Lock-in Amplifier Of Second Harmonic In The TDLAS Gas Detection

In recent years, with the rapid development of industry, gas pollution situation isincreasingly serious. It has become one of the pressing environmental problems. Theharmful gas detection has received more and more attention. Tunable Diode LaserAbsorption Spectroscopy (TDLAS) has the advantages of good selectivity, highsensitivity, high resolution and non-contact measurement. It has become one of theeffective methods of online analysis to trace gases rapidly. A lock-in amplifier isneeded to extract the second harmonic signal which is related to gas concentration inthe TDLAS gas detection.This paper designed a lock-in amplifier used to extract the second harmonicsignal in view of the TDLAS gas detection. First of all, it deeply introduced therelated principles of gas detection based on TDLAS technology，including infraredspectrum absorption principle，TDLAS technology principle，harmonic detectionprinciple and phase-sensitive detection principle.Based on the above principles, usingMATLAB mathematical modeling can theoretically extract the first harmonic and thesecond harmonic signal, and prove the relationship between gas concentration and thesecond harmonic signal amplitude. Theory into practice, I analyzed the signalcharacteristics and built circuit design program. The lock-in amplifier circuitsdesigned for TDLAS gas detection system include a differential input circuit, a phaselocked frequency multiplier circuit, a phase-shift circuit, a phase-sensitive detectorcircuit (multiplying circuit), a low-pass filter circuit, a non-inverting amplifier circuit,a peak detector circuit, an A/D conversion circuit and a LCD display circuit.The experiments are divided into simulation experiment and gas experiment.Simulation experiment: We sampled the MATLAB simulation waveform, with theARM7microprocessor analog issued gas absorption waveform as the input signal. Weused the lock-in amplifier to extract first harmonic signal and the second harmonicsignal. Gas experiment: We built a TDLAS gas detection platform and configured different concentrations of methane, for example, using a dual detector to get adifferential signal as an input signal. We observed and extracted different secondharmonic signals when the gas concentrations were0%,10%and20%. Theexperiments verify the program and the function of the circuit, and show thefeasibility and correctness of the second harmonic detection.