---
title: "Noise, interference, distortion; thermal and high frequency noise"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:noise-effects-thermal-hf"
source: https://www.bctengineer.com/q/semester_6/communication_systems/noise-effects-thermal-hf
site: BCT Engineering
---

# Noise, interference, distortion; thermal and high frequency noise

Communication Systems · Chapter 1: Introduction

- Asked in: 2071 Bhadra (CS I), 3+1+3 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-effects-thermal-hf
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

Define noise, interference and distortion? What will be their effects in communication? Explain briefly thermal and high frequency noise.

## Short answer

Noise is random unwanted signals (thermal, shot) degrading SNR; interference is external man-made signals causing crosstalk; distortion is system-imperfections causing waveform change. Thermal noise: P_n=kTB, white. High-frequency (transit-time) noise arises when carrier transit time approaches signal period, causing excess noise at microwave frequencies.

## Answer

#### Definitions
- **Noise:** random, unwanted electrical signals (from natural or electronic sources) that add to the wanted signal, e.g. thermal noise in resistors, shot noise in transistors, atmospheric static.
- **Interference:** contamination by unwanted signals from **other man-made sources**, usually other transmitters or electrical machines, e.g. a neighbouring radio station, power-line hum, crosstalk between wires.
- **Distortion:** alteration of the signal waveform caused by the **imperfect response of the system itself** (channel, amplifier, filter), e.g. clipping in an amplifier, unequal attenuation of frequencies in a cable. It disappears when the signal is switched off.

#### Effects in communication
- Noise lowers the signal-to-noise ratio, causing hiss in analog systems and bit errors in digital systems; it sets the minimum detectable signal and limits range and channel capacity ($C=B\log_2(1+S/N)$).
- Interference produces crosstalk, whistles and unwanted programmes; it limits frequency reuse and needs filtering, shielding and frequency planning.
- Distortion changes the waveform shape, causes intersymbol interference (ISI) in digital links and poor fidelity in audio; it can be reduced by equalizers.

#### Thermal noise
Thermal (Johnson or Nyquist) noise is produced by the random motion of free electrons in any conductor or resistor above absolute zero. Its power is proportional to absolute temperature and bandwidth, and its spectrum is flat (white) up to very high frequencies:

$$
P_n=kTB,\qquad V_{n(rms)}=\sqrt{4kTBR}
$$

where $k=1.38\times10^{-23}$ J/K, $T$ in kelvin, $B$ in Hz. Example: $R=1\ \text{k}\Omega$, $T=290$ K, $B=10$ kHz gives $V_n\approx0.4\ \mu\text{V}$.

#### High-frequency (transit-time) noise
At very high frequencies, the time taken by carriers (electrons or holes) to travel from input to output of a device, e.g. emitter to collector of a transistor, becomes comparable to the period of the signal. Some carriers then diffuse back toward the input, producing an input admittance whose conductive part grows with frequency and creates random noise currents. This **transit-time noise** increases rapidly above a cut-off frequency, which is why every active device has an upper usable frequency and why microwave receivers use special low-noise devices.

## Check yourself

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

1. Which best distinguishes distortion from noise and interference?
   A. It is caused by the imperfect response of the system itself and disappears when the signal is off
   B. It comes from neighbouring stations
   C. It is random thermal motion
   D. It is only present in digital links
   Answer: A. Distortion is waveform alteration from the system itself. (Key point: Definitions)

2. Thermal noise power is:
   A. kTB, proportional to temperature and bandwidth
   B. k/(TB)
   C. Independent of temperature
   D. Proportional to carrier frequency only
   Answer: A. P_n = kTB; spectrum is flat (white). (Key point: Thermal noise)

3. How does noise limit channel capacity?
   A. Through C=B log2(1+S/N), lower SNR gives lower capacity
   B. It raises the bandwidth
   C. It has no effect on digital systems
   D. It increases capacity
   Answer: A. Shannon's formula depends on S/N. (Key point: Effect of noise)

4. What causes high-frequency (transit-time) noise?
   A. Lightning
   B. Carrier travel time becomes comparable to signal period, so some carriers diffuse back
   C. Random electron motion at low frequency
   D. Power-line hum
   Answer: B. At high frequencies transit time effects give input admittance growing with frequency and random currents. (Key point: Transit-time noise)

## Related in Communication Systems

- [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, 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
- [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
- [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

## Related in other subjects

- [Noise, thermal noise, delay distortion](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Anoise-types-thermal) — Data Communication, chapter 1
- [Short note: Interference in mobile communication](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ainterference-mobile-communication) — Wireless Communication, chapter 2
- [Short note: Noise and its effects](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ashort-noise-effects) — Building Technology, chapter 1
- [Typical noise levels and effects of noise](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Anoise-levels-places-effects) — Environment and Pollution Control, chapter 6