---
title: "Noise; internal vs external; sources of external noise"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:noise-internal-external-sources"
source: https://www.bctengineer.com/q/semester_6/communication_systems/noise-internal-external-sources
site: BCT Engineering
---

# Noise; internal vs external; sources of external noise

Communication Systems · Chapter 1: Introduction

- Asked in: 2072 Magh (CS I), 2+2+2 marks
- Video: [CT#33 NOISE in a communication system || EC Academy](https://www.youtube.com/watch?v=u9nuKs2RAe0) — EC Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/noise-internal-external-sources
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

What is noise? Distinguish between internal and external noise. List various sources of external noise.

## Short answer

Noise is unwanted random disturbance that reduces SNR. Internal noise originates inside the receiver (thermal, shot, etc.), can be calculated; external noise comes from outside (atmospheric, cosmic, industrial) and is hard to predict. External sources: lightning (atmospheric), solar radiation, cosmic/galactic noise, man-made noise from motors and ignition.

## Answer

#### Noise
**Noise** is any unwanted, random electrical disturbance that adds to the desired signal in a communication system and tends to mask it, reducing the signal-to-noise ratio. Example: hiss in a radio, snow on an analog TV picture.

#### Internal vs external noise

| Basis | Internal noise | External noise |
|---|---|---|
| Origin | Inside the receiver/devices | Outside the receiver |
| Examples | Thermal, shot, partition, flicker | Atmospheric, cosmic, industrial |
| Predictability | Can be calculated (e.g. $kTB$) | Hard to predict |
| Control | Low-noise design, cooling, narrow $B$ | Shielding, site choice, directional antenna |

#### Sources of external noise
1. **Atmospheric noise:** lightning discharges and other natural electrical disturbances (static); serious below 30 MHz.
2. **Solar noise:** radiation from the sun, especially during sunspot activity.
3. **Cosmic (galactic) noise:** radiation from distant stars and galaxies, about 8 MHz to 1.5 GHz.
4. **Industrial (man-made) noise:** automobile ignition, electric motors, switching equipment, fluorescent lights, welding machines, high-voltage power lines.

## Check yourself

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

1. Which pair correctly lists internal noise types?
   A. Atmospheric and cosmic
   B. Thermal and shot
   C. Solar and industrial
   D. Industrial and flicker
   Answer: B. Internal noise arises inside the receiver/devices: thermal, shot, partition, flicker. Atmospheric, solar, cosmic and industrial are external. (Key point: Internal vs external noise)

2. Why can internal noise be controlled better than external noise?
   A. It can be calculated (e.g. kTB) and reduced by low-noise design, cooling and narrow B
   B. It always has a fixed frequency
   C. It only occurs above 30 MHz
   D. It is created by the transmitter
   Answer: A. Internal noise is predictable, so cooling, low-noise design and narrow bandwidth reduce it; external noise is hard to predict. (Key point: Predictability)

3. Atmospheric noise is mainly caused by what, and where is it serious?
   A. Distant galaxies, above 1.5 GHz
   B. Lightning discharges, serious below 30 MHz
   C. Car ignition, in the VHF band
   D. The sun, only at night
   Answer: B. Lightning and natural static produce atmospheric noise, which is serious below about 30 MHz. (Key point: Atmospheric noise)

4. Which of the following is an industrial (man-made) noise source?
   A. Sunspot activity
   B. Galactic radiation
   C. Fluorescent lights and welding machines
   D. Lightning
   Answer: C. Ignition, motors, switching gear, fluorescent lights and welding machines are man-made noise sources. (Key point: Industrial noise)

## Related in Communication Systems

- [External vs internal noise; interference sources; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aexternal-internal-noise-interference-needs) — Communication Systems, chapter 1
- [Noise, distortion, interference; internal and external noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-types-internal-external) — Communication Systems, chapter 1
- [How noise limits performance; types of internal noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-limits-internal-noise-types) — Communication Systems, chapter 1

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