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, $A_{m}=10 \,V$ ,
As $\mu=\frac{A_{m}}{A_{c}} $
$\Rightarrow A_{c_{1}}=\frac{A_{m}}{\mu}=\frac{10}{0.5}=20\, V$
When $\mu=0.8$ and $A_{m}=10 V$
$A_{c_{2}}=\frac{A_{m}}{\mu}=\frac{10}{0.8}=12.5 \,V$
Change in carrier amplitude is
$\Delta A_{c}=A_{c_{1}}-A_{c_{2}}=20-12.5=7.5\, V$
Thus, carrier frequency is reduced by $7.5 \,V$.