---
title: "Noise, distortion, interference; internal and external noise"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:noise-types-internal-external"
source: https://www.bctengineer.com/q/semester_6/communication_systems/noise-types-internal-external
site: BCT Engineering
---

# Noise, distortion, interference; internal and external noise

Communication Systems · Chapter 1: Introduction

- Asked in: 2076 Bhadra (CS I), 2+5 marks
- Video: [Noise #2. Internal (Thermal, Transistor & Freq Dependent) Vs External Noise (Pwr Line & Atmospheric)](https://www.youtube.com/watch?v=Mj6oTk8fOh0) — Technologies Discussion
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/noise-types-internal-external
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

Define noise, distortion and interference in communication system. Describe the types and causes of internal and external noise that may affect the communication system.

## Short answer

Noise is unwanted random signal; distortion is change in signal shape from imperfect system response; interference is contamination from other sources. External noise includes atmospheric (lightning), extraterrestrial (solar, cosmic) and industrial (motors, sparks). Internal noise types: thermal (Johnson) noise kTB, shot noise, partition noise, flicker (1/f) noise and transit-time noise.

## Answer

#### Definitions
- **Noise:** any unwanted, random electrical signal that adds to the wanted signal and tends to mask it, e.g. hiss in a radio receiver.
- **Distortion:** an unwanted change in the **shape** of the signal caused by the imperfect response of the system or channel (non-flat gain, non-linear phase, non-linearity). It vanishes when the signal is removed.
- **Interference:** contamination by unwanted signals from **other sources**, usually other transmitters or circuits, e.g. adjacent-channel signals, crosstalk.

#### External noise (generated outside the receiver)
1. **Atmospheric noise (static):** produced by lightning discharges and other natural electrical disturbances; heard as crackling; strong below about 30 MHz, worse in the tropics and in thunderstorm seasons.
2. **Extraterrestrial noise:**
   - *Solar noise:* radiation from the sun's hot surface; increases greatly during sunspot activity.
   - *Cosmic (galactic) noise:* from distant stars and galaxies; noticeable from about 8 MHz to 1.5 GHz.
3. **Industrial (man-made) noise:** from automobile ignition, electric motors, switchgear, fluorescent lamps, welding, high-voltage lines; caused by sparking and switching; most intense in cities, from about 1 to 600 MHz.

#### Internal noise (generated inside the receiver/devices)
1. **Thermal (Johnson) noise:** random motion of free electrons in a conductor due to temperature. Noise power $P_n = kTB$ and rms voltage $V_n = \sqrt{4kTBR}$, where $k = 1.38\times10^{-23}$ J/K. It is white (flat spectrum) and Gaussian.
2. **Shot noise:** random arrival of charge carriers across a junction or at an anode in diodes, transistors and tubes; $I_n = \sqrt{2qI_{DC}B}$.
3. **Partition noise:** random division of current between two or more electrodes (e.g. emitter current dividing into base and collector currents).
4. **Flicker (1/f) noise:** due to surface and material irregularities in devices; its power falls as frequency increases, important below a few kHz.
5. **Transit-time noise:** at very high frequencies, when carrier transit time across a device is comparable to the signal period, causing random input current; it rises with frequency.

| Basis | External noise | Internal noise |
|---|---|---|
| Origin | Outside the receiver | Inside components/devices |
| Examples | Atmospheric, cosmic, industrial | Thermal, shot, flicker |
| Control | Hard; site choice, shielding, directivity | Good design, low-noise devices, cooling |

## Check yourself

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

1. Which description fits distortion?
   A. Thermal agitation of electrons
   B. Unwanted change in signal shape from imperfect system response, which vanishes when the signal is removed
   C. Contamination from another transmitter
   D. Random signal added from outside
   Answer: B. Distortion arises from non-flat gain, non-linear phase or non-linearity and disappears with no signal; noise remains. (Key point: Distortion vs noise)

2. Industrial (man-made) noise is caused mainly by:
   A. Lightning discharges
   B. Solar radiation
   C. Sparking and switching, e.g. ignition, motors, welding
   D. Random electron motion in resistors
   Answer: C. Sparking/switching from ignition, motors and lamps; it is most intense in cities. (Key point: Industrial noise)

3. Thermal noise power in bandwidth B at temperature T is:
   A. kT/B
   B. kTB
   C. k B / T
   D. 2qIB
   Answer: B. P_n = kTB; it is white and Gaussian. (Key point: Thermal noise)

4. Shot noise is caused by:
   A. Crosstalk
   B. Random arrival of discrete charge carriers across a junction
   C. Cosmic radiation
   D. Heat in conductors
   Answer: B. Current is discrete charges arriving randomly, with I_n = sqrt(2 q I_DC B). (Key point: Shot noise cause)

## 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 vs interference vs distortion](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-interference-distortion) — Communication Systems, chapter 1
- [Distortion vs noise vs interference](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adistortion-noise-interference-diff) — Communication Systems, chapter 1
- [Noise, interference, distortion; thermal and high frequency noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-effects-thermal-hf) — Communication Systems, chapter 1
- [Reasons for modulation; noise vs distortion vs interference](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-reasons-noise-distortion-compare) — Communication Systems, chapter 1
- [Distortion, noise, interference; four types of noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adistortion-noise-interference-four-noise) — Communication Systems, chapter 1

## Related in other subjects

- [Internal/external drainage; water logging; Terai surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainternal-external-drainage-water-logging-terai) — Irrigation and Drainage Engineering, chapter 9
- [Noise, thermal noise, delay distortion](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Anoise-types-thermal) — Data Communication, chapter 1
- [Page size impact; internal, external fragmentation](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Apage-size-impact-fragmentation) — Operating System, chapter 4
- [Risk planning; internal, external; risk reduction](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Arisk-reduction-internal-external-planning) — Project Engineering, chapter 5