---
title: "Short note: PAM, PWM and PPM"
subject: "Communication Systems"
chapter: 4
key: "semester_6/communication_systems:short-note-pam-pwm-ppm"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-pam-pwm-ppm
site: BCT Engineering
---

# Short note: PAM, PWM and PPM

Communication Systems · Chapter 4: Pulse Modulation

- Asked in: 2079 Bhadra (CS II), 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 Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-pam-pwm-ppm
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-4

## Question

Write a short note on PAM, PWM and PPM.

## Short answer

PAM varies pulse amplitude, PWM varies width, PPM varies position. PAM is simple but noise sensitive; PWM and PPM offer better noise immunity with more bandwidth.

## Answer

Analog pulse modulation uses a periodic pulse train as the carrier and varies one pulse parameter in step with the sampled message.

```text
m(t)  ____/‾‾‾‾\____/
PAM   |  |‾| |‾‾| |‾|  |   height varies
PWM   ▌  ▐█  ▐██ ▐█  ▌     width varies
PPM   | |   |    | |       position varies
```

**PAM (Pulse Amplitude Modulation)**
- The **amplitude** of each pulse is proportional to the sample value; width and position are fixed.
- Produced by natural or flat-top (sample-and-hold) sampling.
- Simplest; first step of PCM and TDM.
- Poor noise immunity, since noise adds directly to amplitude.
- Demodulated with a low-pass filter (plus equalizer for flat-top).

**PWM (Pulse Width / Duration Modulation, PDM)**
- The **width** of each pulse varies with the sample; amplitude and starting position are fixed.
- Generated by comparing the message with a sawtooth/triangular wave in a comparator (or a monostable).
- Better noise immunity (amplitude can be clipped), but power varies with width, so transmitter power is wasted.
- Used in motor speed control, class-D amplifiers, SMPS.

**PPM (Pulse Position Modulation)**
- The **position** (time shift) of each pulse from its nominal position varies with the sample; amplitude and width are fixed.
- Generated from PWM: the trailing edge of each PWM pulse triggers a short fixed pulse.
- Constant pulse power and best noise immunity of the three, but needs accurate synchronization.
- Demodulated by converting back to PWM and low-pass filtering.

| Feature | PAM | PWM | PPM |
|---|---|---|---|
| Varied parameter | Amplitude | Width | Position |
| Noise immunity | Low | Better | Best |
| Transmit power | Varies | Varies | Constant |
| Sync needed | No | No | Yes |
| Bandwidth | Least | More | More |

## Check yourself

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

1. In PAM, which pulse parameter carries the message?
   A. Width
   B. Position
   C. Amplitude
   D. Frequency
   Answer: C. PAM varies pulse amplitude with the sample; width and position stay fixed. (Key point: PAM)

2. PWM is commonly generated by:
   A. Comparing the message with a sawtooth or triangular wave
   B. Multiplying by a sine carrier
   C. Passing the message through an integrator
   D. Quantizing the message
   Answer: A. A comparator against a sawtooth gives pulses whose width follows the message. (Key point: PWM generation)

3. Which statement about PPM is correct?
   A. Its pulse power varies with the message
   B. It is the most noise-immune of the three but needs accurate synchronization
   C. It needs no synchronization
   D. It is made by varying pulse amplitude
   Answer: B. PPM has constant pulse power and the best noise immunity, but the receiver needs timing reference. (Key point: PPM)

4. Why is PAM the least noise-immune of the three?
   A. Its pulses are too narrow
   B. Noise adds directly to the pulse amplitude that carries the information
   C. It uses the highest bandwidth
   D. It has constant power
   Answer: B. In PAM, information is in the amplitude, so noise corrupts it directly. PWM and PPM allow amplitude clipping. (Key point: Noise immunity)

## Related in Communication Systems

- [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

## Related in other subjects

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