC Number:

O482.53

Xiao-yang ZHANG, Shou-quan YAO, Jun-cheng XU, Yu JIANG. Magnetic Field Locking System Based on Fluxgate and Time Domain Digital Frequency Discrimination[J]. Chinese Journal of Magnetic Resonance, 2022, 39(4): 448-458.

Figures/Tables 7

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测量周期数n	测量时间			测量时间为200 μs的最小频率精度/Hz
测量周期数n	100 μs	200 μs	300 μs	测量时间为200 μs的最小频率精度/Hz
1	10.0 kHz	5.0 kHz	3.333 kHz	0.25
5	50.0 kHz	25.0 kHz	16.667 kHz	1.25
10	100.0 kHz	50.0 kHz	33.333 kHz	2.5

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