EMI Sources

Share Embed Donate

Short Description

Download EMI Sources...


INTRODUCTION The sources of electromagnetic interference are both natural and human-made. Natural sources include sun and stars, as well as phenomena such as atmospherics, lightning, thunderstorms, and electrostatic discharge. On the other hand, electromagnetic interference is also generated during the practical use of a variety of electrical, electronic, and electromechanical apparatus. This interference, which is generated by various equipment and appliances, is human-made. We will mention several sources of electromagnetic interference. This report presents a description of the sources and nature of natural electromagnetic noise. Although electromagnetic pulses (EMPs) caused by nuclear explosions cannot be said to be a natural phenomenon, the electromagnetic disturbances generated by an EMP are analogous to disturbances caused by natural atmospheric phenomena in their most severe and extreme form. It is, therefore, convenient for the purpose of analysis to treat electromagnetic pulses along with natural phenomena like lightning and electrostatic discharge.

ELECTROMAGTECIC INTERFERENCE Electromagnetic Interference (EMI) is everywhere, inside and outside of all electrical equipment. Man-made EMI is on the increase daily, due to the increased use of electrical and electronic equipment. The successful detection and elimination of EMI require a systematic search for EMI sources as well as a knowledge of interference susceptibilities of the equipment. Familiarity with the environment in which the equipment will work as well as possible alternative environments are fundamental to effective EMI reduction to a required minimum level. What is Electromagnetic Interference (EMI)? Every electronic device is a source of radiated electromagnetic fields called radiated emissions. These are often an accidental by-product of the design. Electromagnetic interference (EMI) is the degradation in a device‟s performance due to these fields which make up the “electromagnetic environment.” Every electronic device is susceptible to EMI. Its influence can be seen all around us. The results include: “ghosts” in TV picture reception, taxicab radio interference with a police radio system, power line transient interference with personal computers, and self-oscillation of a radio receiver or transmitter circuit. Electromagnetic compatibility (EMC) is when a device functions satisfactorily without introducing intolerable disturbances to the electromagnetic environment or to the other devices therein. EMC is achieved when electronic devices coexist in harmony, such that each device functions according to its intended purpose in the presence of, and in spite of, the others. EMI is the problem that occurs when unwanted voltages or currents are present to influence performance. EMC is the solution. This is achieved by applying proven design techniques, the use of which assures a system relatively free of EMI problems. Telecommunication is made possible through radio waves which are traveling electromagnetic waves. Every wireless communication device including cell phones, radio, GPS tracking devices and satellite communication devices rely on radio waves for transmission and reception of signals. What leads to electromagnetic interference is the property of these radio waves to superimpose on each other.

As we know from radios, its receiver antenna is tuned to receiving a certain band of radio frequencies. All signals in that range are received by the antenna, irrespective of what source it is radiated from. Any other electric source that transmits electromagnetic radiation in those frequency bands will cause interference in the reception of that antenna. Remembering some elementary physics, every changing electric field gives rise to a magnetic field and every oscillating electric field gives rise to a magnetic field. That is how electromagnetic waves travel through space. They need no transmission medium. Any received electromagnetic wave generates an EMF (electromotive force) in the antenna conductor which leads to noise in the connected circuit. The absence of selectivity in reception of electromagnetic waves leads to interference. This problem plagues amplitude modulated (AM) radio broadcasts, which possess selectivity. However, modern wireless networking technologies (like Wi-Fi) have selectivity in receiving radio waves. A plethora of techniques like frequency hopping, error correction and spread spectrum have been devised that reduce noise due to electromagnetic interference to a minimum. Sources and Types of EMI Let‟s classify EMI in terms of its causes and sources. The cause of EMI may be within the system, in which case it is termed an intrasystem problem or from the outside, in which case it is called an intersystem problem. The term emitter is commonly used to denote the source of EMI. The term susceptor is used to designate a victim device. Both intrasystem and intersystem EMI generally can be controlled by the system design engineer if she or he just follows some design guidelines and techniques. The sources of EMI can be classified as natural and artificial (man-made). The origins of EMI are basically undesired conducted emissions (voltages and/or currents), or radiated emissions (electric and/or magnetic fields). Conducted emissions are those currents that are carried by metallic paths (the unit‟s power cord) and place on the common power net. Here they may cause interference with other devices also connected to this net. Radiated emissions concern the electric fields radiated by the device which may be received by other electronic devices causing interference in those other devices. Figure 1 illustrates the conceptual difference between conducted and radiated path.

