why my fft doesnot give the correct answer??

i have the signal which consist of fundamental frequency and some harmonics, while doing fft i m not getting correct answer like i get in fftanalysis using powergui tool in simulink. the code is as follows. please help me out.

signal= xlsread('Isa.xlsx');
X=signal(:,1);
fs=1e6;
fnyquist=fs/2;
t=[0:1/fs:1];
N=length(t);
X_mag=abs(fft(X));
bin_vals= [0 :(N-1)];
fax_Hz=bin_vals*fs/N;
N_2=ceil(N/2);
subplot(2,1,1)
% abs(fft(X))
plot(fax_Hz,X_mag)
xlabel('Frequency (Hz)')
ylabel('Magnitude');

Answer 1

[Sulaymon Eshkabilov]

Hi,

There are several unclear points, i.e. your time signal and sampling frequency fs match with your signal from sla.xls, etc. They have to match. In other words, they need to have the same values.

Here is a simple example that you can adjust with your data (sla.xlsx), such as fs, t, N:

fs=1e3;     % Sampling frequency 
tend = 5;   % End value of time signal
t=0:1/fs:tend; 
L = numel(t);  % Length of a signal. Note that t has to match with your data from your xlsx file
signal= sin(13*2*pi*t)+cos(113*2*pi*t)+.01*randn(size(t));  % signal=xlsread('sla.xlsx');
N = 2048;   % Block size can be: 4096, 8192, ...
Y1 = abs(fft(signal, N))/L;
f=fs/2*linspace(0, 1, N/2+1);
figure
plot(f, 2*Y1(1:N/2+1), 'r-'), grid on, legend('Y1', 'location', 'SouthEast')
title 'FFT of signal(t)'
shg

Good luck.