-
图7所示相敏检波电路波形图,图中(a)为ICL8038波形发生器输出的正弦调制信号用于驱动振动电容,方波为相敏检波参考信号。将激励信号经功率放大后驱动振动电容器,弱电流调制成交流电压信号,经过放大得到如图7(b)和(c)所示信号,其中(b)为相移0°相敏检波图,(c)为相移120°相敏检波图;(a)和(b)中通道1信号是放大后信号,通道2信号是相敏检波信号,通道3信号是检波后通过低通滤波器的信号;可以看出,在整个周期内相敏检波是对输入信号做了整流,此信号通过低通滤波器可以得到直流电压信号。通过对比(b)和(c)图,当相移120°时,低通滤波后信号的幅度显著减小而且还伴随着明显的噪声,这说明在不合理的相移情况下,不仅会使待测信号严重丢失,而且伴随着噪声的产生。
为了测试微弱电流信号与经过系统处理后电压信号的线性关系,测试采用Keithley 6430 系列超灵敏电流源,分别测试了输入电流信号从0.01到10 pA所对应的电压信号,如图8所示:
实验结果表明,输入弱电流与输出电压之间具有良好线性关系,说明该系统能够有效放大微弱电流信号,电流测量灵敏度达到了10–14 A,量程范围为10–11~10–14 A。引入直流负反馈后,测量弱电流达到稳定时的时间响应如表1所列,从没有引入负反馈的100多秒减小为15秒左右,系统放大倍数提高后,还可以进一步缩短测量响应时间。
电流范围/
pA上升时间(达到峰值
90%的时间)/s电流范围/
pA下降时间(降到峰值
10%的时间)/s0~1 15.45 1~0 15.28 0~5 14.47 5~0 15.36 0~10 14.83 10~0 15.50 为了进一步验证该系统在实际应用中的可靠性,用积分电离室探测氚靶放射出的β射线。考虑到积分电离室输出电流和工作电压之间有饱和特性,实验测量了积分电离室的工作电压增加与系统电路的输出电压之间关系。采用标准电流源标定,将系统电路的输出电压换算成积分电离室的输出电流,可得到积分电离室的输出电流和工作电压之间的饱和曲线图如图9所示。
从图9可知:当工作电压为1 100 V左右时出现了坪区,即饱和区;由于电离室工作在饱和区,所以积分电离室工作时可以选择在1 100 V以后的饱和区。
Development of a Weak Current Amplifier Based on AC Modulation Technology
doi: 10.11804/NuclPhysRev.38.2020049
- Received Date: 2020-07-16
- Rev Recd Date: 2020-09-20
- Publish Date: 2021-03-20
-
Key words:
- AC modulation technique /
- phase-sensitive detection /
- weak current
Abstract: In the measurement of ray intensity, the current signal from the integral ionization chamber is in the range of 10-10~10-15 A, which needs to be converted and amplified before it can be collected. A weak current signal amplification system is designed for the integral ionization chamber in this paper, using AC modulation technology to modulate the weak DC current signal to be measured into AC voltage signal, and then obtains the DC voltage signal after amplification, phase-sensitive detection, filtering and other circuit processing, and finally the data is collected. At the same time, the DC negative feedback is introduced into the system to shorten the response time of current measurement. The linear and response characteristics of the system were tested by the standard current source, and the β ray of tritium target was detected by an integral ionization chamber. The test results show that the system can effectively amplify the weak current signal in the range of 10-14~10-11 A with good linearity and response time characteristics, and the current measurement sensitivity is 10-14 A, the system has a good prospect of engineering application.
Citation: | Pengpeng ZHANG, Lei YANG, Caifeng LAI, Shuyi SUN, Bin LU, Dongcang LI. Development of a Weak Current Amplifier Based on AC Modulation Technology[J]. Nuclear Physics Review, 2021, 38(1): 61-65. doi: 10.11804/NuclPhysRev.38.2020049 |