Here is the code I use to plot a function in frequency domain in Matlab:
dt = 1/10000; % sampling rate et = 0.1; % end of the interval t = 0:dt:et; % sampling range y = 2+sin(2.*pi.*50.*t)+18.*sin(2.*pi.*90.*t)+6.*sin(2.*pi.*180.*t); % sample the signal subplot(2,1,1); % first of two plots plot(t,y); grid on % plot with grid xlabel('Time (s)'); % time expressed in seconds ylabel('Amplitude'); % amplitude as function of time Y = fft(y); % compute Fourier transform n = size(y,2)/2; % 2nd half are complex conjugates amp_spec = abs(Y)/n; % absolute value and normalize subplot(2,1,2); % second of two plots freq = (0:100)/(2*n*dt); % abscissa viewing window stem(freq,amp_spec(1:101)); grid on % plot amplitude spectrum xlabel('Frequency (Hz)'); % 1 Herz = number of cycles/second ylabel('Amplitude'); % amplitude as function of frequency
The problem is, when I zoom in my graph I don't see peaks exactly on 50Hz, 90Hz and 180Hz.
What did I do wrong in my code?