A Basic Overview of Electro Magnetic Interference

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A very basic introduction to EMI and the associated issues
   EMI EMI 1 A Basic Overview of Electromagnetic Interference  by Chuck Easttom   EMI EMI 2 Electromagnetic interference (EMI) is a disturbance that affects an electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source. All network transmissions mediums are susceptible to EMI to one degree or another. According to Faber and Rybinski (1997), this is a particular concern for unshielded twisted pair cable. Their study measured the sensitivity of UTP cabling to a variety of common sources of EMI. For testing purposes they utilized a microwave oven, a fluorescent light, and an electric hand drill. Those tests illustrate that fact that any item which utilizes electricity can be a source of electromagnetic interference. This would include appliances, other network cables, power cables, fluorescent lighting, and literally any source of electrical conductivity. This can be a significant concern when selecting and installing network wiring in any building. Shider (2008) explains that EMI is caused by stray current coupling between electronic systems. EMI essentially adds unwanted electrical energy and signal to a given transmission. So when a particular network cable is transmitting voltages representing data on a network, nearby electrically active equipment can add unwanted voltages to that signal thus interfering with the transmission of the data. All manufactures establish an acceptable level of EMI their particular  product can tolerate. Shider (2008) further explains that attenuation, a closely related concept, is when the voltage of a given signal degrades as the distance it travels increases. Thus the limits on various network mediums. For example twisted pair cable (shielded or unshielded) is limited to 100 meters before attenuation begins to make the signal unusable. As transmission speeds increase, concern over EMI also increases. Faber and Rybinski (1997) define Electromagnetic compatibility (EMC) as a systems ability to minimize radiated energy levels and EMI. The measurement of EMC is a valuable asset to network administrators attempting to select an appropriate transmission medium.   EMI EMI 3 Conley (2007) discusses the implications for EMI on department of defense systems. His discussion of EMI goes beyond the items typically considered in corporate environments. Since communications are the primary concern regarding EMI and military systems, Conley (2007) examines the impact of such diverse sources of EMI as geomagnetic interference and atmospheric conditions. Most corporate environments limit their concerns about EMI to other electronic devices, power lines, and similar man made concerns. However when the corporate network involves a wide area network, then it can be susceptible to the same interference that defense department communications are. According to Nazerain and Massould (2005), crosstalk is caused by coupling, the transfer of electrical energy between conductors. In other words electrical signal from one source is transmitted to another medium. Or one wires signal bleeds over into an adjacent wire. The three main types of coupling are capacitive, inductive, and conductive. Capacitive coupling occurs when two separate conductors are close enough together to act as a capacitor. The close  proximity of the conductors, in effect, creates a capacitor. Inductive coupling occurs when the current in one conductor induces a similar current in another conductor. Conductive coupling occurs when there is physical contact between conductors. This is often the case in terminal cross talk, which is often related to twisted pair wiring, according to Sheffield (2009). In general EMI and cross talk differ in that cross talk is EMI specifically generated by nearby communications wiring. Crosstalk is in effect, a special case of EMI. Crosstalk results when a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel Benzacar (2008) discusses six ways to reduce interference due to cross talk. In his paper he first and foremost addresses the use of filtering mechanisms to reduce cross talk EMI. In fact the all of his six methods involve selecting   EMI EMI 4 and testing the appropriate filter. A filter can be as simple as shielding on cable or something more complex, depending on what the individual situation calls for. A point made very clear by Benzacar (2008) is the need to test such filtering mechanism in a high stress environment rather than in a passive environment. He states that without a high stress level, it is possible that one would get a positive outcome from a test, but still not have adequate filtering for the real world. Smithfield (2009) explains that terminal crosstalk is cross talk that occurs at the termination of an electronic communication medium. This can be on either end of the communication. This often occurs in twisted pair cable, due in part to the twisting of the cable at the termination point. Smithfield (2009) discusses the desirability of a high signal to noise ration as defined as the amount of information signal divided by the amount of noise interference. Signal to Noise Ratio (SNR) is also sometimes referred to as Carrier to Interference Ratio (CIR) Cross talk and electromagnetic interference are two major problems for any network communication. It can be caused due to overlapping electronic signals, electrical conduits or even the wiring itself. In order to ameliorate the interference produced by cross talk one must select the appropriate medium for network transmissions. Closely related to crosstalk is the topic of electromagnetic interference. This is often caused by natural phenomena. The best way to combat this is through proper filtering technologies.   EMI EMI 5 References Benzacar, S. (2008). Six ways to avoid cross talk interference http://www.eetindia.co.in/STATIC/PDF/200809/EEIOL_2008SEP25_RFD_SIG_ TA_01.pdf?SOURCES=DOWNLOAD. Conley, C. (2007).  Electromagnetic interference, an Achilles heel.  Retrieved November 10, 2009 from http://www.chips.navy.mil/archives/01_fall/electromagnetic_interference.htm Faber,W., Rybinski,. V. (1997).  UTP cabling and the effects of EMI.  Retrieved  November 10, 2009 from http://www.siemon.com/us/whte_papers/97-10-02- presentation.asp  Nazerain, S. Massould, P. (2005). An Empirical Study of Crosstalk in VDSM Technologies. Retrieved November 12, 2009 from http://atrak.usc.edu/~massoud/Papers/xtalk-observations-glsvlsi05.pdf Shinder, D. (2009). Understanding interference and attenuation . Retrieved November 8, 2009 from http://www.toadworld.org/Student%20Info/Training/Articles/Understanding%20Interference%20and%20Attenuation.pdf Smithfield, N. (2009).  Physical Layer Propagation . Retrieved November 10 2009 from www.apsu.edu/smithfieldn/4C03R1.PPT
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