help me in FFT normalization!!

fft normalization

hi

please help me , if anyone knows

Q1: how to normalize the FFT plot.

Q2: how to convert fft normalize frequency in HERTZ along x-axis

please help me in these questions....

best regards

normalize fft

It sometimes depends on who's FFT function you are using.
Here is a MATLAB example. Notice the fs/N and 2/N.
If you are unfamiliar with MATLAB, the abs() function returns the magnitude of a complex number.

matlab fft normalization

DSB-AM

fs=1000 ;
ts=0,001;
n=[0:1/fs:1];
fc=150;
fm=15;
faz=0;


t=cos(2*pi*n*fc+faz); % carrier signal

m=cos(2*pi*n*fm); % message signal





figure;
subplot(2,1,1);
plot(n,t);
axis([0 0.05 -1 1]);
title('carrier signal');
subplot(2,1,2);
plot(n,m);
axis([0 0.5 -1 1]);
title('message signal');




tf=fft(t)/length(t);

tfg=abs(tf);

tfg=fftshift(tfg);

eks=[-fs/2:1:fs/2];



mf=fft(m)/length(m);

mfg=abs(mf);

tfg=fftshift(tfg);






r=1*m.*t;

rf=fft(r)/length(r);

rfg=abs(rf);

rfg=fftshift(rfg) ;



figure;
subplot(3,1,1)
plot(eks,tfg)
title('frequency spectrum of carrier signal')

subplot(3,1,2);
plot(eks,mfg);
title('frequency spectrum of message signal')

subplot(3,1,3);
plot(eks,rfg);
title('frequency spectrum of DSB-AM modulation signal')





figure;
plot(m,'r:*');
axis([0 100 -1 1]);
hold on;
plot(r,'k');
axis([0 100 -1 1]);
title('time domain spectrum of message signal and carrier signal')

fft normalize

echo47 said:

It sometimes depends on who's FFT function you are using.
Here is a MATLAB example. Notice the fs/N and 2/N.
If you are unfamiliar with MATLAB, the abs() function returns the magnitude of a complex number.

Code:

N  = 64;        % number of points
fs = 4000;      % sample rate
f1 = 750;       % signal 1 frequency
f2 = 1000;      % signal 2 frequency
a1 = 1.5;       % signal 1 amplitude
a2 = 2.3;       % signal 2 amplitude
t = (0 : N-1) / fs;
y = a1*cos(2*pi*f1*t) + a2*cos(2*pi*f2*t);
h = fft(y);
% Discard duplicate upper half. Scale frequency and amplitude.
freq = fs/N * (0 : N/2);
plot(freq, 2/N * abs(h(1 : N/2+1))); xlabel('Hertz');

Click to expand...

Hi echo47,

I used your MATLAB code, but changed the values of N and fs:
N = 4096;
fs = 10000;

The result turns out to be strange (see the figure).

**broken link removed**

What's wrong with it?

Thanks

normalize fft matlab

Can anyone show me what's wrong in the result in the previous post please?

I used MATLAB code of echo47, and just modified the first 2 lines:


But the result (as shown in the figure) is so strange:
- The 1st component: f = 749.5Hz, amplitude = 1.406 !? (It should be 750Hz, 1.5)
- The 2nd component: f = 1001Hz, amplitude = 1.743 !? (It should be 1000Hz, 2.3)

Thanks