fft fast transform fourier problem ?

Hi,

I am having trouble plotting the fft (fast fourier transform) of this signal :

% Create square wave for ii = 1 : length(x) if x(ii) < 0 y(ii) = 0; elseif x(ii) > 1 y(ii) = 0; else y(ii) = 1; end end

% Plot square wave subplot(1,1,1) plot(x,y) grid;

i wanna this fft .

Answer 1

[buzz]

What's exactly the matter? You have a rect impulse, use the fft function implemented in matlab:

Y = fft(y);

You should have a sinc function into fourier domain. Remember the fft is normalized to the length of the signal to be transformed. So, in order to view the amplitude correctly:

A = abs(Y)/length(y);

About the frequency axis (normalized frequency):

N = length(y);
df = 1/N;
f_axis = 0:df:1-df
plot(f_axis,y);

That's all. Waiting for more detail about the question.

Answer 2

[benzouine yassine]

Thks for your answer,but when i use the fft i don't get a sinc of my signal , why ? this is my final code :

 clc, clear

Fs = 1000; % Sampling Frequency x = -5 : 1/Fs : 5; % Time vector % Create square wave for ii = 1 : length(x) if x(ii) < 0 y(ii) = 0; elseif x(ii) > 1 y(ii) = 0; else y(ii) = 1; end end

% Plot square wave %subplot(3,2,1) plot(x,y) grid; axis([-5 5 -2 2])

figure;

Y = fft(y);

A = abs(Y)/length(y);

N = length(y); df = 1/N; f_axis = 0:df:1-df plot(f_axis,y); grid;

figure1;