I have my data obtained from A Resonant Frequency Testing using impact hammer. The time domain waveforms from both impact and acceleration responses were processed using signal processing software. To get the Frequency Response Function (FRF), the fourier transform was preformed in the signals, but this data is the real part not the imaginary part. So, what I need to do is to plot the mode shapes (Normalized amplitude vs. Length of the specimen) by using the imaginary part of the transfer function. Can I get your help please to get the imaginary part from the real part of my data using Excel?