Question

In amplitude modulation, the modulation index $m$, is kept less than or equal to 1 because

• A. $m>1$, will result in interference between carrier frequency and message frequency, resulting into distortion
• B. $m>1$ will result in overlapping of both side bands resulting into loss of information
• C. $m>1$ will result in change in phase between carrier signal and message signal
• D. $m>1$ indicates amplitude of message signal greater than amplitude of carrier signal resulting into distortion

Answer 1

Answer: (A), (D)

The modulation index $(m)$ of amplitude modulated wave is
$m=\frac{\text{ amplitude of message signal }\left(A_m\right)}{\text{ amplitude of carrier signal }\left(A_c\right)}$
If $m>1$, then $A_m>A_c$. In this situation, there will be distortion of the resulting signal of amplitude modulated wave. Maximum modulation frequency $\left(m_f\right)$ of $AM$ wave is
$m_f=\frac{\Delta v_{\max }}{v_{m(\max )}}$
$=\frac{\text{ frequency deviation }}{\text{ maximumfrequency value of modulating wave }}$
If $m_f>1$, then $\Delta v_{\max }>v_m$. It means, there will be overlapping of both side bands of modulated wave resulting into loss of information.