Posted on: July 1, 2014

What is Noise?

Noise is the undesirable electrical signal which interferes with, or distorts the desired signal. There are different types of noise; some are permanent such as the hum in power circuits or temporary such as transient noise from lightning.

The noise, which is either internally generated or exterior from other sources, affects the signals in the equipment leading to errors or malfunction. In addition, the internally generated noise can affect other interconnected equipment and systems in the vicinity. The electronic noise occurs in analog and digital systems and can degrade the quality of useful signals or data. The effect of electrical noise depends on the nature of noise and the circuit.

What is noise
Figure 1: Noise signal (top) noise over AC power – Image Credit

Characteristics of noise

Even though there are many types of noise, some characteristics are common across the various types of noise. The noise spreads across the frequency spectrum, some are low frequency while others are high, and the amplitude may be the same or vary with time. The different categories based on frequency are:

  • White noise: This has a flat amplitude at all frequencies
  • Pink noise: does not have a flat response, its power density decreases with increase in frequency.
  • Band limited noise: The noise frequency band can be reduced by the circuit through which the noise passes, or by filters

Types of noise

There are two main categories of noise, based on the source; the internal noise, generated within the equipment or device, and the external noise that come from other sources outside the equipment.

Internal noise

  • Thermal noise: This noise is generated during the energy transfer between electrons and ions. This occurs due to the collisions caused by random vibrations of the free electrons and ions in conductors. As they move around, some random current flows inside the conductor and constitutes to the noise voltage which is referred to as thermal noise.
  • Transit time noise: The random noise occurs due to the movement of charge carriers during the transit time. It is directly proportional to the frequency, hence more significant at higher frequencies
  • Shot noise: This is the most common type of noise in electronic devices; it is generated by the random movement of holes and electrons across PN junctions. Even with the correct bias that ensures the current flow in one direction, some holes and electrons still move randomly due to the discontinuities in the device such as the contact between the semiconductor material and the copper lead.
  • Miscellaneous internal noise: such as flicker noise, partition noise and transistor thermal noise.

External noise

  • Man made noise such as industrial noise, switch gear, electrical motors, aircraft and automotive ignition, fluorescent lights, leakage from high voltage lines etc. This type of noise is produced due to the arc discharges.
  • Atmospheric noise: this is random and comes from lightning discharges and other naturally occurring electrical disturbances. The atmospheric noise spreads over a wide frequency range and affects the communication equipment most.
  • Extraterrestrial noise

Effects of noise in electrical systems

The noise interferes with good signals to cause:

  • Computer malfunctions such as shutdowns, computer locks, download gaps, network problems, data drops, internet errors, etc
  • Power quality issues such as equipment failure, resets, voltage drops, etc
  • Inaccurate readings in measuring instruments
  • Video quality issues such as display bars and stripes, distorted pictures, etc
  • Audio: sound with hamming and buzzing noises
  • AC drive burnouts and errors
  • Errors in precision instruments

Reducing the effect of electrical noise

Most of external and internal noise can electrical noise can be suppressed by capacitors, metal oxide varistors as well as a variety of filter circuits.

However some noise cannot be eliminated completely, but their effect can be minimized. For example, the effects of external noise are minimized by shielding or changing the location of the equipment affected by the noise. For the internal noise, proper design and use of better components can help in eliminating or minimizing the noise.

What is noise
Figure 2: Waveform with noise and after noise suppression – Image Credit

Summary

Electronics and electrical equipment are prone to undesired noise inherent in the systems, electrical power systems, and in the atmosphere. The internally generated noise in semiconductors arises due to collisions or random movement of charge carriers. The externally generated noise includes the switching surges and lightning. Most of the noise can be minimized by proper design and use of filters to suppress the noise signal.