- #1
dmorris619
- 42
- 0
I understand that for the FFT the resolution or bin size is a function of the number of samples in your signal and that while padding the signal with zeros will make the graph look more precise it will not enable you to resolve between two frequencies if they are contained in the same bin. What I do not understand is how the amplitude of each bin is determined. For example, so I have a bin size of 5 Hz and I have two cosine signals of equal magnitude in that bin separated by 1 Hz(lets say 1000 and 1001) why does the magnitude of that reported bin not equal 2? As they are moved further apart the amplitude becomes even less and also starts impacting the bin next to it as well. I imagine this has something to do with the fact that in the rectangularly windowed FFT cosine and sine are sincs rather than diracs, but am not exactly sure why the amplitude comes out to some seemingly(to my inexperienced eyes) random value. This then leads me to ask two more questions. The first is whether there is some kind of formula to determine the amplitude give the frequency and the amplitude of my cosines. The second and more ignorant question, is there anything I can do, like windowing, to make it so that the amplitude seems more logical for a bin size(so two equal magnitudes are twice the magnitude). Again the second question really is based in the fact that I don't fully understand what the meaning of each bin's amplitude is, i.e. those seemingly arbitrary numbers actually correlate to some important part of the FFT.