基于可调谐激光光谱吸收法的红外气体检测, 为了从差分信号中有效提取出一次和二次谐波信号, 研发了一种谐波信号正交锁相放大器.采用正交锁相放大及谐波检测原理, 利用Simulink软件平台构建了模拟实验系统, 对其功能做了仿真和验证.采用数字信号处理器作为核心控制器, 设计并制作出了谐波信号锁相放大器的系统实物, 它主要由90°移相器、两路模拟乘法器、两路低通滤波器、差分信号放大器、模数转换器等构成.实验中, 首先利用幅度可调的标准正弦信号作为待测信号, 对锁相放大器的输出信号幅度与标准正弦信号的幅度做实验测量与对比, 二者线性拟合优度高达0.999 94, 最大误差小于4%, 具有良好线性度; 其次, 将模拟吸收产生的差分信号作为待测信号, 利用基频方波和二倍频方波分别作为参考信号, 提取谐波信号, 因二次谐波信号微弱易受噪音干扰, 其误差在5%以内, 一次谐波最大误差小于3.5%.系统具有良好的稳定性和性价比, 在红外气体检测中具有较好的应用前景.

In order to efficiently extract the first and second harmonic signals from the differential signal in infrared gas detection based on tunable laser absorption spectroscopy, an orthogonal phase lock-in amplifier was developed. According to the principle of orthogonal lock-in amplifier and harmonic detection, the simulative experiment system based on Simulink platform was set up, and its functions were simulated and verified. The hardware system is designed and developed by adopting a digital signal processor as its core controller, which is mainly composed of a 90° phase shifter, two analog multiplier, two low pass filter, a differential signal amplifier and an AD converter. In experiment, firstly, by using standard sine-wave signals with tunable amplitudes as the signals to be measured, the amplitudes of the output signals from the fabricated lock-in amplifier and the standard amplitudes are both tested and compared. Good linear relationship is found between them, the linear fitting degree is as high as 0.999 94, and the maximum error is less than 4%. Secondly, using the simulated differential signal with gas absorption as the signal to be measured, and by using fundamental square wave signal and frequency-doubled square wave signals as reference signals, both first and second harmonic signal are extracted. Because the second harmonic signal is too weak and easy to be polluted by noises, its detection error is less than 5%, and that of the first harmonic signal is less than 3.5%. The proposed system has good applicative prospects in infrared gas detection due to good stability and high price ratio.