Digital signal processing (DSP) techniques for FT-IR multiple modulation measurements using a photoelastic modulator (PEM)

摘要

Digital signal processing (DSP) techniques for performing multiple modulation measurements with a polarization photoelastic modulator (PEM) in a step-scanning FT-IR spectrometer. The frequency and phase of the PEM drive signal are extracted from the digitized data collected for the actual measurement. This can then be used to perform the desired analysis of the polarization signals (e.g., CD, LD, DIRLD). This is accomplished by successively refining an initial estimate of the PEM frequency (typically starting at the nominal PEM frequency l SB0/SB, or at the value determined from the previous step). This is done by using the current estimate of the PEM frequency to compute a phase error, and then using the computed phase error to refine the estimate of the PEM frequency. The phase errors are computed using different sets of samples in the sampling interval.