Correlated and Uncorrelated Sources of Noise in Communication Systems

Electrical noise is defined as any unwanted electrical energy that falls within the signal’s passband.

Noise can be broadly classified into two types:

Correlated noise

Correlation denotes a connection between the signal and the noise. As a result, correlated noise exists only when there is a signal.

Uncorrelated noise

Uncorrelated noise exists whether or not there is a signal. Uncorrelated noise is further classified into two broad categories:

  1. External
  2. Internal

External Noise Sources

Atmospheric Noise

Natural disruptions in the earth’s atmosphere, with lightning, and discharges being the most significant causes. It is frequently in the form of an impulse that spreads energy throughout an extensive frequency range.

Extraterrestrial Noise (Space Noise)

Space noise is the noise that originates outside the earth’s atmosphere (outer space).

Divided into two categories:

Solar noise – caused by the sun’s heat. The sun emits a wide range of frequencies, including those used for broadcasting.

Cosmic noise – noise that comes from stars other than the sun.

Human-made Noise

Produced by humans. Produced by spark-producing equipment. For example, a car engine, switching equipment, or fluorescent light.

Internal Noise Sources

Shot Noise

Caused by the random arrival of carriers (holes and electrons) in semiconductor pn junctions. The carrier is not flowing in a continuous and constant flow, but rather in a random direction.

Transit-time Noise

Noise is generated in semiconductors when the transit time of carriers crossing a junction is near the signal period and some of the carriers diffuse back to the semiconductor’s source or emitter. The time it takes for a carrier to travel through a device causes irregular and random noise variations.

Thermal Noise (White Noise or Johnson Noise)

Generated by the agitation and interaction of electrons in a conductor due to heat.

White Noise – another name for thermal noise because its frequency content is uniform across the spectrum.

Johnson Noise – another name for thermal noise, first studied by J.B. Johnson.

Correlated Noise

It is a type of internal noise that is associated with the signal and cannot exist in a circuit unless there is a signal, i.e. NO SIGNAL, NO NOISE!

Nonlinear amplification produces harmonic and intermodulation distortion, both of which are types of nonlinear distortion.

Nonlinear distortion generates undesired frequencies that interfere with the signal and hinder performance.

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