(11.24)
Now, let us compare the spectrum of a square wave of period T as shown in Figure 11.22 and that of a square pulse shown in Figure 11.24. From equation (11.19) and (11.24) that the Fourier coefficients in Figure 4 have the following relationship to 
(11.25)
where 
is the fundamental frequency. Therefore, the envelope of the quantities obtained by multiplying 
to the line spectra of the Fourier coefficients of the square wave gives the continuous spectra of the Fourier transform 
of the square pulse. As shown in Figure 11.25, multiplying the spectrum for the square wave with period T = 8 in Figure 11.24 gives the spectrum for the square pulse in Figure 11.25.