---
title: "Define AM and FM with waveforms"
subject: "Basic Electronics Engineering"
chapter: 5
key: "semester_2/basic_electronics_engineering:am-fm-waveforms"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/am-fm-waveforms
site: BCT Engineering
---

# Define AM and FM with waveforms

Basic Electronics Engineering · Chapter 5: Communication System

- Asked in: 2069 Bhadra, 2+3 marks
- Video: [What is Modulation ? Why Modulation is Required ? Types of Modulation Explained.](https://www.youtube.com/watch?v=mHvV_Tv8HDQ) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/am-fm-waveforms
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-5

## Question

Define amplitude modulation and frequency modulation and draw the necessary waveforms.

## Short answer

Amplitude modulation varies carrier amplitude according to the message signal, keeping frequency constant, with envelope shape of the message. Frequency modulation varies carrier frequency according to the message, keeping amplitude constant; the carrier cycles are dense at positive message peaks and sparse at negative peaks.

## Answer

#### Amplitude modulation

In **amplitude modulation**, the amplitude of the carrier changes according to the instantaneous value of the message signal, while its frequency stays constant.

vAM = Vc [1 + m sin ωm t] sin ωc t, with m = Vm/Vc.

#### Frequency modulation

In **frequency modulation**, the frequency of the carrier changes according to the instantaneous value of the message signal, while its amplitude stays constant.

vFM = Vc sin[ωc t + mf sin ωm t], with mf = Δf/fm, where Δf is the frequency deviation.

#### Waveforms

```text
 Message (modulating):
   /\    /\    /\
  /  \  /  \  /  \
 /    \/    \/    \

 Carrier:
 /\/\/\/\/\/\/\/\/\/\/\

 AM (amplitude varies, frequency fixed):
    /\            /\
 /\/  \/\/\    /\/  \/\
 \/\  /\/\/  \/\/  \  /
    \/            \/
 (envelope follows message)

 FM (frequency varies, amplitude fixed):
 |||| | |  |   |  | | ||||
 dense when message high,
 sparse when message low
```

In the AM wave, the envelope has the shape of the message. In the FM wave, the carrier peaks stay at the same height, and the carrier cycles become closer together when the message is at its positive peak and spread out when it is at the negative peak.

## Check yourself

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

1. What changes in an AM wave?
   A. Frequency changes; amplitude constant
   B. Amplitude changes with the message; frequency is constant
   C. Both vary
   D. Neither varies
   Answer: B. The envelope follows the message. (Key point: AM definition)

2. What changes in an FM wave?
   A. Amplitude changes; frequency constant
   B. Phase only; frequency fixed
   C. The message frequency changes
   D. Frequency changes with the message; amplitude is constant
   Answer: D. FM carrier peaks stay at the same height. (Key point: FM definition)

3. In an FM wave, when are carrier cycles closest together?
   A. At the negative peak
   B. At the positive peak of the message
   C. When the message is zero
   D. Never; spacing is fixed
   Answer: B. Frequency is highest when message is at positive peak. (Key point: FM waveform)

4. What is mf?
   A. Vm/Vc
   B. fm/Δf
   C. Δf × fm
   D. Δf/fm, deviation divided by message frequency
   Answer: D. Modulation index for FM is the ratio of frequency deviation to message frequency. (Key point: FM index)

## Related in Basic Electronics Engineering

- [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
- [Short note: Amplitude modulation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ashort-note-am) — Basic Electronics Engineering, chapter 5
- [Communication system and AM block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-am) — Basic Electronics Engineering, chapter 5

## Related in other subjects

- [ISB modulation; AM vs FM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aisb-am-vs-fm) — Communication Systems, chapter 2
- [Angle modulation; single tone FM time and frequency domain](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aangle-mod-single-tone-fm-time-freq) — Communication Systems, chapter 3
- [Frequency modulation](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afm-explain) — Data Communication, chapter 5
- [AM versus FM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aam-vs-fm-compare) — Data Communication, chapter 5, asked 2×