can I use FFT to determine the variable frequency?

Hi All.

Can I use Fourier transform to determine the variable frequency?Below is the code. If the frequency perturbation(rand/10) is small,the main frequency array(xx) contains the same value. If the frequency perturbeation(rand/10) is large, such as rand,the value in main frequency array(xx) does NOT make sense.

Is there any way I can find the variable frequency?

I appreciate any of your advice.

Ivy

clear all;
clc;

Fs=1000; 
T=1/Fs;
L=1000;
t=(0:L-1)*T;

for i=1:1:10000          %variable frquency signal.  
x(i)=0.7*sin(2*pi*(50+rand/10)*T*(i-1));  
end

for k=1:1:5000
NFFT =1024;
Y=fft(x(k:1024+k),NFFT);
f= 100*linspace(0,1,100);
[C,index]=max(abs(Y(1:100)));
xx(k)=f(index);
end

Answer 1

[Rick Rosson]

You may want to try the spectrogram function. For more information:

   >> doc spectrogram

HTH.

Rick

[Ivy]

Thank you, Rick.

I will look through it.

[Ivy]

Hi Rick,

I have use the following code to get the variable frequency.But I can't get the result(frequency increase from 50 to 10050). Could you please correct me?

Thanks!

Ivy

clear all;

clc;

Fs=1000;

T=1/Fs;

L=1000;

t=(0:L-1)*T;

for i=1:1:10000 %variable frquency signal.

x(i)=0.7*sin(2*pi*(50+i)*T*(i-1));

end

NFFT =1024;

[S,F,T,P] = spectrogram(x,256,255,256,1E3);

surf(T,F,10000*log10(P),'edgecolor','none'); axis tight;

view(0,90);