Figure 1. Differences between conducted and radiated emissions

Natural electromagnetic noise sources existing since the origin of the Earth may be influencing the evolution of living systems. Different sources generate natural noise inside the magnetospheric cavity at different frequencies. A primary source of noise is given by the interaction of particles and waves coming from outer space with the magnetosphere plasma at various altitudes; another source is inside the ionospheric cavity where atmospheric lightning discharges produce several interesting propagating phenomena. At higher frequency radio noise originated in the atmosphere becomes less important and cosmic noise is prevalent up to the millimetric wavelength. Human technologies implanted for power transmission and communications are the well known causes of man-made noise. Man-made noise, mainly due to communication and broadcasting systems, electric energy transport systems, automotive ignition, industrial thermal processes and instruments for scientific/medical appliances is distributed, albeit not uniformly, in all bands. In VLF-HF band atmospheric noise is still larger than man-made noise in the order of a tenth of dB in rural areas and simply overcomes man-made noise in business areas. In general man-made noise is concentrated in particular frequencies in association with technological applications. Both contributions, natural and man-made sources, affect technological applications such as communication, remote sensing, etc. Figure 2 shows a taxonomy of the different sources of electromagnetic interference. A brief description of each category will be given below.






Conducted Coherent Radiated Broadband Conducted Incoherent Radiated Man-made Intentional Conducted Unintentional Narrowband






Figure 2. Taxonomy of EMI Sources 

 

Natural EMI sources - Sources that are associated with natural phenomena. They include atmospheric charge/discharge phenomena such as lightening and precipitation static and extraterrestrial sources including radiation from the sum and galactic sources such as radio stars, galaxies, and other cosmic sources. As shown in the above diagram, all natural sources are classified as broadband, incoherent, radiated, and unintentional. Man-made EMI sources - Sources associated with man-made devices such as power lines, auto ignition, fluorescent lights, etc. Broadband EMI - Electromagnetic conducted and radiated signals whose amplitude variation as a function of frequency extends over a frequency range greater than the bandwidth of the receptor. Narrowband EMI - Electromagnetic conducted and radiated signals whose amplitude variation as a function of frequency extends over a frequency range narrower than the bandwidth of the receptor. Coherent broadband signals - Neighboring components of the signal (in the frequency domain) has a well-defined amplitude, frequency, and phase relationship.

  

 

Incoherent broadband signals - Neighboring components of the signal (in the frequency domain) are random or pseudo-random (bandwidth limited) in phase or amplitude. Conducted EMI - Noise signals transmitted via electrical conduction paths (i.e. wires, ground planes, etc.). Radiated EMI - Electric and magnetic fields transmitted through space from source to receptor. Intentional radiating emitters - Emitters whose primary function depends on radiated emitters. Examples include electronic licensed communication systems. These include communication, navigation, and radar systems. Unintentional (incidential) radiating devices - Devices that radiate radio frequencies but is not considered their primary function. Restricted radiating devices - Devices that intentionally use electromagnetic radiation for purposes other than communication or data transfer. (i.e. garage door operating systems, wireless microphones, etc.)

Man-made noise is originated by human technologies. It is strongly dependent on the distance from the sources (power lines, radio, TV communication installations and other) which can be very variable, on frequency and on emitted power. Power and frequency in their time and spatial distribution are the quantities that best describe all man-made radio sources. Other important characteristics are the type of emitted waves, continuous or impulsive, their modulation and wave polarization. Terrestrial man-made sources are mainly located in business, industrial and residential areas. In rural (countryside) and „quiet rural‟ areas the sources become scarce. In spite of its relevance, man-made noise power, depending on frequency band, is on average 20-30 dB lower, in „quiet rural‟ areas than the business and residential area. Of course, starting from the 1960‟s, satellite communication systems also contribute to man-made noise measurable on Earth. As in the case of other frequencies, man-made noise emissions are due to electronic devices for indoor use, industrial and radio communication apparatuses, including broadcasting radios, since all this equipment employs electrical oscillators that become unwanted sources of artificial radio emission. The main localized sources are radio AM broadcasting (see Figure 3) and some industrial equipment. Among the latter relevant sources are heaters that exploit magnetic induction and dielectric losses in the material. Other weaker but numerous sources, such as automotive ignition, heavily contribute to the background noise. We have also immeasurable indoor sources such as electrical household appliances, PC monitors TV, etc. Other scientific and medical devices

are irrelevant in the background radio noise configuration in the considered range of frequency.

Figure 3. Power emitted by the most common communication systems.

