Phase-sensitive Detection For Quasi-continuous Signal And Its Application To Spectral Analysis

Quasi-continuous signal is an important modulation form, which has wide applicationin many areas. Quasi-continuous modulated spectrum measurement technology is fit to beapplied in gas monitoring for the industrial process due to its advantageous features ofspeediness, accuracy and stability. However, there is no phase-sensitive detection methodfor quasi-continuous signal and the application of traditional lock-in amplifier can result inerrors in the results of quasi-continuous modulated spectrum measurement. Hence, theprincipal line of scientific research in this paper lies in the phase-sensitive detection methodfor quasi-continuous signal and the application of it in spectral analysis with specificcontents as follows:Firstly, the rules of invalid signal filtering and interference signal correcting ofquasi-continuous signals are established. According to this method, the software realizedlock-in amplifier is designed for quasi-continuous signal phase-sensitive detection. Thepaper used this phase lock to emulate spectrum measurement process so as to verify thefeasibility and effectiveness of software phase lock. The formula for the relationship ofscanning frequency, modulation frequency and phase lock time constant in the wavelengthmodulation spectral measurement was achieved through several TDLAS experiments,which has laid theoretical foundation for the increase in TDLAS measuring efficiency andits practical popularization.Secondly, the paper has established quasi-continuous modulated spectrummeasurement system and analyzed the performance of software phase lock amplifier andcompared the results in quasi-continuous signal demodulation of software phase lock andtraditional lock-in amplifier (SR844).The results showed that software phase lock can solveproblems such as wave form distortion, losing lock and so on, and in particular, it canimprove the signal to noise ratio. Allan variance analysis showed that compared withtraditional digital phase lock, software phase lock has a lower detection limit. The paperhas also established wavelength modulation measuring system with this software phaselock and compared the continuous and quasi-continuous spectral absorption measurementresults under different concentration gradient and different modulation frequency. Theexperimental results showed that the results of quasi-continuous modulation laserabsorption spectrum measurement are superior to the software demodulation measuring results of continuous modulation spectrum in response coefficient, signal to noise ratio,limit of detection and other indexes. It has laid significant theoretical foundation forfollow-up application of this technology in the aerometry of practical industrialenvironment.Finally, the paper applied this software phase lock in practical industrial environmentand optimized the measurement technology of CO, one of the major by-products in cokingindustry. The paper has completed CO measurement experiments under differentconcentration gradient, pressure and temperature respectively and achieved the formula forthe relationship between second harmonic signal and CO concentration. These correctionschemes can enable the system to automatically have corresponding temperaturecompensation and pressure compensation for second harmonic signal according to thetemperature and pressure value of real-time measurement during its field application.