| In recent years, gas pollution has become one of the factors that threaten the health of people. The importance of strengthening the gas monitoring efforts is quite obvious.Tunable Diode Laser Absorption Spectroscopy(TDLAS) has been widely used in greenhouse gas monitoring because of its high resolution and high sensitivity, in which the second harmonic detection technology in gas monitoring has achieved rapid high-sensitivity measurements. A lock-in amplifier is needed to extract the second harmonic signal which is related with faint gas concentration information in the TDLAS gas detection.In this paper, we designed a lock-in amplifier used to extract the weak second harmonic signal in view of the TDLAS gas detection. Firstly, it introduced the related principles of gas detection based on TDLAS technology, including infrared spectrum absorption principle,TDLAS technology principle,harmonic detection principle and digital lock-in amplifier principle. Based on the above principles, using System Generator modeling the various components of digital lock-in amplifier, Including the DDS which is used to generate a reference quadrature signal, multiplier and a low pass filter. Various modules are verified, and to build a dual-channel digital lock-in amplifier model for simulation and verification.Secondly, based on the FPGA using VHDL language to design digital lock-in amplifier components, including the DDS module, the multiplier, the distributed low-pass filter, the root module. Simulation is conducted for each module, and using schematic diagram design method and implements a two-channel digital lock-in amplifier in ISE.Finally, according to the requirements of the system selected Spartan6 series XC6SLX45 T type chip as the main chip of our system, and according to the demand of the lock-in amplifier circuit board is designed.The code of a dual-channel digital lock-in amplifier is downloaded to the design of the circuit board. using specific signals for test, the test results show that the digital lock-in amplifier is able to extract the amplitude of the signal under test and has a good stability. |
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