The user can conduct the FFT for each data frame partitioned in step 1. The basic principles and calculation methods of the FFT are widely known. RecurDyn uses an algorithm from Numerical Recipes (NR) to calculate the data for the Campbell diagram. The signal process does not include interpolation or extrapolation, so the system uses the current signal data. You can use multiple windows to conduct FFT. To extract the magnitude, you can use the amplitude and the PS (power spectrum) or the PSD (power spectrum density).