how to get "fft" of sin wave?

hi

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))

Answer 1

[Star Strider]

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)))

That plots the peak at 4 cycles/time unit, that appears to be what you want to do.

[ADNAN KIRAL]

many thanks @star strider.

[Star Strider]

As always, my pleasure.

[ADNAN KIRAL]

a quick question please, I want to get the highest frequency output, I used "round" but it is working for an only integer number. it is not working for 2.5 so on.

how can I solve this problem?

thanks in advance

[Star Strider]

I am not certain what you intend by ‘highest frequency output’. The highest frequency you can use (the highest frequency it is possible to uniquely resolve in a sampled signal) is the Nyquist frequency, given here by ‘Fn’.

[ADNAN KIRAL]

please let me make it clear. i just do not want to see the small frequencies given the image. i want to ignore them. can I remove from my data ?

[Star Strider]

Those are artefacts of the fft computation.

You can eliminate them from the plot by thresholding:

plot(Fv, abs(ff(Iv)).*(abs(ff(Iv)) >= 0.15))

That will force everything below 0.15 to 0 while leaving the rest of the plot unchanged.

[ADNAN KIRAL]

many thanks. appriciate that. @ Star Strider

[Star Strider]

As always, my pleasure.