Near-infrared spectroscopy using digital phase-sensitive detection

ABSTRACT

Scanning near-infrared (NIR) spectrometers commonly employ analog lock-in amplifiers for demodulation that require complex circuitry and are sensitive to electromagnetic interference. Here, a new sampling system based on digital phase-sensitive detection is reported that used a quadrature sampling algorithm to establish the sampling frequency as an integral multiple of four times the optical signal frequency. A complex programmable logic device was used to control the analog-to-digital converter. A digital signal processor was used to receive data, complete the quadrature calculations, and transfer the results to a computer. Consequently, the circuitry was simplified with reduced electromagnetic interference. Spectra were obtained using instrumentation based on analog and digital sensitive detection that showed the latter provided more accurate results. The optimum signal-to-noise ratio of 66.34 dB was higher than the value obtained by the analog system.

KEYWORDS:

• Digital phase-sensitive detection
• digital quadrature algorithm
• near-infrared spectrometer

Acknowledgments

The authors are grateful for Zhiwei Liu, Xiaochao Chen, and Cheng He for sharing their expertise in preliminary work, and thank lecturer Jing Zhao and Professor Tingting Lin for putting forward modification suggestions for this article.