The Basics of Electrical Noise

Electrical noise is interfering and unwanted current or voltages in an electrical device or system. Electrical noise, or more simply noise, has a significant effect on the design and operation of almost all electrical and optical systems which are used to communicate or process information. Noise is responsible for the familiar static observed on home radio receivers, the clicking sounds on frequency-modulation (FM) radios operating in fringe (near-threshold) areas, and the "snow"-type granularity on the picture tube of a television receiver displaying a weak signal. In general, noise provides the fundamental limitation to the range over which radio or optical signals can be transmitted and received with integrity. Noise is, therefore, of great importance to engineers who design and operate such systems.

It's convenient to differentiate between noise which results from human activity and that which is naturally occurring. Noise which results from human activity, such as that generated by an electrical appliance or an automotive ignition, can usually be eliminated or minimized by good design practice (shielding, filtering, equipment location, etc.). Naturally-occurring noise can be further subdivided into that which is irregular or erratic in nature and that which is more or less continuous.

An example of noise which is irregular or erratic is that associated with an electrical storm. This type of noise is sometimes dealt with in the system design, but since it's only occasionally present, it does not ordinarily constitute a design limitation. On the other hand, naturally occurring noise which is essentially continuous in time is responsible for the fundamental limitation cited above. The remainder of the article therefore concentrates on this type of noise.

Most noise generation is a consequence of the spontaneous fluctuations which occur within matter at the microscopic level. In electrical circuits these fluctuations give rise to what are commonly referred to as thermal noise and shot noise. Thermal noise is generated by the random motion of free electrons in a resistor or any conductor with resistance. The random motion, and thus the noise generated, is proportional to the temperature of the medium. At absolute zero temperature on the Kelvin scale (-459.67'F), all motion ceases and no noise is generated. Shot noise is most commonly identified with the fluctuations in the current of a vacuum tube caused by the random emission of electrons from its heated cathode. Shot noise is also observed in semiconductor devices as random fluctuations in carrier density when an electric field is applied. There are other types of noise associated with electrical circuits, but shot noise and thermal noise are by far the most important.

In a system in which signals are transmitted through the atmosphere [for example, amplitude-modulation (AM) or FM radio broadcast, or satellite communications], the receiving system will always receive noise as well as the desired signals. This noise is a result of thermal radiation from the Earth, planets, Sun, Moon, the galaxy (galactic noise), radio-emitting stars, and atmospheric gases. In addition, there is a small background level of thermal radiation, uniformly distributed, which is believed associated with the big bang origin of the universe. All of these noise sources, weighted by the directional characteristics of the receiving antenna, will contribute to the overall system noise.

In an optical communications system, a signal level is represented by a number of energy packets called photons. The mean arrival rate of the photons at the detector is proportional to the optical intensity or signal strength. At the detector (a photodiode), the photons are absorbed, each creating a hole-electron pair and thus a current in which the electrons are randomly positioned in time and in which the mean number of electrons is proportional to the optical intensity. The statistical nature of this process gives rise to fluctuations in the number of photons representative of a given level and , subsequently, the number of electrons generated to represent that level. If the detector has internal gain as in an avalanche photodiode, each hole-electron pair can create additional hole-electron pairs. This process, however, is statistical in nature, resulting in a mean value of gain but giving rise to additional fluctuations in the generated current.

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