Abstract
Signal processing has always been a critical aspect of interferometric spectroscopy. The main value of Fourier Transform Spectroscopy (FTS) is to provide the maximum information per unit time. This means that careful signal processing is required to maximize the signal to noise ratio of the result. Digital signal processing (DSP) is not new in FTS. DSP for apodization, Fourier transform, phase correction, and some digital filtering have been used for many years. The first difficulty encountered in using DSP is the required transformation of the continuous time signal to a sequence of finite precision numbers, that is a discrete time, discrete magnitude representation of the continuous signal. Today, sampling and digitization with conventional techniques are reasonably straightforward, with digitization noise of the order of 2−16. Advances in several aspects of FTS technology, including step scanning, have created the need to lower the digitization noise. The processing of signals from multiple modulation FTS experiments, specially step-scan signals, has presented serious limitations until recently, due primarily to the performance restrictions of the available hardware, and the fundamental limitations of the analog systems used. The application of DSP techniques to reduce the sampling and digitization errors are reviewed, and practical solutions that can provide digitization noise lower than 2−24 with low cost systems are described. Some of the causes and effects of sampling error are reviewed, to clarify common misunderstandings. DSP applications in the detection of multiple modulation step-scan experiments and photoacoustic spectroscopy are described in detail. These applications allow the measurement of the signal during the entire step time, eliminate interference of secondary effects of phase modulation, and integrate the entire experiment in the data system, eliminating all the analog adjustments (knobs) needed when lock-in amplifiers are used.
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