Question

A signal of frequency 20 kHz and peak voltage 5 V is used to modulate a carrier wave of frequency 1.2 MHz and peak voltage 25 V. Choose the correct statement

• A. Modulation index = 5, side frequency bands at 25 V
• B. Modulation index = 0.2, side frequency bands at 1400 Hz and 1000 kHz
• C. Modulation index = 0.4
• D. Modulation index = –0.2

Hint:

AM modulation index = message amplitude / carrier amplitude.

Answer 1

The Correct Option is B

Step 1: Modulation index \[ m=\frac{V_s}{V_c}=\frac{5}{25}=0.2. \]
Step 2: Side band frequencies in AM: \[ f_{SB}=f_c\pm f_s=1.2\,\text{MHz}\pm20\,\text{kHz}. \]
Step 3: Only option (B) has correct index 0.2. Hence → (B).