Hi I am trying to program excel to take the DFT of a signal, then bring it back to the time domain after a low pass filter. I have a code that can handle simple data for example t = [ 0, 1, 2, 3] y = [2, 3, -1, 4] So I think everything is great and so I plug in my real signal and things go off the rails. Firstly my transform doesn't seem to work. In the simple cases I have a start frequency of 0 and an end frequency of 4. Everything works. I know my data has equal spacing, but I do not know my frequency to start or stop so I pick 0 and some arbitrary numbers. I have the number of points within the frequency interval as variable. My output looks like no transform I have ever seen and bringing it back to time is equally bad. Am I missing something when I move to real data? Now I was following this blog http://excelunusual.com/spectral-analysis-a-fourier-transform-tutorial/, as a basis for my code and I found that gave me a decent looking tranform of my real data but failed on a simple problem as stated above. The expression for the transform is different. G(f) = ∫g(t)e-2π*j*f*t where as most text books have n/N in the exponent. Not sure what the disconnect is but looking for some opinions on this. Thanks in advance.