f1 = 4*1000
t1 = linspace(-1/f1, 1/f1, 100*1000);
FT_x_c1 = fft(x_c1);
and result is
It is obvious that FFT of sinc function in time domain is rect function in frequency domain.
But as you see, there's something wrong with the result.
What should I do to solve this situation?