Design Of Extremely Weak Photocurrent Measurement System And Frequency Locking Error Correction Method

Photocurrent measurement is a method that measures the weak current from the photoelectric effect.It is widely used in scientific research fields such as electrochemistry,astronomy,and space science,as well as industrial detection such as water quality detection and atmospheric detection.In the field of photocurrent measurement,the weak signal measurement technology is widely used because of the low signal-to-noise ratio and weak of measured signal of photocurrent.As the development of detection devices and the increase of detection requirements,the measured current signal becomes more and more weaker and technology is more and more mature.However,there are several issues to be solved in the applications.First,the general weak signal measurement equipment is expensive and bulky.It is not suitable for industrial environments,mass production and cannot embedded into industrial inspection systems.Second,for extremely weak photocurrent measurement system based on lock-in amplifier,frequency locking error will occur when modulation frequency drifts in some applications.The error will reduce measured accuracy and limit the application range.In response to the above problems,this thesis designs a weak photocurrent measurement system,which is miniaturized,high resolution and can be embedded in various measurement devices.Make the system applicable to industrial or outdoor environments.For frequency locking error problem,this thesis analyzes the principle of lock-in amplifier and proposes a correction method that make the amplifier still accurately measures in the presence of frequency error.The main contributions and innovations of this thesis include:1.For the problem of miniaturization and industrial application of extremely weak photocurrent detection systems.In the design of extremely weak photocurrent detection system,the system is divided into front-end analog part and digital processing part.In the front-end analog part,the photocurrent detection and modulation drive light source are integrated design,and BPF is added to the detection part to improve the signal noise.Than,and use power isolation measures to avoid extra power.In the digital processing part,the MCU is used to implement a locking amplification algorithm to reduce system complexity.In actual tests,the system's detection sensitivity can reach 5FA,fully meeting the needs of general photocurrent detection.2.For the reduction of measured accuracy because of frequency locking error,locking frequency tracking method and LPF flat top optimization method are proposed.The locking frequency tracking method corrects the internal reference frequency of the lock-in amplifier in real time to correct the frequency lock error.This method can effectively improve the measured accuracy.And LPF flat top optimization method is to optimize the LPF passband characteristics inside the lock-in amplifier,which increases the acceptable range of the frequency locking error.3.For most the general commercial lock-in amplifier instruments,the frequency locking error cannot be corrected.An auxiliary frequency tracking system is designed based on the proposed frequency tracking method that help the general instruments correct the lock error.This auxiliary reads the parameters during the measurement process through the RS-232 communication interface and correct the parameters after calculation.The auxiliary system can effectively correct the frequency locking error and increase the accuracy to the maximum of the instrument.