Asked by aaa
on 24 Apr 2012

Hi,

I am having trouble plotting the frequency spectrum of a sine wave. For this code, i expect the main frequency component to be centered around 1/(2*pi), but they are not. Is there something I am missing in my code?

x = [0:0.1:2*pi]; y = sin(x); z = fft(y); z = fftshift(z); N = length(y); f = [-N/2:N/2-1]/N; plot(f,abs(z))

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Answer by Rick Rosson
on 24 Apr 2012

Accepted answer

Please try:

%% Time specifications: Fs = 100; % samples per second dt = 1/Fs; % seconds per sample StopTime = 1; % seconds t = (0:dt:StopTime-dt)'; N = size(t,1);

%% Sine wave: Fc = 12; % hertz x = cos(2*pi*Fc*t);

%% Fourier Transform: X = fftshift(fft(x));

%% Frequency specifications: dF = Fs/N; % hertz f = -Fs/2:dF:Fs/2-dF; % hertz

%% Plot the spectrum: figure; plot(f,abs(X)/N); xlabel('Frequency (in hertz)'); title('Magnitude Response');

HTH.

Rick

Answer by Elige Grant
on 24 Apr 2012

Your "f" looks off. For your configuration (which uses fftshift):

N = length(y); dt = x(2)-x(1); % Time increment Nyq = 1/(2*dt); % Nyquist frequency df = 1/(N*dt); % Frequency increment if mod(N,2) == 0 % N is even f = -Nyq : df : Nyq-df; else % N is odd f = [sort(-1*(df:df:Nyq)) (0:df:Nyq)]; end

Answer by phanindra reddy
on 21 May 2015

Hi every one

could you please plot spectrum for the variables that are in EXCEL file which is attached.?

thanks.

Please provide me code also.

Phanindra

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