What do we mean by saying that the spectrum of rectangular pulse is

sinc(x) function?

In case you are just wondering what the whole idea is, at all...

A Fourier transform can be just a frequency domain representation of a

time domain function. Time and frequency are... kin and conjugate to

each other.

The spectral width in one domain times the spectral width in the other

domain will be greater than a finite constant (1, usually), which

implies that something narrow in terms of time information will be

wide in terms of frequency information and the converse. The

transform of the rectangle, 2*a*t*sinc(2*PI*f*t), exhibits this

spectral area product in the "f*t" unitless term inside the sinc()

function. Narrowing 't' spreads out the result over broader 'f'.

Jon