Re: problem with calculating period with FFT


Hi Ehsan:

You may want to expand the code a bit, as follows:

% Sampling Rate:
Fs = 500; % Hertz
dt = 1/Fs; % seconds

% Time Domain:
StartTime = -1; % seconds
StopTime = 1; % seconds
t = StartTime:dt:StopTime-dt; % seconds

% Frequency domain:
N = length(t);
df = Fs/N; % Hertz
f = -Fs/2:df:Fs/2-df; % Hertz

% Carrier Frequency:
Fc = 10; % Hertz

% Signal:
x = cos(2*pi*Fc*t);

% Spectrum:
X = fftshift(fft(fftshift(x)));


% Plotting:

figure;

subplot(2,1,1);
plot(t,x);
grid on;
xlabel('time (seconds)');
title('Time Domain');

subplot(2,1,2);
plot(f,abs(X));
grid on;
xlabel('frequency (hertz)');
title('Frequency Domain');


Notice that the plot of the spectrum in the Frequency Domain shows two
impulses at -10 Hz and +10 Hz, as you would expect for a pure cosine wave
with carrier Fc = 10 Hz.

I hope that helps.

Thanks.

Rick



"ehsan mirrahimi" <ehsan.mirrahimi@xxxxxxxxxxxxx> wrote in message
news:fdm4l5$38$1@xxxxxxxxxxxxxxxxxxxxx
hi suppose this is my signal

t=linspace(-1,1,1000);x=cos(20*pi*t);plot(abs((fft(x))))

http://imageupload.com/out.php/i27113_fft.jpg

the sampling frequency is:1000/2=500Hz
the maximum FFT value indext is 981
so what is the period of the signal?
can anyone help me?



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