Weak Photoelectric Signal Detection Research Based On The Digital Lock-in Amplifier

In the photoelectric signal detection system, the surface of photodetector is irradiated by the light, which can produce weak electric current, these electrons flow is very weak and Disturbed by the strong noise signal, the traditional weak current signal detection method can not be used. In order to reduce the impact of strong noise and 1/f noise on the weak photoelectron flow, weak photoelectric signal detection system based on digital phase-locked amplifier is designed in this paper. Firstly, through chopper, the current is modulated into cycle optical signal by optical signal, the cycle optical signal can generate a periodic weak current signal by photodetector, then a series of post-processing are made, which include signal amplification, current-voltage conversion, signal conversion and signal filtering and so o on, and the sampled digital signal is input to the digital signal demodulation system, Finally,getting the information of photoelectric signal amplitude through mathematical signal demodulation.First of all, this thesis analyzes the principle of function autocorrelation and cross-correlation, researches the mathematical model deeply, by comparing the two kinds of mathematical model, the latter has better capability on anti-jamming, so the phase-locked amplifier is designed by using cross-correlation principle. Its periodic signal is demodulated through external reference signal, the information of original signal amplitude and phase under strong noise is obtained, and Matlab software simulation is made. In order to make the weak photoelectric signal input to the digital phase-locked amplifier signal demodulator, it must do amplification and filtering for the weak photoelectric signal, the second order amplifying circuit’s magnification is 1012, the second-order anti aliasing filter’s cut-off frequency is 8 KHz, all components in the signal amplifying circuit and filter circuit parameter are calculated, and simulation verification is made by Multisim simulation software.Then, based on Multisim software simulation results, hardware circuit is designed, the first-stage amplifier circuit realizes IV conversion, the weak photoelectric signal is converted into a voltage signal and amplified 1011 times, which is the most important part of the hardware circuit, and has a direct impact on the entire system detection performance. In this paper, comparing high precision and low noise op-amp chip of the current market, the first-stage and second-stage amplifier circuit respectively selects LMC6062 and LF442 for amplifier design. After amplification, signal can be input into the anti-aliasing filter to filter,to filter out high frequency noise interference, then the signal is input to eight high-speed AD,finally after AD transformation, the signal is input into the FPGA to realize the modulation of signals.Finally, the digital signal demodulation system based on FPGA is designed by using DSP Builder toolkit, including four aspects:(1) Designing of extracting the measured signal frequency, mainly realizing FFT for the measured signal, after FFT, the measured signal has maximum amplitude-frequency response, and the corresponding frequency information is extracted.(2) Designing of the reference signal based on DDS function signal generator,mainly providing the input sine and cosine reference signal of digital signal demodulation, the frequency information is extracted by the measured signal frequency components.(3)Designing of multiplier of digital signal processing, the main function is to achieve phase sensitive detection between measured signal and reference signal, namely for the multiplication in the digital signal processing.(4) Designing of low-pass filter based on FIR,main function is to realize the replacement of digital signal integral. The individual modules are joined together to debug, and the compiled project file is downloaded into the FPGA chip to complete the design of digital signal demodulation.Testing the whole photoelectric system, Firstly, testing the hardware amplifier circuit,the test results show that the first stage I/V conversion circuit can amplify the-2pA current signal into 0.2V voltage signal, and the error range is approximately 1.5%, secondly, testing the mathematical demodulation system based on FPGA, using ATTEN function signal generator to generate the standard input test signals, the test shows that mathematical demodulation system and weak photoelectric signal detection system based on FPGA have stable performance and high detection precision, when the RSNR ≈-26 dB, the minimum detectable precision can reach 0.1pA, finally connecting the weak signal detection amplification circuit and the mathematics demodulation system based on FPGA, testing the effect of photoelectric detection system on the actual photoelectric signal detection results, the test show that the testing amplitude error of photovoltaic system is less than 0.5%, the frequency testing error of the system is less than 0.2%. Therefore, getting the conclusion that the designed optical detection system can detect the actual photoelectric signal detection better, the reference signal frequency has good tracking performance, and the photoelectric signal detection accuracy is improved.