---
title: "External vs internal noise; interference sources; need for modulation"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:external-internal-noise-interference-needs"
source: https://www.bctengineer.com/q/semester_6/communication_systems/external-internal-noise-interference-needs
site: BCT Engineering
---

# External vs internal noise; interference sources; need for modulation

Communication Systems · Chapter 1: Introduction

- Asked in: 2075 Bhadra (CS I), 2+2+4 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/external-internal-noise-interference-needs
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

Distinguish between external and internal noise. List out the sources of interferences. Write the needs of modulation in communication system.

## Short answer

External noise originates outside the receiver (e.g., atmospheric, industrial), while internal noise is generated within components (thermal, shot). Interference sources include co-channel, adjacent-channel, image frequency, crostalk, intermodulation products, power-line hum, multipath, and spurious emissions. Modulation is needed for practical antenna size, multiplexing, noise immunity, long-range propagation, and narrowbanding.

## Answer

#### External vs internal noise

| Basis | External noise | Internal noise |
|---|---|---|
| Source | Generated outside the receiver | Generated within receiver components |
| Types | Atmospheric, solar, cosmic, industrial | Thermal, shot, partition, flicker, transit-time |
| Cause | Lightning, sun, stars, motors, ignition | Random electron motion, carrier arrival |
| Frequency range | Mostly significant at LF–VHF | Present at all frequencies (thermal is white) |
| Measurement | Difficult to predict | Can be calculated, e.g. $kTB$ |
| Reduction | Shielding, site selection, directional antennas | Low-noise devices, cooling, narrow bandwidth |

#### Sources of interference
1. **Co-channel interference:** another transmitter on the same frequency (e.g. distant cells reusing the frequency).
2. **Adjacent-channel interference:** strong signals in neighbouring channels leaking through imperfect filters.
3. **Image-frequency interference** in superheterodyne receivers.
4. **Crosstalk** between neighbouring wire pairs or cables.
5. **Intermodulation products** from non-linear amplifiers mixing several signals.
6. **Power-line hum**, switching supplies and electrical machines.
7. **Multipath echoes** and reflections.
8. Harmonics and spurious emissions from other transmitters.

#### Needs of modulation
1. **Practical antenna size:** antenna length should be about $\lambda/4$. A 10 kHz signal has $\lambda = 30$ km (7.5 km antenna), but on a 10 MHz carrier the antenna is only 7.5 m.
2. **Multiplexing:** different messages on different carriers can share one channel (FDM), e.g. many radio stations.
3. **Noise and interference reduction:** wideband modulation (FM, spread spectrum) improves SNR.
4. **Narrowbanding:** reduces the high-to-low frequency ratio, easing antenna and amplifier design.
5. **Long-range propagation** and fitting the signal into the allotted frequency band of the channel.

## Check yourself

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

1. Which pair is correctly classified?
   A. Both are internal
   B. Atmospheric: internal; thermal: external
   C. Both are external
   D. Atmospheric: external; thermal: internal
   Answer: D. Atmospheric, solar, cosmic, industrial are external; thermal, shot, partition, flicker are internal. (Key point: External vs internal noise)

2. Co-channel interference arises from:
   A. Image frequency of superheterodyne
   B. Another transmitter on the same frequency
   C. Neighbouring channel leaking through filters
   D. Wire-pair coupling
   Answer: B. Co-channel means same frequency, e.g. distant cells reusing a frequency. (Key point: Co-channel interference)

3. For a 10 kHz signal (lambda = 30 km), a lambda/4 antenna is 7.5 km. On a 10 MHz carrier it becomes:
   A. 7.5 km
   B. 75 m
   C. 750 m
   D. 7.5 m
   Answer: D. lambda = 30 m at 10 MHz, so lambda/4 = 7.5 m. (Key point: Antenna example)

## Related in Communication Systems

- [Noise; internal vs external; sources of external noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-internal-external-sources) — Communication Systems, chapter 1
- [Noise vs interference; minimizing effects; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-interference-minimize-modulation-need) — 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
- [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
- [Communication system block diagram; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acomm-block-diagram-needs-modulation) — Communication Systems, chapter 1
- [Noise vs interference and their limitations](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-interference-limitations) — Communication Systems, chapter 1

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