Digital demodulation method based on multichannel combined acquisition for near-infrared spectrometer

Abstract

Abstract With the evolution of digital technology, the digital demodulation method has become widely employed in near-infrared (NIR) spectrometers due to its advantages of reducing the volume, enhancing the reliability, and increasing the flexibility of instruments. However, digital demodulation has a lower signal-to-noise ratio (SNR), which can be improved by lowering the measurement precision, speed, and stability. To improve the SNR without sacrificing the measurement speed or stability, an improved digital demodulation method was developed based on multichannel combined acquisition. To validate the proposed method, a multichannel combined acquisition digital lock-in amplifier demodulation system was designed for portable raster scanning NIR spectrometers. In this system, six ADC channels were used in conjunction to acquire the modulated sampling signal. Afterwards, comparative experiments were conducted to compare the conventional and novel methods. The experimental results revealed that the SNR increased from 2499 to 3007 with a measurement time of 16.75 s and 17.08 s, respectively. Furthermore, scanning the reference reflector resulted in a baseline drift of 0.064% and 0.060% with a scanning time of 128.27 s and 128.21 s. Scanning the kaolin sample resulted in a baseline drift of 0.27% and 0.23% with a scanning time of 128.44 s and 128.53 s. In brief, the multichannel combined acquisition method can improve the SNR of spectrometers without compromising their stability or measurement time.