MATLAB Central Newsreader - FFT on MATLAB with Graphical Illustration

FFT on MATLAB with Graphical Illustration

Kola Ogidi — Sat, 23 May 2009 19:32:01 -0400

Hello,

I am trying to produce the FFT for a 16Hz sine wave sampled at 1024Hz on MATLAB and also illlustrate this graphically on MATLAB.

I have tried the following code:
fs=1024;
f=16;
t1=0;
tstep=(1/1024);
t2=4/1024;
t=[t1:tstep:t2];
x=sin(2*pi*f*t);
X=fft(x);
Sampleresp=abs(X);
N=length(X);
F=[0:N-1]*fs/N;
stem(F,Sampleresp);

However, my output figure is not showing a peak at 16Hz as I expect. Could you please explain to me what changes I need to make to my code to yield the correct output/graph.

Thank you.

Re: FFT on MATLAB with Graphical Illustration

Sadik — Sat, 23 May 2009 20:09:02 -0400

Please replace line starting with t2 by the following:

t2 = 63/1024;

This is because, although we think that we have only one period of the signal, in the discrete world, things are considered to be periodic. Therefore, you should have a full period of your sine wave. Your period is T = 1/16 which is equal to 64/1024. But since you started from 0/1024, which will have the identical signal amplitude with 64/1024, we will exclude 64/1024 and end the sequence at 63/1024.

"Kola Ogidi" <jreal_kkoo@hotmail.com> wrote in message <gv9ivh$97d$1@fred.mathworks.com>...
> Hello,
>
> I am trying to produce the FFT for a 16Hz sine wave sampled at 1024Hz on MATLAB and also illlustrate this graphically on MATLAB.
>
> I have tried the following code:
> fs=1024;
> f=16;
> t1=0;
> tstep=(1/1024);
> t2=4/1024;
> t=[t1:tstep:t2];
> x=sin(2*pi*f*t);
> X=fft(x);
> Sampleresp=abs(X);
> N=length(X);
> F=[0:N-1]*fs/N;
> stem(F,Sampleresp);
>
> However, my output figure is not showing a peak at 16Hz as I expect. Could you please explain to me what changes I need to make to my code to yield the correct output/graph.
>
> Thank you.

Re: FFT on MATLAB with Graphical Illustration

Kola Ogidi — Sat, 23 May 2009 20:51:01 -0400

Thank you so very much, Sadik. My output figure is now showing a peak at approximately 16Hz. I believe I just need to tweak around with the MATLAB equivalent for zoom to confirm my results. Thanks again.

Kola