Strictly speaking, the FFT analyzer is intended for use with harmonically related signals that are stationary in time. In other words, all signals are in the center of the bin and never change with time, at least the analysis time. We have learned how to live with non bin centered data by the use of windows. We still have a way to go in how to handle very noisy data. The path is even longer in being able to extract useful data from transient vibration data. The secret to obtaining useful data in these cases is an understanding of the FFT process and how it may be used to provide good approximations of transient data. This paper will discuss several cases where great care must be taken in setting up the analyzer or very misleading data will be produced.
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