| Recently,with the rapid development of industry and technology in China,people’s living standards have improved significantly.However,technological development is also a double-edged sword.As time goes by,more and more leaks of polluted gases remind people to solve the problem of air pollution urgently.Therefore,it has become extremely important to carry out related research on detecting polluting gases accurately.Among the many gas detection methods,tunable diode laser absorption spectroscopy(TDLAS)has the advantages such as fast response,high sensitivity,and high precision.However,when using TDLAS to detect gas concentration,to reduce the 1 / f noise,it is necessary to combine TDLAS with wavelength modulation spectroscopy(WMS).In WMS,a function generator is used to generate scanning signals and modulation signals,and the synchronous signal of the modulation signal is transmitted to lock-in amplifier.The lock-in amplifier is then used to extract the second harmonic signal related to the target gas concentration,which can achieve a higher accurate measurement.In traditional TDLAS gas sensing systems,the function generator and lock-in amplifier are independent units,which not only increases the size of the system,the power consumption,and cost,but also the communication of data between devices may introduce additional noise and reduce the sensitivity.This thesis describes the development of a miniature digital lock-in amplifier designed to detect target gas concentration using TDLAS,based on the characteristics and application requirements of TDLAS.The miniature digital lock-in amplifier can achieve the relevant function of a dual-channel function generator and a high-precision lock-in amplifier.It can provide a sawtooth wave scanning signal and a sine wave modulation signal to the laser and can extract the second harmonic signal based on the sine wave modulation signal to derive the target gas concentration.Firstly,the principle of lock-in amplifier and the basic principle of using TDLAS combined with WMS to detect gas concentration are introduced.Based on the above principles,the hardware circuit of the miniature digital lock-in amplifier with MSP430F1611 as the control core is designed,including the main controller circuit,function generation circuit for generating a sawtooth wave scanning signal and a sine wave modulation signal,low noise pre-amplifier circuit,data acquisition circuit and data transmission circuit.Secondly,a software platform and a data display host computer are designed.The software platform controls the reference clock source,phase-sensitive detection,data acquisition and transmission through program algorithms,while the host computer completes the display and storage of the demodulated data.Meanwhile,the miniature digital lock-in amplifier verifies the correctness of the signal demodulation using phase-sensitive detection by demodulating its own output sawtooth wave scanning signal and sine wave modulation signal.Finally,a methane detection platform based on the miniature digital lock-in amplifier is built to verify the performance.Using the miniature digital lock-in amplifier to achieve the wavelength scanning of a tunable diode laser source with a central wavelength of 1653.7 nm and extracts the second harmonic signal from the detected signal by a photodetector.The target gas concentration can be derived from the second harmonic signal.In order to enhance the sensitivity of this device,a multi-pass cell with an effective optical length of4.2 m is used and placed between the light source and the photodetector.In the experiment,methane with concentrations of 2.5 ppm,5 ppm,10 ppm,15 ppm,20 ppm,25 ppm,and 30 ppm are filled into the multi-pass cell,respectively.In order to validate the feasibility and accuracy of the miniature digital lock-in amplifier,the spectrums of methane with different concentrations are obtained by the home-made miniature digital lock-in amplifier and a commercial lock-in amplifier(Stanford,SR830),respectively.These results verify the consistency of the homemade and the commercial lock-in amplifiers.Compared to traditional lock-in amplifiers,this miniature digital lock-in amplifier has a smaller size,lower power consumption,and is more conducive to system integration.It will greatly promote the application of gas sensing based on TDLAS in various fields. |
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