Ali - what is the line (or lines) of code that you are using to do the FFT? What is the block size, N?
% define the function
func = @(t)100*sin(2*pi*50*t) + 1*sin (2*pi*123*t) + 2*sin (2*pi*203*t) + ...
3*sin(2*pi*223*t) + 4*sin(2*pi*331*t) + 5*sin(2*pi*2812*t) + ...
6*sin(2*pi*5752*t) + 7*sin(2*pi*7993*t);
% set the sampling rate
Fs = 50000;
% set the block size
N = 50000;
% create the time vector
t = linspace(0,1-1/Fs,Fs);
% create the time domain data
y = func(t);
% do the fft
Y = fft(y,N);
% determine the frequencies
f = Fs/2*linspace(0,1,N/2+1);
% plot single-sided amplitude spectrum.
plot(f,abs(2*Y(1:N/2+1)))
title('Single-Sided Amplitude Spectrum of y(t)')
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')
A figure is created that clearly describes the frequency components of the input. To get the amplitudes of each frequency component, just do
% get Ym which is just the magnitude of the first N/2+1 elements
Ym = abs(Y(1:N/2+1))';
% find those whose magnitude is greater than one
[idcs] = find(Ym>1);
% display "table" of frequency vs amplitude (subtract one since
% frequencies start at 0; to get amplitude, divide by N/2 since real input)
[idcs-1 Ym(idcs)/(N/2)]
ans =
50 100
123 0.99999999999999
203 2
223 2.99999999999994
331 4.00000000000001
2812 5
5752 6.00000000000004
7993 6.99999999999998
The above is near identical to the input amplitudes.
