Scaling x-axis after using IFFT on part of the FFT

Hi,

I have a measurement signal (from accelerometer), of which I want to know what kind of influence certain frequencies have on the signal. To this end, I take the FFT of the signal, remove all frequencies except the ones I want to know more about, and then use IFFT to go back to accelerations.

However, using FFT, the x-axis represents frequencies, and using IFFT this should convert to time again. The problem is, since I take out a lot of the signals, my signal length goes from 2390 to 251 samples. Now my question is, what does the x-axis represent of this IFFT? Or how can I scale it properly?

My code:

length_vector=length(X_measure);
NFFT = 2^nextpow2(length_vector);
freq = Fs_imu/2*linspace(0,1,NFFT/2);

X_fft = fft(X_measure,NFFT)/length_vector;

%Plots
figure
plot(X_measure)
title('X acc measured')
figure
plot(freq(50:300),2*abs(X_fft(50:300)));
title('FFT X acc 50:300')
figure
plot(length_vector*ifft(X_fft(50:300),'symmetric'))
title('IFFT of FFT X Acc 50:300')

With the above code, I'm mostly confused about the x-axis for: length_vector*ifft(X_fft(50:300),'symmetric') .

Thanks in advance!

Kind Regards,

Roland