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Help on IFFT

Started by bell...@ion.chem.utk.edu September 15, 2007
Hi, All
I have original mass spectrom which is time domain signal. Then I use fft
in Matlab to fourier transform into powerspectrum. Code as below:

a=textread(C:\notebook\070111.txt')
%load data into a single column matrix called "a"
At(a,65536); %built in m file, fft of matrix a for 2^16 element.
Pa=A.*conj(A)/65536; %calculate the intensity of A
f = (0:32768)/65536; % create the frequency of x-axis
h=ones(1,32769); %create a vector of 1
g=h./f; %the frequency of x-axis
m=g/16;%factor the number of sample points.

plot(m,Pa(1:32769));

Now I have question on how to perform inverse FFT for just part of the
result of FFT in the region of (200:5000)? And how to calculate the x
axis?
Thanks for advance.

Bella