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Electromagnetic Interference EMI
ELECTROMAGNETIC INTERFERENCE: Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics and electrical equipment. It can be induced intentionally, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like.
EMI is said to exist when unwanted voltages or currents are present so that they adversely affect the performance of an electrical device or electronic system. These voltages/currents can reach the victim circuit or device by conduction or by non ionizing radiation. In all cases, EMI occurs because of a combination of three factors:
EMI control refers to the process of making design changes or adjustments of the signal or noise levels in order to achieve EMC.
Interference Sources: Designers do not plan and design their equipment with the intention of being sources of interference. However, unintentionally, what is a desired signal in one path is an undesired signal (considered noise) in an inadvertent coupling. Interference may pose as an arc discharge, radiation from a lightning strike, a corona discharge from power lines, or a noise caused by a sudden change in current flow in a conductor.
Functional interference often includes: sine waves, computer clock pulses, speech or video waves, or pulses forming data trains. An example of this interference type is signal leakage from cable TV systems. Fluorescent lamps, commutators, car ignition systems, and industrial, scientific and medical equipment all constitute sources of interference - and this includes EMP that accompanies a nuclear explosion.
EMI always starts with current flow through a conductor and also shows up in the victim equipment in the form of a current or voltage. The coupling path (including a conducting gas or air) might be a conduction or radiation path. The actual paths can include common wiring, capacitance between devices, mutual inductance between adjacent wiring, nonionizing radiation, or wires in an EM field. This type of coupling is aided by the fact that all conductors exhibit resistance and inductance.
Nth = kTBwherek = Boltzmann's constantT = the absolute temperature of the device, in KelvinB = the bandwidth, in Hz
Nth = kTB
k = Boltzmann's constant
T = the absolute temperature of the device, in Kelvin
B = the bandwidth, in Hz
Types of Interference:
Equipment Malfunctions due to EMI
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