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:
- A source
- A transmission path
- A response (at least one response is unplanned)
EMI control refers to 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.
Grounding/Bonding: Grounding is the establishment of an electrically conductive path between an electical or electronic element of a system and a reference point or plane referenced to ground. Grounding is basically considered a circuit concept, whereas, bonding generally refers to the physical implementation of the grounding concept. Grounding also refers to an electrical connection made to the earth.
Grounding for currents whose wavelength is long compared to the circuit dimensions are best made as a single point connection. For current wavelengths the same, or shorter than the circuit dimensions, a multipoint ground should be used. A multipoint ground must be used to avoid the situation where the ground lead might be a significant fraction of a 1/4-wavelength, and not serve as a low-impedance conductor.
Shields/Shielding: The purpose of shielding is to confine radiated energy to a specific area, or to prevent radiated energy from entering a specific area. The most effective shield is a solid metallic enclosure. However, the problem with a solid enclosure is that it does not permit light, air, water, or other substances to be passed through it, so shields with holes (screens, braids, honeycomb arrangements) as well as conductive glass may be required. The use of plastic enclosures has made thin film shields vital in achieving the needed shielding effectiveness.
Electrical Noise: As stated earlier, electrical noise is basically interfering and unwanted currents or voltages in an electrical device or system. Electrical noise has a significant effect on the design and operation of almost all electrical and optical systems used to communicate or process information. Electrical noise is responsible for the familiar static heard in radios. Noise provides the fundamental limitation to the range over which radio or optical signals can be transmitted and received with integrity. Noise therefore, is of great importance to the designers of electronic equipment and systems.
Noise Sources: Most noise generation is a consequence of the spontaneous fluctuations that occur within matter at a microscopic level. In electrical circuits, these fluctuations give rise to what is commonly referred to as thermal noise. Thermal noise is generated by the random motion of free electrons in a resistor or any conductor with resistance. The random motion and resulting noise generated is proportional to the temperature of the medium. At absolute zero Kelvin (-459.67 degrees F), all motion ceases and no noise is generated.
In a system where signals are transmitted through the atmosphere, the receiving system will always receive noise as well as the desired signal. This noise is a result of thermal noise from the Earth, planets, Sun, moon, and galactic noise like radio-emitting stars. In addition, there is a small background-level uniform thermal radiation thought to be caused at the time of the origin of the universe. All these sources combined with the directional characteristics of a receiver antenna will contribute to the overall system noise.
Man-made Radio Noise - originates from a wide variety of sources, examples include car ignition systems, high-voltage power line corona discharges, arc weld ears, fluorescent lights to just name a few. The amplitude of radio noise from a man-made source decreases with distance from the source. Therefore, it can be said that the amplitude of the man-made noise at the receiver is dependent on spatial parameters, temporal variations in the noise source, and the directional properties of both the noise-radiating elements and the receiving antenna. Most cases of harmful man-made noise involve only one or two noise sources. These sources are located relatively close to the victim receiver. Generally, man-made noise levels decrease with increasing frequency. Sometimes, man-made radio noise is inductively coupled or conducted from its source into a receiver.
Thermal Noise - is an electronic circuit is defined as the random noise associated with the thermodynamic interchange of energy necessary to maintain thermal equilibrium between the circuit and its surroundings. The noise is caused by thermal agitation in a dissipating e body. The name comes from the fact that such random motion depends on the temperature of the material. Thermal noise has a uniform power spectral density, for all practical purposes over all frequencies. The thermal noise power, in Watts, available in a given bandwidth is given by the equation:
Nth = kTB
where
k = Boltzman's constant
T = the absolute temperature of the device, in Kelvin
B = the bandwidth, in Hz
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Radio Sources - Astronomy
EMI-Related Technical Articles
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