Why I could not convert time domain to frequency domain data?

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Muna Shehan 2017-3-15

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https://ww2.mathworks.cn/matlabcentral/answers/329995-why-i-could-not-convert-time-domain-to-frequency-domain-data

评论： Muna Shehan 2017-3-16

Hi all; i try tho covert the time domain to frequency domain by using fft function, but the acceleration vs Frequency plot is incorrect. So kindly can anyone explain what is the wrong in the code. Thanks in advance

Lenght = size(x_time,2);
NFFT = 2^nextpow2(Lenght(1,1)-1); % Next power of 2 from length of b
Y1 = fft(Acc1,NFFT)/(Lenght(1,1)-1);
f = Fs/2*linspace(0,1,NFFT/2+1);
yPower1 = 2*abs(Y1(1:NFFT/2+1));       %magnitude
ydB1 = 20*log10(abs(Y1(1:NFFT/2+1))); %power
figure(51);
plot(f,yPower1);
xlabel('Frequency (Hz)','fontsize',14)
ylabel('Acc.','fontsize',14)
%axis ([0 10 0 10])
figure(61);
plot(f,ydB1);
xlabel('Frequency (Hz)','fontsize',14)
ylabel('Power (dB)','fontsize',14)

where x_time is the time vector Acc1 is the acceleration vector

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Star Strider 2017-3-15

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That looks correct to me.

The details will be more apparent if you add this line after each plot:

set(gca, 'XLim',[0 30])

since there doesn’t appear to be anything above 30 Hz.

Muna Shehan 2017-3-16

Thanks for your comment.Based on your comment, the code for fft is correct even if the acceleration vs time profile is repeated sine wave signal as shown in the first plot .I followed your suggestion and the frequency response is as in the below plots.