OTO2005 Elektrik ve Elektronik OTO Dr. Bar ERKU 2013 EMI Elektromanyetik Giriim Magnetic Field Magnetic fields are produced by electric currents, which can be macroscopic currents in wires, or microscopic currents associated with electrons in atomic orbits. Paralel Plakalar ve Elektrik Alan: How does the wave propagation occur? Electromagnetic radiation involves electric (E) and magnetic (H) fields. Any change in the flux density of a magnetic field will produce an electric field change in time and space (Faraday's Law). This change in an electric field causes another change in the magnetic field due to the displacement current (Maxwell). A time-varying magnetic field produces an electric field and a time-varying electric field results in a magnetic field. This forms the basis of electromagnetic waves and time-varying electromagnetics (Maxwell's Equations). Wave propagation occurs when there are two forms of energy and the presence of a change in one leads to a change in the other. Energy interchanges between electric and magnetic fields as the wave progresses. Faraday's Law Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by - changing the magnetic field strength, - moving a magnet toward or away from the coil, moving the - coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. Teta Asn Derece Olarak Alnz. 2 levhal kondansatr ok levhal kondansatr Capacitive Coupling The utility of the lumped circuit equivalent of coupling channels can be seen now. An electric field coupling is modeled as a capacitance between the two circuits. The equivalent capacitance C ef is directly proportional to the area of overlap and inversely proportional to the distance between the two circuits. Thus, increasing the separation or minimizing the overlap will minimize C ef and hence the capacitive coupling from the noise circuit to the signal circuit. Other characteristics of capacitive coupling can be derived from the model as well. For example, the level of capacitive coupling is directly proportional to the frequency and amplitude of the noise source and to the impedance of the receiver circuit. Thus, capacitive coupling can be reduced by reducing noise source voltage or frequency or reducing the signal circuit impedance. The equivalent capacitance C ef can also be reduced by employing capacitive shielding. Capacitive shielding works by bypassing or providing another path for the induced current so it is not carried in the signal circuit. Proper capacitive shielding requires attention to both the shield location and the shield connection. The shield must be placed between the capacitively coupled conductors and connected to ground only at the source end. Significant ground currents will be carried in the shield if it is grounded at both ends. For example, a potential difference of 1 V between grounds can force 2 A of ground current in the shield if it has a resistance of 0.5 . Potential differences on the order of 1 V can exist between grounds. The effect of this potentially large ground current will be explored further in the discussion of inductively coupled noise. As a general rule, conductive metal or conductive material in the vicinity of the signal path should not be left electrically floating either, because capacitively coupled noise may be increased. Inductive Coupling As described earlier, inductive coupling results from time-varying magnetic fields in the area enclosed by the signal circuit loop. These magnetic fields are generated by currents in nearby noise circuits. The induced voltage V n in the signal circuit is given by the formula: V n = 2f B A Cos (1) where f is the frequency of the sinusoidally varying flux density, B is the rms value of the flux density, A is the area of the signal circuit loop, and is the angle between the flux density B and the area A. The lumped circuit equivalent model of inductive coupling is the mutual inductance M as shown in Figure 16(b). In terms of the mutual inductance M, V n is given by the formula: V n = 2f M I n (2) where I n is the rms value of the sinusoidal current in the noise circuit, and f is its frequency. Because M is directly proportional to the area of the receiver circuit loop and inversely proportional to the distance between the noise source circuit and the signal circuit, increasing the separation or minimizing the signal loop area will minimize the inductive coupling between the two circuits. Reducing the current I n in the noise circuit or reducing its frequency can also reduce the inductive coupling. The flux density B from the noise circuit can also be reduced by twisting the noise source wires. Finally, magnetic shielding can be applied either to noise source or signal circuit to minimize the coupling. Noise Noise is caused by random variations in current or voltage caused by the random movement of charge carriers (usually electrons) carrying the current. Flywheel of test engine Inductive sensor output and MCU input signals at approximately (a) 300 rpm and (b) 3750 rpm engine speeds. Electromagnetic Interference (EMI) is caused by undesireable radiated electromagnetic fields or conducted voltages and currents. The interference is produced by a source emitter and is detected by a susceptible victim via a coupling path. The coupling path may involve one or more of the following coupling mechanisms: 1. Conduction - electric current 2. Radiation - electromagnetic field 3. Capacitive Coupling - electric field 4. Inductive Coupling - magnetic field Adjacent: Birbirine yakn Digital Signals and Harmonic Components A digital signal shows a rectangular voltage waveform, which is formed by overlaying many sine waves. The frequencies of these sine waves are integer times the repetition frequency of the digital signal. A sine wave with a frequency equal to the repetition frequency is called a fundamental wave, and those with a frequency n times the repetition frequency are called n th -order harmonics. The harmonics included in the digital signal are considered a principal cause of EMI noise emission from the electronic circuit. Because of the high frequencies, harmonics radiate easily. If a harmonic frequency is close to the frequency of a radio or TV broadcast signal, the harmonics will be superimposed on the radio wave, causing receiving interference. Power supply noise Power supply noise is considered as another cause of EMI noise emission from electronic circuits. Digital IC's use DC power supplies, and the DC current on the digital IC's power supply terminal will be interrupted according to the IC operation. Such a sporadic change in current causes EMI noise. The charts above show the voltages, measured with an oscilloscope and a spectrum analyzer, on a power supply terminal of an IC that will operate at 5MHz. According to the IC operation timing, the power supply terminal outputs an oscillation waveform, and the spectrum analysis data on this oscillation waveform proves that harmonics are included in the waveform. These harmonic components cause EMI noise. Fourier Serileri Zaman Domenindeki Mkemmel ve Periyodik (Srekli) Sinyallerin Frekans Domenindeki Durumlarna likin rnekler even:ift Filters DESBEL: Bir vericinin gc 1W`tan 2W`a kartlrsa, gteki desibel cinsinden art; N=10 log (2/1) = 3 dB imdi elimizde 5 kW`lk bir verici olsa, biz bunun gcn 10 kW`a kartrsak desibel cinsinden art, glerin deiik olmasna ramen nceki rnekle ayndr. N=10 log (10/5) = 3 dB Bu rneklerlerden bir sonu karacak olursak gteki iki katlk bir art +3 dB, yar yarya azal ise -3 dB ile ifade edilir. Grltl sinyal ve grltlerin filtre edilmesi A f = 1 A f = R 2 de: Pratikte 9k ohm diren bulunmadndan, 9.1 kohm luk diren zmde kullanlmtr.