Two times fft of the signal

Ouput of the first fft is a sinc function. Now i need to convert the frequency axis of first fft into time domain. After that I need to take fft again and get a rectangular waveform. I have doubt that when i map the frequeny axes of the first fft into time axes at that time, the amplitude of fft remains in the frequency domain only. I am just changing the axes from frequency to time. So when i take the second fft so indirectly i am taking the fft two times of the original signal? And result shoud be rectangular waveform?

Answer 1

[Thorsten]

Yes. It's a property of the Fourier Transformation: the FT of a box is a sinc, and the FT of a sinc is a box. So if you compute fft(fft(box)) you end up with a box function.