---
title: "PAM vs PPM; generation of PWM"
subject: "Communication Systems"
chapter: 4
key: "semester_6/communication_systems:pam-ppm-pwm-generation"
source: https://www.bctengineer.com/q/semester_6/communication_systems/pam-ppm-pwm-generation
site: BCT Engineering
---

# PAM vs PPM; generation of PWM

Communication Systems · Chapter 4: Pulse Modulation

- Asked in: 2081 Bhadra (CS II), 3+4 marks
- Video: [Conclusion : PAM PWM & PPM I Pulse Modulation I Communication System | GATE NET ESE I](https://www.youtube.com/watch?v=_jMPnx5sn-o) — Dopamine
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/pam-ppm-pwm-generation
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-4

## Question

Differentiate between pulse amplitude and pulse position modulation. Explain the generation of pulse width modulation.

## Short answer

PAM varies pulse amplitude; PPM varies pulse position. PAM has poor noise immunity while PPM is robust. PWM is generated by comparing the message with a sawtooth: a comparator outputs a high pulse while the ramp is below the message, making pulse width proportional to the sample value.

## Answer

#### PAM vs PPM

| Point | PAM | PPM |
|---|---|---|
| Parameter varied | Amplitude of pulses | Position (time) of pulses |
| Pulse width and amplitude | Width fixed, amplitude varies | Both fixed |
| Noise immunity | Poor (noise adds to amplitude) | Good (amplitude noise can be clipped) |
| Transmitter power | Varies with signal | Constant |
| Bandwidth | Depends on pulse width; lowest | Large (needs sharp, narrow pulses) |
| Synchronisation | Not critical | Needs accurate timing reference |
| Generation | Sample-and-hold | Derived from PWM (differentiate trailing edges) |
| Detection | LPF / hold | Convert to PWM, then LPF |

#### Generation of PWM

**Pulse width modulation (PWM)** keeps amplitude and position of the leading edge fixed and varies the **width** of each pulse in proportion to the message sample.

```text
             +-----------+
m(t) ------->|  (+)      |       +------------+
             |   sum     |------>| Comparator |--> PWM
saw-tooth -->|           |       | (vs. Vref) |
(fs)         +-----------+       +------------+

saw  /|  /|  /|      m(t) + saw compared with Vref
    / | / | / |
   /  |/  |/  |
PWM  ___    _____    __
    |   |  |     |  |  |      width ~ m(nTs)
 ___|   |__|     |__|  |__
```

**Working (comparator method):**

1. A sawtooth (ramp) generator runs at the sampling frequency $f_s$.
2. The message $m(t)$ is added to the ramp (or compared directly with the ramp).
3. A comparator outputs HIGH while the ramp is below the message (or the sum exceeds the reference) and LOW otherwise.
4. The time at which the crossing occurs depends on the message value, so the **trailing edge** moves and the pulse width is proportional to $m(t)$.

Alternative: a **monostable multivibrator** triggered at $f_s$ whose timing is controlled by the message voltage (e.g. 555 timer with the message on the control pin).

**PPM from PWM:** differentiate the PWM and use the trailing-edge spikes to trigger a fixed-width monostable; pulse positions then follow the message.

## Check yourself

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

1. Which has better noise immunity?
   A. PPM, since amplitude noise can be clipped
   B. PAM, since amplitude carries information
   C. Both are equal
   D. PAM, due to constant power
   Answer: A. PPM information is in timing, not amplitude. (Key point: PAM vs PPM)

2. How is PWM generated in the comparator method?
   A. Ramp (sawtooth) at fs plus/vs message into a comparator
   B. Differentiating PPM
   C. Multiplying by a carrier
   D. Passing through a quantizer
   Answer: A. The crossing time depends on the message, so width follows m(t). (Key point: PWM generation)

3. What varies in PWM?
   A. Frequency
   B. Pulse position only
   C. Amplitude
   D. Pulse width
   Answer: D. Width is proportional to the sample value. (Key point: PWM parameter)

4. How is PPM derived from PWM?
   A. Integrate PWM
   B. Square PWM
   C. Low-pass filter PWM
   D. Differentiate PWM and trigger a fixed-width monostable from trailing edges
   Answer: D. Trailing-edge spikes mark positions. (Key point: PPM from PWM)

## Related in Communication Systems

- [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
- [ISB modulation; AM vs FM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aisb-am-vs-fm) — Communication Systems, chapter 2
- [DPCM vs delta modulation; E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adpcm-vs-dm-e1) — Communication Systems, chapter 4

## Related in other subjects

- [Single-pulse PWM: modulation index and Fourier series](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Asingle-pulse-pwm-fourier) — Power Electronics, chapter 5
- [What is sinusoidal pulse-width modulation](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Awhat-is-spwm) — Power Electronics, chapter 5
- [PAM versus PWM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apam-vs-pwm) — Data Communication, chapter 5
- [PWM half-bridge inverter switching instants](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Apwm-inverter-switching-instants) — Power Electronics, chapter 5