how to get "fft" of sin wave?

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ADNAN KIRAL 2019-6-24

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https://ww2.mathworks.cn/matlabcentral/answers/468669-how-to-get-fft-of-sin-wave

评论： Star Strider 2019-6-25

I have the following code for "fft" but I could not get the exactly frequency-amplitude response. where did I make a mistake ? can u please help me with that?

I want to get this figure; amplitude 2.5 and frequency 4Hz.

final_point=3;
time=1:0.01:final_point;
omega=2*pi*4;
y1=2.5*sin(omega*time);
% %FFT
acc_inpt=y1;
n_fft=length(acc_inpt);
 
ff=fft(acc_inpt,n_fft);
num_points=n_fft; 
ff=ff/(num_points/2);
figure;
plot(abs(ff))

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采纳的回答

Star Strider 2019-6-24

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The amplitude appears to be correct. You need to define the frequency axis to get the frequencies to be correct:

final_point=3;
time=1:0.01:final_point;
omega=2*pi*4;
y1=2.5*sin(omega*time);
% %FFT
Fs = 1/mean(diff(time));                            % Sampling Frequency
Fn = Fs/2;                                          % Nyquist Frequency
Fv = linspace(0, 1, numel(y1)/2+1)*Fn;              % Frequency Vector
Iv = 1:numel(Fv);                                   % Index Vector
acc_inpt=y1;
n_fft=length(acc_inpt);
 
ff=fft(acc_inpt,n_fft);
num_points=n_fft; 
ff=ff/(num_points/2);
figure;
plot(Fv, abs(ff(Iv)))