Question

In amplitude modulation, the modulation index should be:

• A. Less than 0.5
• B. Equal to 1
• C. Greater than 1
• D. Between 0 and 1

Hint:

Over-modulation (\( m>1 \)) causes envelope distortion in AM signals — always keep \( m \leq 1 \).

Answer 1

The Correct Option is D

Step 1: Understanding modulation index in AM.
In amplitude modulation (AM), the modulation index \( m \) is defined as the ratio of the amplitude of the modulating signal to the amplitude of the carrier signal. It indicates the extent of modulation applied to the carrier.
Step 2: Acceptable range of modulation index.
For distortionless AM transmission, the modulation index must satisfy:
\[ 0<m \leq 1 \] If the modulation index exceeds 1, the signal becomes over-modulated, leading to distortion in the received signal.
Step 3: Analyzing the options.
(A) Less than 0.5: This is not necessary; modulation can be effective even when \( m \) is greater than 0.5.
(B) Equal to 1: This represents 100% modulation, which is acceptable but not the only valid case.
(C) Greater than 1: This causes over-modulation and distortion, so it is incorrect.
(D) Between 0 and 1: Correct — this range ensures proper modulation without distortion.
Step 4: Conclusion.
To avoid distortion and ensure faithful signal transmission, the modulation index in AM must lie between 0 and 1.