Celestial Electromagnetic Noise It is well known that celestial bodies like the sun, stars, and galaxy are at a very high temperature. The electromagnetic radiation from these bodies can be attributed to the random motion of charged ions resulting from thermal ionization at very high temperatures. The process of burning has subsided in celestial bodies like planets and moon. However, for some interval of time, one side of these bodies is exposed to the sun and is heated to extremely high temperatures as it captures thermal radiation from the sun. These heated parts of the celestial bodies emit thermal noise. The characteristics of such emissions depend upon the temperature attained by these bodies. The sources of extraterrestrial emissions have an approximately continuous as well as discrete distribution. Potential sources of discrete emission are the sun, moon, and Jupiter. They emit broadband as well as narrowband electromagnetic noise. Radiation from the sun changes drastically during solar flares and sunspot activity. Continuous sources like the galaxy normally emit broadband electromagnetic noise.

The level of electromagnetic noise emitted by a cosmic source does not vary appreciably with time, unless the source itself undergoes a change which results in a corresponding variation in the emitted electromagnetic noise. However, the cosmic noise received at a given point on earth varies with the time of the day because earth rotates around the sun and also revolves around its own axis. The broad spectrum noise extending from meter wavelengths to centimeter and lower wavelengths was originally linked with our own galaxy. It was subsequently discovered to emanate from all directions in the cosmos. Hence, what was originally described as galactic noise was later given the more general name of cosmic noise. EFFECTS OF EMI There are numerous factors that affect radio wave propagation, which include transmission power, antenna gain, attenuation factors, and noise. In addition, there are ionospheric and tropospheric conditions that also have a dramatic effect on the propagation of radio transmission through free-space. The Basic Effects are:  Atmospheric Effects The transmission of High Frequency HF signals (3 MHz - 30 MHz) for distances greater than approximately 100 miles depends on sky wave propagation, due to the ionization levels that exist in the ionosphere. The effects of the ionosphere on signals in the medium and high frequency regions are more pronounced than at VHF/UHF ranges (30 MHz–300 MHz/300 MHz –3 GHz) .The transmission of VHF/UHF signals depend on ground wave propagation and almost always follow line-of-sight paths, which requires no obstructions between the transmitting and receiving stations. In effective satellite communication the calculation of free-space transmission loss is the primary consideration effect. In addition, noise levels from terrestrial, man-made, and cosmic sources can also interfere and negatively affect signal propagation.  Transients due to EMI Electrical disturbances pose a great threat to electrical equipment and data. Electrical disturbances go by various names such as spikes, surges, and transient surge voltages. Regardless of the name, the effects of these disturbances remain the same: disruption, degradation and damage. With the increasing popularity of computer networking, the effect of transient surges on

communication lines is also of great importance. Transient surges may cause serious damage to communication interfaces inside a building. There are different ways a transient surge may be created, a single surge suppression layer applied to incoming lines may not be appropriate to completely shield the internal lines and equipment from transient voltages. Transients can be present on any conductor such as utility power lines, telephone, data, and signal lines. Spikes, a type of transient surge, are short-term over-voltages, which are usually measured in milliseconds. This unwanted excess of electrical energy could be created easily on any conductive line. The energy content of transients can be enormous and can damage equipment, or cause it to malfunction by giving faulty signals due to inaccurate voltage levels. Equipment driven by microprocessors and other integrated circuits (IC) are especially vulnerable to transient voltage surges. Inductive coupling, created from various sources, is usually the cause for transients specific to data lines. A transformer produces a magnetic field extending from a coiled wire in the primary, which induces a voltage in the coiled wire of the secondary. Under the same principles, wires that run adjacent to one another within a building can magnetically couple transients, as shown in Figure 4. This coupling can be caused by a power line, which induces a voltage in an adjacent data line, or from one data line to another (which is usually referred to as crosstalk).

Figure 4. Inductive coupling.

Lightning can cause a much more powerful kind of magnetic coupling, which can cause sudden damages to multiple items in a single strike. Figure 5 shows a lightning bolt striking the ground. This lightning bolt is surrounded by a very powerful magnetic field. In much the same way a magnetic field from one conductor can induce transients on an adjacent conductor, the magnetic field of

a lightning strike can induce power in an external power line without actually striking the line directly.

