The spectrum of a signal is the magnitude of each frequency components of a signal plotted against frequency. A spectrum is the frequency domain representation of a signal. A time domain signal can be changed to spectrum by using fourier transform. In this case the frequency domain representation of the signal is already given which is the spectrum itself. Plot the function (sinc in this case) against frequency (f) to obtain the spectrum.
f=1:10:100000;
y=sinc(f/5000);
plot(f,y);
