Design Of Digital Lock-in Amplifier Based On FPGA And Its Application In Wavelength Modulation Spectrum

In recent years,the rapid development of science and technology has brought a series of environmental problems,PM2.5,greenhouse gas emissions have a great impact on people’s health,and the detection of dangerous gases in highways,underground mines and other scenes has also received extensive attention from all walks of life,so trace gas detection plays a very important role in various fields.Laser spectroscopy technology is currently a trace gas detection of an advantage technology,which as a laser as a light source,with high sensitivity,fast response,good stability and other advantages,of which wavelength modulation spectroscopy(WMS)technology simple structure,easy to operate,widely used in the field of trace gas detection,and lock-in amplifier is the core device of wavelength modulation spectrum,its performance directly determines the sensitivity of gas detection.This paper mainly designs an FPGA-based digital lock-in amplifier,which is developed using XC7Z020 chip produced by XILINX,using 16-bit high-speed transmission A/D,D/A chip for data acquisition and transmission,the overall size is only14cm*9.5cm*6cm,weight 420 g,highly integrated,miniaturized,a variety of optional measurement parameters can meet the measurement needs in a variety of complex environments.Starting from the technical principle of TDLAS,this paper introduces in detail the basic theory of direct absorption spectroscopy technology and wavelength modulation spectroscopy technology of Beer-Lambert’s law,and analyzes the basic principle of lock-in amplifier to extract weak signals from noise.Secondly,the hardware implementation scheme of this design is introduced,mainly including the design of power module,A/D module and D/A module,as well as the chip selection,notch circuit,op amp circuit,attenuation and protection circuit,differential to single-ended signal circuit,single-ended to differential signal circuit,bias voltage circuit and other design schemes in the detailed design of the module,and shows the physical diagram and functional parameters of the design.In addition,the software implementation scheme of this design is introduced,using verilog HDL language programming and Vivado software as development tools,the design scheme and functions of DDS module,multiplier module,CIC filter and FIR filter in multirate signal processing are respectively shown simulation.Finally,an experimental platform is built,a laser with a central wavelength of 1653.7nm is used to measure methane gas,and the functions of the design are tested,and the second harmonic signal with different modulation amplitude,demodulation phase and filter bandwidth is measured respectively to obtain the optimal parameters,and then different concentrations of methane gas are measured,and the detection limit analysis is carried out,and the detection limit of the design for methane gas in a gas cell of 35 cm is 1.07 ppm.