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摘要:
本文基于可编程逻辑门阵列(field programmable gate array，FPGA)快速处理数字信号及多线程优势，结合波长调制可调谐半导体激光吸收光谱(WM-TDLAS)技术，研制可编程门阵列WM-TDLAS二氧化碳浓度检测系统。根据应用功能需求，利用FPGA芯片的逻辑门阵列可编程特性，设计了具备信号采集及调制、谐波解调功能的数字化锁相放大器(digital lock-in amplifier，DLIA)。为验证其性能进行谐波提取测试、
Q 值、抗噪声实验，结果表明目标频率提取线性度达99.99%，Q 值可达45。在对不同信噪比信号进行谐波提取实验中，当信噪比为43 dB时，均值最大相对误差仅为0.91%。采用中心波长1572 nm分布式反馈激光器作为光源，覆盖选定的6360 cm−1处吸收线，密集多通气体吸收池有效光程14 m，开展了气体浓度检测实验。系统检测浓度与二次谐波幅值拟合线性度为99.982%，通过提升扫描波长频率，系统可在0.1 s获取浓度值。艾伦(Allan)方差结果表明，当积分时间为44 s时，系统的检测下限为1.86 ppm。实验结果表明，该可编程门阵列WM-TDLAS检测系统具有检测精度高、响应速度快、稳定性强和小型化的特点，可用于实际应用中浓度实时监测。Abstract:An FPGA-based field programmable gate array (FPGA) and wavelength-modulated tunable diode laser absorption spectroscopy (WM-TDLAS) technique have been combined to develop a programmable gate array WM-TDLAS CO2 concentration detection system. Leveraging the programmable nature of FPGA chips, a digital lock-in amplifier (DLIA) with signal acquisition and modulation, as well as harmonic demodulation functions, was designed to meet the application requirements. To validate its performance, harmonic extraction tests,
Q -factor assessments, and anti-noise experiments were conducted. The results revealed a linearity of 99.99% for the target frequency extraction and aQ -factor of up to 45. In the harmonic extraction experiments for signals with different signal-to-noise ratios, the maximum relative error in the mean value was only 0.91% when the signal-to-noise ratio was 43 dB. Using a distributed feedback laser with a center wavelength of 1572 nm as the light source, covering the absorption line at 6360 cm−1, and utilizing an effective optical path of 14 m in a dense multi-pass gas absorption cell, gas concentration detection experiments were carried out. The system demonstrated a fitting linearity of 99.982% between the detected concentration and the amplitude of the second harmonic. By increasing the scanning wavelength frequency, the system was capable of obtaining concentration values in 0.1 seconds. The Allan variance results showed that the detection limit of the system was 1.86 ppm when the integration time was 44 seconds. The experimental results indicate that the developed WM-TDLAS detection system based on an FPGA array features high detection accuracy, rapid response, strong stability, and miniaturization, making it suitable for real-time concentration monitoring in practical applications. -
Overview: In view of the above situation, this paper designs a digital lock-in amplifier (DLIA) with high-frequency, long-bit-wide signal acquisition, signal modulation, and harmonic demodulation functions based on FPGA, and applies it to the carbon dioxide (CO2) concentration detection system of WM-TDLAS technology. For the concentration detection system, the hardware parameters of the lock-in amplifier and the key data processing algorithms were optimized. 1) The high-low-pass cooperation scheme is used to filter out the low-frequency clutter signal in the input data to improve the signal-to-noise ratio of the system. To reduce resource consumption and optimize system performance, a Cascaded Integeator-Comb Filter (CIC) filter is designed to perform downsampling of signals with a high sampling rate and reduce the required order of low-pass filter to achieve a lower cut-off frequency. 2) Direct Digital Frequency Synthesis (DDS) technology based on external input clocks is introduced to generate high-frequency synchronous clock reference signals, which can reduce the distortion and harmonic peak jitter caused by clock offset jitter in non-homologous digital systems. 3) The FPGA generates the laser scan signal required for wavelength modulation and the high-frequency clock required for the analog signal acquisition circuit, simplifying the peripheral circuit. This paper designs a digital quadrature lock-in amplifier based on programmable logic gate array (FPGA) programmable features. Harmonic signal demodulation is realized, and the frequency of the high-frequency laser modulation signal can be tuned. The quadrature lock-in amplifier can effectively extract the weak signal in the background noise, through the different signal-to-noise ratio of the signal to be measured under the harmonic extraction and Q value experiments. The signal-to-noise ratio of the signal under test is 43 dB with a maximum error of only 0.91%, and the Q value is 45, indicating that the lock-in amplifier has good frequency response and noise immunity. To test the performance of the designed digital quadrature phase locker in the WM-TDLAS detection, build based on the WM-TDLAS carbon dioxide experimental system to carry out the concentration detection, stability, and response time test, the amplitude of the harmonic signal extracted by the lock-in amplifier and the CO2 concentration has a good linear relationship (R2 = 0.99982), the system acquires the concentration value of the time of 0.1 s, Allen's square indicates that the detection limit is 1.86 ppm. The experimental results show that the WM-TDLAS detection system based on FPGA digital lock-in amplifier has the advantages of digital signal modulatability, high detection sensitivity, and strong noise immunity, and can be used for real-time monitoring of concentration in practical applications.
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图 2 可编程门阵列WM-TDLAS二氧化碳浓度检测系统结构
Figure 2. Structure of the FPGA-based WM-TDLAS CO2 concentration detection system
图 4 检测系统线性度实验。(a)正弦信号振幅提取结果；(b)提取振幅与实际值的线性拟合
Figure 4. Detection system linearity experiment. (a) Amplitude extraction result of sinusoidal signal； (b) Linear fitting of extracted amplitude with actual value
图 5 抗噪声及Q值实验。(a)不同信噪比谐波提取结果；(b) DLIA频幅响应
Figure 5. Anti-noise and Q-value experiment. (a) Harmonic extraction results with different signal-to-noise ratio； (b) Frequency amplitude response of DLIA
图 7 二氧化碳浓度检测实验结果。(a)二次谐波波形图；(b)二氧化碳浓度反演曲线
Figure 7. Experimental results of carbon dioxide concentration detection. (a) Second harmonic waveforms; (b) Carbon dioxide concentration inversion curve
图 9 系统稳定性实验。(a) 二氧化碳浓度波动；(b) Allan方差
Figure 9. System stability experiment. (a) Carbon dioxide concentration fluctuates; (b) Allan variance
表 1 锁相放大器体积对比 (L：长；W：宽；H：高)
Table 1. Volume comparison of lock-in amplifier (L: Length; W: Width; H: Height)
Type L×W×H/cm Volume/cm3 OE1022 44.8×47.5×13.3 28302 MFIA 28.3×23.2×10.2 6696 LIA-BVD 18×10×1.5 270 DLIA 12.6×8.5×1.5 160 
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