Quadrature Lock-in Amplifier Research Based On DSP And Its Infrared Gas Sensing Applications

This project is supported by the National Science and Support TechnologyProgram“Infrared carbon monoxide detector (sensor) research and development inmine environment”.Incomplete combustion of coal will product methane and CO gases, It willmake people poisoning and cause fire accident. In recent years, the fire accidenthappened frequently, and it is a serious threat to the safety of people’s lives andproperty.With the high speed development of semiconductor technology, thedistributed feedback semiconductor laser (DFB) based on it has been widelyused, especially in the infrared gas detection based on the Bouguer–Lambert–Beerlaw. Tunable diode laser absorption spectroscopy (TDLAS) has high sensitivity andselectivity, and the second harmonic has related information with concentration, andit needs to be drawn through the lock-in amplifier, so the research of this subjecthas great significance.In the infrared gas detection, this paper designs an orthogonal lock-in amplifierto extract the harmonic signal base on the principle of tunable diode laser absorptionspectroscopy. First of all, simulink the whole experimental system on the Simulinkplatform of MATLAB, it aims to test the function of hardware system.Then, thesystem will be designed in this paper with DSP (digital signal processor) as the corecontroller, it is a quadrature lock-in amplifier.Finally, verify the function of hardwaresystem through the experiment.First, the standard sine signal is used as the signal to be measured. By comparingthe amplitude of the input signal and the amplitude of the output signal, we can getthe linear fitting degree which is up to0.99994and the maximum measurement errorwhich is less than4%; Secondly, using the signal to be measured--differential signaland reference signal which is produced with the use of ARM to simulate the absorpti-on of gas–as the fundamental frequency square wave and frequency-doubled square wave respectively to extract the second harmonic signal. The maximum error of thefirst harmonic is less than3.5%. The second harmonic error is less than5%because itis very vulnerable to all kinds of noise interference. Finally, we change the input con-centration and extract the corresponding second harmonic signal respectively. Thesystem has a broad application prospects because of its high stability and Capability/Price Ratio.The innovations of this article:1, Research and realizate a portable orthogonal lock-in amplifier to extracte theharmonic signals independently. The reference signal does not need to have the samefrequency and phase with the measured signal. Remove the complex phase shift stepand simplify the complexity of the hardware design system.2, This portable instrument can be used in any gas detection based on TDLAStechnology, such as methane, carbon monoxide and water vapor etc.