Realization of high-precision measurement technology for frequency signal of optically pumped magnetometer
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摘要: 针对井下弱磁观测环境狭小，测量精度要求高，但方便实时上传数据，可实现自动测量等特点，设计并制作了一种适用于井下绝对观测的氦光泵磁力仪单片机的频率计。频率计基于Cortex-M3内核的ARM芯片，通过定时器的外部时钟模式进行定时计数，在中断函数中进行计算，得到信号频率。多次实验后，为进一步提高测量精度，使用32 MHz有源温补晶振为芯片提供主频信号，提高了主频精度，减少程序对CPU的资源占用率。实验结果表明：频率计精度较高，满足项目需求。系统误差稳定，在840.70 kHz—1.96 MHz的弱磁测量范围内，误差均为1 Hz，易结合误差原因通过软件补偿实现高精度测量。Abstract: We designed and made a kind of microcontrollers frequency meter for borehole geomagnetic absolute observation by helium optically pumped magnetometer, which is suitable for the narrow space, high measurement accuracy, an easy access to upload data and automatic measurement. This frequency meter is based on ARM Cortex-M3 microcontroller, using timers’ external clock mode to count and calculate the frequency result in the interrupt service function. After many experiments, in order to further improve the measurement accuracy, the 32 MHz active temperature-compensated crystal oscillator is used to provide the main frequency signal for the chip, which improves the main frequency accuracy and reduces the code’s resource occupation rate of the CPU. The results show that the frequency meter has enough high precision that can meet the project requirements. The system error is stable, and the systemerror is 1 Hz in the range of 840.70 kHz to 1.96 MHz. Considering the specific reason for this error, we can easily compensate it by software codes and then realize a high-precision measurement.
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表 1 二定时器法主要配置参数
Table 1 Main configuration parameters of two timers method
寄存器名称 TIM3 TIM2 TIMx_ARR 65536-1 60000-1 TIMx_PSC 0 1200-1 
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表 2 四定时器法主要配置参数
Table 2 Main configuration parameters of four timers method
寄存器名称 TIM3 TIM4 TIM8 TIM2 TIMx_ARR 65536-1 65536-1 50000-1 20-1 TIMx_PSC 0 0 72-1 0
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表 3 同源实验结果
Table 3 Experiment result using the same signal source
输入信号/kHz 二定时器法 四定时器法 测量值/Hz 绝对误差/Hz 相对误差 测量值/Hz 绝对误差/Hz 相对误差 62.5 62 500 0 0 62 500 0 0 250 250 000 0 0 25 0000 0 0 500 499 999 1.0 2.0×10−6 500 000 1.0 2.0×10−6 1000 999 997 3.0 3.0×10−6 999 999 1.0 1.0×10−6 2000 1 999 993 7.0 3.5×10−6 1 999 999 1.0 5.0×10−7
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表 4 独立有源温补晶振作为信号源时的实验结果
Table 4 Experiment results using independent active TCXO as signal source
输入信号/kHz 实际测量值/Hz 绝对误差/Hz 相对误差 1 000 999 999 1 1.0×10−6 100 000 0 0 2 000 1 999 999 1 5.0×10−7 3 000 2 999 998 1 3.3×10−7 2 999 999 2 6.7×10−7
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表 5 高精度信号发生器作为信号源时的实验结果
Table 5 Experiment results using high-precision signal generator as signal source
输入信号/kHz 四定时器法测量/Hz 绝对误差/Hz 相对误差 输入信号/kHz 四定时器法测量/Hz 绝对误差/Hz 相对误差 500 500 000 0 0 2 200 2 199 999 1.0 4.5×10−7 600 599 999 1.0 1.7×10−6 2 300 2 299 999 1.0 4.3×10−7 700 699 999 1.0 1.4×10−6 2 400 2 399 999 1.0 4.2×10−7 800 799 999 1.0 1.3.×10−6 2 500 2 499 999 1.0 4.0×10−7 900 899 999 1.0 1.1×10−6 2 600 2 599 999 1.0 3.8×10−7 1 000 999 999 1.0 1.0×10−6 2 700 2 699 999 1.0 3.7×10−7 1 500 1 499 999 1.0 6.7×10−7 2 800 2 799 999 1.0 3.6×10−7 2 000 1 999 999 1.0 5.0×10−7 2 900 2 899 999 1.0 3.4×10−7 2 100 2 099 999 1.0 4.7×10−7 3 000 2 999 999 1.0 3.3×10−7
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表 6 微波频率计超高频段测量的实验结果
Table 6 Experiment results of microwave frequency meter in ultra-high frequency measurement
输入频率/Hz 实测频率/Hz 绝对误差/Hz 相对误差 5 000 000 000 5 000 000 373.12 373.12 7.5×10−8 5 100 000 000 5 100 000 326.17 326.17 6.4×10−8 5 270 000 000 5 269 999 878.63 121.37 2.3×10−8 5 520 000 000 5 520 000 061.08 61.08 1.1×10−8 5 603 000 000 5 603 000 047.03 47.03 8.4×10−9
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表 7 CPLD频率计中频测量的实验结果
Table 7 Experiment results of frequency memter of CPLD in intermediate frequency measurement
输入频率/Hz 实测频率/Hz 绝对误差/Hz 相对误差 56 000.000 56 000.00 0 0 104 523.687 104 522.00 1.687 1.6×10−5 504 258.741 504 252.03 6.711 1.3×10−5 774 519.638 774 508.00 11.638 1.5×10−5 1 000 000.000 999 986.00 14.000 1.4×10−5
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表 8 FPGA频率计中低频段测量的实验结果
Table 8 Experimental results of FPGA frequency memter in low frequency range measurement
输入频率/Hz 实测频率/Hz 绝对误差/Hz 相对误差 1 000 999 1.0 1.0×10−3 10 000 10 000 0 0 20 000 20 001 1.0 5.0×10−5 200 000 200 004 4.0 2.0×10−5
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表 9 软件校正后以独立的有源温补晶振作为信号源
Table 9 Independent active TCXO as signal source with software calibration
输入信号/kHz 实际测量值/kHz 绝对误差/Hz 相对误差 1 000 1 000.000 0 0 2 000 2 000.000 0 0 3 000 2 999.999 0 0 3 000.000 1.0 3.3×10−7
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