FFT of a time-valued signal, then IFFT doesn't return the same signal

Hello everybody,

I've got a problem using matlab fft/ifft: I have a .dat file from which I read the first two columns (data).

I make the fft of the signal and then the ifft to see if they correspond. They do not. Does anyone know why?

My code:

fs=200000;

time=0:1/fs:2-1/fs;

data=load(['S09H4R2.dat']);

Ffrapp=2*fft(data(:,1))./length(data(:,1));

Fsarf=2*fft(data(:,2))./length(data(:,2));

antiFfrapp=ifft(FFTfrappF./2.*length(data(:,1)),'symmetric');

antiFsarf=ifft(FFTfrappF./2.*length(data(:,2)),'symmetric');

figure

plot(time,data(:,1),'b');hold on;

plot(time,data(:,2),'c');

plot(time,antiFfrapp,'r -s');

plot(time,antiFsarf,'g -*');

xlim([0 0.02]);

THANK YOU in advance

Answer 1

[Peng Li]

fs=200000;
time=0:1/fs:2-1/fs;
data=load(['S09H4R2.dat']);
Ffrapp=2*fft(data(:,1))./length(data(:,1));
Fsarf=2*fft(data(:,2))./length(data(:,2));
antiFfrapp=ifft(FFTfrappF./2.*length(data(:,1)),'symmetric'); % what is your FFTfrappF? you were using Ffrapp in above lines.
antiFsarf=ifft(FFTfrappF./2.*length(data(:,2)),'symmetric');
figure
plot(time,data(:,1),'b');hold on;    
plot(time,data(:,2),'c');            
plot(time,antiFfrapp,'r -s');
plot(time,antiFsarf,'g -*');
xlim([0 0.02]);