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- 2070 Chaitra · 4 marks
- 2069 Asar · 4 marks
AM versus FM
Short answer
AM: modulation index μ = Am/Ac (0–1), bandwidth fixed at 2fm. FM: modulation index β = Δf/fm (any value), bandwidth = 2(Δf+fm) = 2fm(β+1), increasing with β.
Differentiate AM and FM based on modulation index and bandwidth.
| Point | AM | FM |
|---|---|---|
| Varies | Carrier amplitude | Carrier frequency |
| Modulation index | μ = Am/Ac, between 0 and 1 (μ > 1 is over-modulation and distorts) | β = Δf/fm = kf·Am/fm, can be any value (more than 1 for wideband FM) |
| Bandwidth | BW = 2fm (narrow) | BW = 2(Δf + fm) = 2fm(β + 1) by Carson's rule (wide) |
| Effect of modulation index on bandwidth | None: bandwidth is fixed by fm | Increases with β |
| Noise immunity | Poor | Good |
| Power efficiency | Low (sidebands carry at most one third of the power) | Higher; the transmitted power is constant |
| Typical use | MW/SW broadcast, aircraft | FM radio, TV sound |
✓ Answer checked by Claude Sonnet 5.5
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