---
title: "AM versus FM"
subject: "Data Communication"
chapter: 5
key: "semester_5/data_communication:am-vs-fm-compare"
source: https://www.bctengineer.com/q/semester_5/data_communication/am-vs-fm-compare
site: BCT Engineering
---

# AM versus FM

Data Communication · Chapter 5: Data Encoding and Modulation

- Asked in: 2070 Chaitra, 4 marks; 2069 Asar, 4 marks (asked 2 times)
- Video: [Difference between AM and FM](https://www.youtube.com/watch?v=cCQh2q1neSI) — Engineering Basics
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/am-vs-fm-compare
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-5

## Question

Differentiate AM and FM based on modulation index and bandwidth.

## 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.

## Answer

| 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 |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. AM modulation index range:
   A. Any value
   B. 0 to 10
   C. 0 to 1; above 1 is over-modulation
   D. Always above 1
   Answer: C. mu = Am/Ac. (Key point: AM index)

2. FM bandwidth by Carson's rule:
   A. 2fm
   B. 2(Δf + fm)
   C. fm
   D. Δf only
   Answer: B. Wide. (Key point: FM bandwidth)

3. Effect of modulation index on bandwidth:
   A. Increases in AM; none in FM
   B. Increases in both
   C. None in AM; increases with beta in FM
   D. None in both
   Answer: C. AM fixed by fm. (Key point: Bandwidth vs index)

4. Which has better noise immunity?
   A. AM
   B. Same
   C. Neither
   D. FM
   Answer: D. Constant amplitude. (Key point: Noise)

## Related in Data Communication

- [AM wave power and bandwidth numerical](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aam-numeric-40-carrier-power) — Data Communication, chapter 5
- [AM numerical: 10 sin1000πt on 50 sin200000πt](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aam-numeric-10sin1000pi-800) — Data Communication, chapter 5
- [AM numerical: 20 sin2π500t on 60 sin2π10⁶t](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aam-numeric-20sin500) — Data Communication, chapter 5
- [Single tone FM parameters](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afm-single-tone-numeric) — Data Communication, chapter 5
- [AM numerical: 10 sin1000πt on 50 sin 2π×10⁵t](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aam-numeric-10sin1000pi) — Data Communication, chapter 5
- [Frequency modulation](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afm-explain) — Data Communication, chapter 5

## Related in other subjects

- [Double-tone AM expression, bandwidth and modulation index](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adouble-tone-am-bw-index) — Communication Systems, chapter 2
- [FM numerical: deviation, index, 98% bandwidth, power](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afm-numerical-10cos-80cos-400hz) — Communication Systems, chapter 3, asked 2×
- [Carson's rule; when FM bandwidth equals AM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acarson-rule-fm-am-bandwidth) — Communication Systems, chapter 3
- [Modulation; AM and FM modulated waves](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amodulation-am-fm-wave) — Basic Electronics Engineering, chapter 5