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

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

[Adam]

What are you expecting 'Acceleration vs Frequency' to look like? What you plotted is frequency vs power (i.e. the power spectrum), not frequency vs acceleration

[Muna Shehan]

Thanks for your comment. I need to know the code for fft is correct or not. and to be honest I do not have idea how acce. vs frequency profile look like ( I already have time and acceleration vectors for my dynamic model which is in time domain and know I try to figure out how to transfer this time domain data to frequency domain). In this case can you please suggest how i can get frequency vs acceleration plot. Regards

[Star Strider]

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]

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.