---
title: "PAM versus PWM"
subject: "Data Communication"
chapter: 5
key: "semester_5/data_communication:pam-vs-pwm"
source: https://www.bctengineer.com/q/semester_5/data_communication/pam-vs-pwm
site: BCT Engineering
---

# PAM versus PWM

Data Communication · Chapter 5: Data Encoding and Modulation

- Asked in: 2070 Asar, 5 marks
- Video: [Performance Comparison of PAM, PWM and PPM in Digital Communication by Engineering Funda](https://www.youtube.com/watch?v=90dizh1Sl3E) — Engineering Funda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/pam-vs-pwm
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-5

## Question

Differentiate between PAM and PWM.

## Short answer

PAM varies pulse amplitude proportional to message, width constant; PWM varies pulse width, amplitude constant. PAM is simpler but more noise sensitive; PWM offers better noise immunity and is used in motor control. PAM is a step in PCM, PWM needs synchronisation.

## Answer

**PAM (Pulse Amplitude Modulation)**: the amplitude of a train of constant-width pulses is varied in proportion to the instantaneous message amplitude. **PWM (Pulse Width Modulation)**, also called pulse duration modulation, varies the width of the pulses, while the amplitude and position stay constant.

| Point | PAM | PWM |
|---|---|---|
| Parameter varied | Pulse amplitude | Pulse width (duration) |
| Pulse width and position | Constant | Width varies; position of one edge fixed |
| Noise immunity | Poor: noise adds directly to amplitude | Better: amplitude can be clipped |
| Bandwidth | Lower | Higher (needs steep edges) |
| Transmitted power | Varies with message | Varies with width (average) |
| Generation / detection | Simple: sample and hold | Needs a ramp/comparator circuit |
| Synchronisation | Not needed | Needed to find the edge |
| Use | First step of PCM, TDM | Motor speed control, DC-DC converters, servo |

## Check yourself

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

1. What does PWM vary in proportion to the message?
   A. Pulse amplitude
   B. Pulse width (duration)
   C. Pulse frequency
   D. Carrier phase
   Answer: B. PWM varies the width of the pulses; amplitude and position stay constant. (Key point: Parameter varied)

2. Why does PWM have better noise immunity than PAM?
   A. It uses lower bandwidth
   B. Information is in the width, so amplitude noise can be clipped off
   C. It needs no synchronisation
   D. It has constant power
   Answer: B. Noise adds to amplitude; PWM carries information in width so amplitude can be clipped. (Key point: Noise immunity)

3. Which statement on bandwidth is correct?
   A. PAM needs higher bandwidth than PWM
   B. Both need the same bandwidth
   C. PWM needs higher bandwidth because of steep edges
   D. PWM needs no bandwidth
   Answer: C. PWM needs steep pulse edges, hence higher bandwidth than PAM. (Key point: Bandwidth)

## Related in Data Communication

- [PAM versus QAM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apam-vs-qam) — Data Communication, chapter 5

## Related in other subjects

- [Short note: PAM, PWM and PPM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-pam-pwm-ppm) — Communication Systems, chapter 4
- [PAM, PWM, PPM; PCM of 4.8 MHz TV signal with 512 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apam-pwm-ppm-tv-4-8mhz) — Communication Systems, chapter 4
- [PAM vs PPM; generation of PWM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apam-ppm-pwm-generation) — Communication Systems, chapter 4