Q.Draw the frequency spectrum of an AM wave.

A frequency spectrum is a graph of amplitude against frequency. Because an AM wave is the sum of a carrier and two side frequencies, its spectrum is just three vertical lines: the carrier at $f_c$ with height $V_C$, a lower side frequency at $f_c - f_m$ and an upper side frequency at $f_c + f_m$, each of height $m_a V_C/2$, spaced equally on either side of the carrier.

The two side frequencies are the sidebands — the sum-and-difference signals produced by modulation. A key idea for the II PUC Electronics exam is that the carrier itself carries no information; the entire message resides in the sidebands. That is why the transmission of sidebands is essential — suppressing both sidebands would leave nothing but an information-free carrier.

The bandwidth of the AM wave is the frequency range from the lowest to the highest side frequency:

$BW = (f_c + f_m) - (f_c - f_m) = 2 f_m$ …