Figure 5. Magnetic field created from a lightning strike

There are other major sources of coupling that can be detrimental to data infrastructure. When planning or inspecting the layout of data lines, the following sources of inductive coupling should be addressed:  Data lines that are draped over power conduits.  Running data cables near lightning down conductor (down conductors are lines or structures in a building designed to convey lighting discharge current in a building to ground).  Running data cables near building steel (especially in the vicinity of lightning down conductors).  Running data lines too close to fluorescent lighting (which emit EMI). Much of the electrical equipment seen today is built on integrated circuit and microprocessor technology. Because of certain characteristics common to integrated circuits and microprocessors, this equipment is especially sensitive to transient voltage surges. Microprocessor based and controlled devices can be found in almost every setting. Some of this electronic equipment includes computers and their peripherals, computer and data networks (such as LANs), telecommunication equipments, medical diagnostic equipments, CNC production machinery, radio equipment, televisions, satellite television equipment, electronic cash registers, copy machines, fax machines, etc. These equipments are commonly found connected to some form of data line for communication purposes.

Common failures produced by transients The most common failures produced by transients within electronic devices are disruptive, dissipative, and destructive.  Disruptive effects: are usually encountered when a transient enters the equipment by inductive coupling (either over data or power lines). The electronic components then try to process the transient as a valid logic command. The result is system lock-up, malfunctions, erroneous output, lost or corrupted files, and a variety of other undesirable effects.  Dissipative effects: are associated with repeated stresses to IC components. The materials used to fabricate ICs can withstand a certain number of repeated energy level surges, but not for an extended period. Long-term degradation will eventually cause the components to be rendered inoperable.  Destructive effects: include all conditions where transients with high levels of energy cause equipment to fail immediately. Often, there is physical damage apparent, like burnt and/or cracked PC boards and components, melting of electronic components, or other obvious indications. Receptors of EMI Any EMI situation requires not only an emission source but also a receptor. A receptor is also called a "victim" source because it consists of any device, when exposed to conducted or radiated electromagnetic energy from emitting sources, will degrade or malfunction in performance. Many devices can be emission sources and receptors simultaneously. For example, most communication electronic systems can be emission and receptor sources because they contain transmitters and receivers. Figure 6 shows taxonomy of different receptors that are susceptible to EMI. Similar to the emission source taxonomy, receptors can be divided into natural and man-made receptors. A brief description of each category will be given below.

Figure 6. Taxonomy of EMI receptors  

Natural receptors: Natural receptors include humans, animals, and plants. Man-made EMI receptors: Man-made receptors can be categorized into 4 categories: communication electronic receivers, amplifiers, industrial and comsumer devices, and RADHAZ. 

 

Communication electronic receivers: These receivers include broadcast receivers, communication receivers, relay communication receivers, and radar receivers. Amplifiers: Amplifiers include IF, video, and audio amplifiers. Industrial and consumer receptors: Industrial receptors include digital computers, industrial process controls, electronic test equipments, biomedical instruments, and public address systems and intercoms. Consumer receptors include radio and TV receivers, hi-fi stereo equipment, electronic musical instruments, and climate control systems. RADHAZ: This category includes radiation hazards to electro-explosive devices and fuels. RADHAZ is an acronym for RADiation HAZards, the name given by the U. S. Department of Defense to the program that is determining the extent of radiation hazards and methods for controlling them.

Setting the ground rules. No single operating agency has jurisdiction over all systems to dictate the action needed to achieve EMC. Thus, EMC is usually achieved by industrial association, voluntary regulation, government-enforced regulation and negotiated agreements among the affected parties. Frequency plays a significant role in EMC. For frequency allocation purposes, the world is divided into three regions:

Region 1 - Europe, Africa, the formerly USSR portion of Asia, Asia-Minor, and the Arabian Peninsula. Region 2 - North and South Americas. Region 3 - Australia and the rest of Asia including Hawaii. Frequency allocations and assignments are made according to the constraints established by the international treaties. The Radio Regulations resulting from such international treaties are published by the International Telecommunication Union (ITU). The Federal Communications Commission (FCC) has the authority over radio and wire communications in the United States. The FCC has set limits on the radiated and conducted emissions of electronic devices including electronic typewriters, calculators, televisions, printers, modems, and personal computers. It is illegal to market an electronic device in the United States unless its radiated and conducted emissions have been measured and do not exceed the limits of FCC regulations. Therefore, any electronic device designed today that is designed without incorporating EMC design principles will probably fail to comply with the FCC limits. EMI control techniques. Traditional design practices typically address EMC when an incompatibility is discovered in a production prototype. Solutions to problems at this stage are often superficial. Quick fixes late in the design cycle become more costly and difficult as the complexity of the electronic system increases. Experience has shown that efficient design occurs when EMC considerations begin early in the design phase and continue through every stage. This includes component selection, the design of printed circuit boards and interconnections, and the physical packaging and location of the systems. This approach results in EMC being designed into the system, not added on at a later time. To control or suppress EMI, the three common means employed are grounding, shielding, and filtering. Although each technique has a distinct role in system design, proper grounding may sometimes minimize the need for shielding and filtering. Also, proper shielding may minimize the need for filtering.

