Question

In amplitude modulation, the bandwidth is :

• A. Equal to the signal frequency
• B. Twice the signal frequency
• C. Thrice the signal frequency
• D. Four times the signal frequency

Answer 1

Answer: (B)

Twice the signal frequency

Lower side frequency of Amplitude modulated (AM) wave is $\frac{{\omega }_c-{\omega }_m}{2}$
Upper side frequency of Amplitude modulated wave is $\frac{{\omega }_c+{\omega }_m}{2}$
${\omega }_c,{\omega }_m$ are carrier and modulated signal frequencies respectively.
Signal frequency $\frac{{\omega }_m}{2}$ is present in the side bands.
Thus AM wave has bandwidth of $\frac{{\omega }_c+{\omega }_m}{2}-\frac{{\omega }_c-{\omega }_m}{2}$ $=2\frac{{\omega }_m}{2}$
i.e. twice the signal frequency is produced.