Research And Implementation Of Digital Lock-in Amplifier

Weak signal detection has been focused on issues of measurement, and also cutting-edge issues in today's science and technology. The primary task of weak signal detection is to improve the signal to noise ratio, which requires the use of electronics, information theory, computer and physical methods. Based on the theory of correlation detection principle, digital lock-in amplifier (DLIA) is used to detect weak periodic signal. Because of its fast response, high accuracy and wide measurement range and low cost, it's widely used on electrical, optical, thermal and biological aspects.The purpose of this research is to develop a DLIA with wide frequency range,high accuracy,stable performance.First, this paper summarized the basic method of weak signal detection, and analyzed the weak signal detection theory.Then the principle and composition of LIA is introduced in details.Finally, the trends of DLIA is analysis based on the development at domestic and abroad.According to the requirements and principles, A novel orthogonal DLIA is proposed based on digital signal processor (DSP).In order to ensure real-time and stability,DLIA design a Double-Buffer Structure with variable sampling rate.Combined with the structure, an efficient demodulation algrithm is proposed.Experiment shows the system maximum sampling rate is up to 640 KHz without losing data.The third chapter details the hardware architecture and its basic working principles, then simulates and analysis the key part of the circuit.In Chapter IV, the demodulation algorithm is introduced.It consists of two key parts: the generaion of orthogonal reference sequence and narrowband low-pass filter. A high efficient narrowband LPF is proposed on the theory of decimation. At the sampling rate 500 KHz, the cutoff frequency is as low as 0.45Hz.The attenuation of the LPF is up to-65dB at 1.2Hz.Finally,the test platform is set up to test DLIA in the fallowing aspects:low-pass filter characteristics, noise immunity, linearity and stability of the system. By analysis the test results, some improvements are proposed.The test result shows:when the amplitude of white noses is 1000 times higher than input signal, the system error is less than 0.5%.The DLIA has a good linearity When the test signal is in range of 1mV to 6mV and the system relative error is less than 0.4%.When test signal is from luV to 40uV, the relative error is less than 2%.