Question

The modulation index used in amplitude modulation of a message signal of peak voltage $10$ volts is $0.5$. By how much the peak voltage of the carrier signal be changed so as to obtain the modulation index of $0.8$, keeping the peak voltage of the message signal same as before.

• A. increase by 7.5 V
• B. reduce by 7.5 V
• C. increase by 12.5 V
• D. reduce by 12.5 V

Answer 1

Answer: (B)

Initially modulation index, $\mu =0.5$
and message amplitude, $A_m=10V$ ,
As $\mu =\frac{A_m}{A_c}$
$\Rightarrow A_{c_1}=\frac{A_m}{\mu }=\frac{10}{0.5}=20V$
When $\mu =0.8$ and $A_m=10V$
$A_{c_2}=\frac{A_m}{\mu }=\frac{10}{0.8}=12.5V$
Change in carrier amplitude is
$\Delta A_c=A_{c_1}-A_{c_2}=20-12.5=7.5V$
Thus, carrier frequency is reduced by $7.5V$.