 Grounding Grounding establishes an electrically conductive path between two points. This is one to connect electrical and electronic elements of a system to one another or to some reference point, which may be designated the ground. An ideal ground plane is a zero-potential, zero-impedance body. It can be used as a reference for all signals in associated circuitry, and be the point to which any undesired current can be transferred. Bonding is the establishment of a low impedance path between two metal surfaces. Grounding is a circuit concept, while bonding denotes the physical implementation of that concept. The purpose of a bond is to make a structure homogeneous with respect to the flow of electrical currents, thus avoiding the development of potentials between the metallic parts; such potentials may result in EMI. Bonds provide protection from electrical shock, power circuit current return paths, and antenna ground plane connections. They also minimize the potential difference between the devices. Bonds have the ability to carry large fault current. There are two types of bonding - direct and indirect. The direct bond is a metalto-metal contact between the connecting elements, while an indirect bond is a contact via conductive jumpers. The following guidelines are recommended for effective grounding: 1) All grounding must be done via excellent electrical connection between the ground reference and the item to be grounded. Grounding joints of dissimilar materials must be avoided. 2) In multiconductor cables, ground all unused lines at one end. 3) Ground the shell of all connectors in high EM1 areas. 4) If ground potentials exist between distant areas to be interconnected and cannot be removed by use of grounding techniques, consider using a transformer for isolation. 5) Running ground wires in the most direct route with as few bends and loops as possible will minimize self-inductance and improve the ground.  Shielding The purpose of shielding is to confine radiated energy to a specific region or to prevent radiated energy from entering a specific region. Shields may be in the

form of partitions and boxes as well as in the form of cable and connector shields. Shield types include solid, nonsolid (e.g. screen), and braid, as is used on cables. In all cases, a shield can be characterized by its shielding effectiveness. The following guidelines are recommended for effective cable shielding: 1) When terminating shield cable, always keep the unshielded portion as short as possible (normally less than 25 mm). 2) Never terminate the shield of a balanced line at both ends. Connect the shield at the load (input) end of the line. 3) Always terminate the insulating (floating) end of a shielded cable with an insulating sleeve that ensures that it cannot become inadvertently grounded. 4) The shield must be completely insulated and not become grounded or short-circuited to another cable shield as ground loops will be created. 5) Avoid or minimize the breaks in shields, such as at junction boxes, and always maintain shield continuity and isolation from ground, through all boxes or multipin connectors. 6) Multiconductor shield twisted pair cable should have individual insulated shield and drain wires for each shield.  Filtering An electrical filter is a network of lumped or distributed constant resistors, inductors, and capacitors that offers comparatively little opposition to certain frequencies or dc, while blocking the passage of other frequencies. A filter provides the means whereby levels of conducted interference are substantially reduced. The most significant characteristic of a filter is the insertion loss (IL) which is defined as

where V1 is the output voltage of a signal source with the filter in the circuit, and V2, is the output voltage of the signal source without the use of the filter. Lowpass filters are commonly used in EMC work. The insertion loss for the Lowpass filters is given by



and f is the frequency.


  

EMI is a major problem in the development of embedded systems. Since embedded systems often exist in very noisy environments, even more attention must be paid to EMC. EMC must be taken into consideration during the design stage. Designing for EMC is a long process that starts early in the life cycle and proceeds through the testing stage and even in the post-production stage. Therefore, EMC is a concern for engineers at all phases of the development of an embedded system. Electromagnetic compatibility is achieved when a device functions satisfactorily in its electromagnetic environment without introducing intolerable disturbances. Achieving EMC in electronic devices requires deliberate consideration at the earlier stages of the design cycle. Such an approach and control plan permits all avenues to EMC to be evaluated so that effective and economical solutions can be applied. Environmental reliability testing is used to eliminate potential problems that the system can experience when it is operating in its natural environment. There are many EMC standards used in the regulation of products that may cause EMI. EMC is a growing field because of the ever increasing density of electronic circuits in modern systems for computation, communication, control and so forth. Basic knowledge of EMC is particularly useful to students interested in microwaves, electronics, or computer engineering. EMC is a very important issue that embedded systems designers have to deal with. Even though it is a very difficult topic, there are many practical design techniques that can be used to design for EMC. This will greatly assist designers who are unfamiliar with EM theory to be confident in their design for EMC.

View more...


Copyright ©2017 KUPDF Inc.