Say you are given a vector t =[...] for the time to be used to generate a sine wave of a certain frequency say FHz. I have all the data for t so I can conclude what dt is (my dt = 1d-5). Now I am to do an FFT analysis and plot magnitude vs frequency so I expect to get my original amplitude and also get back the original phase This what I do:
frequency = ...; %This in Hz dt = t(2) - t(1); %which is equal to 1d-5 fs = 2*frequency; % should this be 1/dt? or something else? Nfft = fs; intensity = sin(2*pi*frequency*t); FFT_intensity =fft(intensity,Nfft)/Nfft; magIntensity = abs(FFT_intensity); f=linspace(-fs/2, fs/2, Nfft); plot(f,fftshift(magIntensity))
The results I get are sometimes correct and sometimes incorrect, it seems like when I chop the length of the t vector or change fs (say fs = 1/dt instead) I get the wrong amplitude ( and wrong phase, not shown here)... In general how to relate FHz, Fs and t? I can actually include the numbers I am using if that makes any difference. Thank you for any input.