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10-2016

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Fachzeitschrift für Hochfrequenz- und Mikrowellentechnik

RF & Wireless EMI Noise

RF & Wireless EMI Noise Suppression and Intra-System EMI-Control Techniques EMI control techniques involve both hardware implementations and methods and procedures. They may also be divided into intra-system and intersystem EMI control. Our major concern in this Application Note is intra-system EMI control, however, an overview of each may be appropriate at this time. The Figure above illustrates the basic elements of concern in an intra-system EMI problem. The test specimen may be a single box, an equipment, subsystem, or system (an ensemble of boxes with interconnecting cables). From a strictly near-sighted or selfish point-of-view, the only EMI concern would appear to be degradation of performance due to self jamming such as suggested at the top of the figure. While this might be the primary emphasis, the potential problems associated with either • susceptibility to outside conducted and/or radiated emissions or • tendency to pollute the outside world from its own undesired emissions, come under the primary classification of intrasystem EMI. Corresponding EMI-control techniques, however, address themselves to both self-jamming and emission/susceptibility in accordance with applicable EMI specifications. The techniques that will be discussed include filtering, shielding, wiring, and grounding. Inter-system EMI distinguishes itself by interference between two or more discrete and separate systems or platforms which are frequently under independent user control. Culprit emissions and/or susceptibility situations are divided into two classes: • antenna entry/exit and • back-door entry/exit More than 95% of inter-system EMI problems involve the antenna entry/exit route of EMI. We can group inter-system EMIcontrol techniques by four fundamental categories: frequency management, time management, location management, and direction management. The first step in locating a solution is to identify the problem as either an inter-system or intra-system EMI situation. Generally, if the specimen has an antenna and the problem develops from what exits or enters the antenna from another specimen or ambient, then the problem is identified as an inter-system EMI one. Otherwise, it is an intra-system EMI situation which we will discuss now. Shielding Shielding is used to reduce the amount of electromagnetic radiation reaching a sensitive victim circuit. Shields are made of metal and work on the principle that electromagnetic fields are reflected and/or attenuated by a metal surface. Different types of shielding are needed for different types of fields. Thus, the type of metal used in the shield and the shield‘s construction must be considered carefully if the shield is to function properly. The ideal shield has no holes or voids, and, in order to accommodate cooling vents, buttons, lamps, and access panels, special meshes and “EMI-hardened” components are needed. Once a printed-circuit board design has been optimized for minimal EMI, residual interference can be further reduced if the board is placed in a shielded enclosure. A box‘s shielding effectiveness in decibels depends on three main factors: its skin, the control of radiation leakage through the box‘s apertures or open areas (like cooling holes), and the use of filters or shields at entry or exit spots of cables. A box skin is typically fabricated from sheet metal or metallized plastic. Normally sheet metal skin that is 1 mm thick is more than adequate; it has a shielding effectiveness of more than 100 Texas Instruments, Application Report AN-643, EMI/RFI Board Design, Chapter 11 & 12, www.ti.com Inter-System EMI Figure 1: Filtering 66 hf-praxis 10/2016

RF & Wireless Figure 2. Common Ground Impedance Coupling Figure 3. Common-Mode, Radiated Field-to-Cable Coupling dB throughout the highfrequency spectrum from 1 MHz to 20 GHz. Conductive coatings on plastic boxes are another matter. Table 1 shows that at 10 MHz the shielding effectiveness can be as low as 27 dB if a carbon composite is used, or it can run as high as 106 dB for zinc sprayed on plastic by an electric arc process. Plastic filled materials or composites having either conductive powder, flakes, or filament are also used in box shielding; they have an effectiveness similar to that of metallized plastics. In many cases shielding effectiveness of at least 40 dB is required of plastic housings for microcontrollerbased equipment to reduce printed-circuit board radiation to a level that meets FCC regulations in the United States or those of the VDE in Europe. Such skin shielding is easy to achieve. The problem is aperture leakage. The larger the aperture, the greater its radiation leakage because the shield‘s natural attenuation has been reduced. On the other hand, multiple small holes matching the same area as the single large aperture can attain the same amount of cooling with little or no loss of attenuation properties. Filtering Filters are used to eliminate conducted interference on cables and wires, and can be installed at either the source or the victim. Figure 1 shows an AC power-line filter. The values of the components are not critical; as a guide, the capacitors can be between 0.01 and 0.001 µF, and the inductors are nominally 6.3 µH. Capacitor C1 is designed to shunt any highfrequency differential-mode currents before they can enter the equipment to be protected. Capacitors C2 and C3 are included to shunt any common-mode currents to ground. The inductors, L1 and L2, are called common-mode chokes, and are placed in the circuit to impede any commonmode currents. Wiring Now that the equipment in each box can be successfully designed to combat EMI emission and susceptibility separately, the boxes may be connected together to form a system. Here the input and output cables and, to a lesser extent, the power cable form an “antenna farm” that greatly threatens the overall electromagnetic compatibility of the system. Most field remedies for EMI problems focus on the coupling paths created by the wiring that interconnects systems. By this time most changes to the individual equipment circuits are out of the question. Let us address five coupling paths that are encountered in typical systems comprised of two or more pieces of equipment connected by cables. These should adequately cover most EMI susceptibility problems. They are: • A common ground impedance coupling – a conducting path in which a common impedance is shared between an undesired emission source and the receptor. • A common-mode, radiated field-to-cable coupling, in which electromagnetic fields penetrate a loop formed by two pieces of equipment, a cable connecting them, and a ground plane. • A differential-mode, radiated field-to-cable coupling, in which the electromagnetic fields penetrate a loop formed by two pieces of equipment and an interconnecting transmission line or cable. • A crosstalk coupling, in which signals in one transmission line or cable are capacitively or inductively coupled into another transmission line. • A conductive paththrough power lines feeding the equipment. The first coupling path is formed when two pieces of equipment are connected to the same ground conductor at different points, an arrangement that normally produces a voltage difference between the two points. If possible, connecting both pieces of equipment to a single-point ground eliminates this voltage. Another remedy is to increase the impedance along a loop that includes the path between the ground connections of the two boxes. Examples include the isolation of printed-circuit boards from their cabinet or case, the use of a shielded isolation transformer in the signal path, or the insertion of an inductor between one or both boxes and the ground conductor. The use of balanced circuits, differential line drivers and receivers, and absorbing ferrite beads and rods on the interconnecting cable can further reduce currents produced by this undesirable coupling path. Figure 2 illustrates common ground impedance coupling. A balanced circuit is configured so its two output signal leads are electrically symmetrical with respect to ground, as the signal increases on one output the signal on the other decreases. Differential line drivers produce a signal that is electrically symmetrical with respect to ground Electromagnetic Interference Fixes 1. Insert Filter in Signal Source 2. Insert Filter in Signal Receptor 3. Insert Filter in Power Source 4. Insert Filter in Power Receptor 5. Twist Wire Pair 6. Shield Cable 7. Use Balanced Circuits 8. Install Differential Line Drivers and Receivers 9. Float Printed Circuit Board(s) 10. Separate Wire Pair 11. Use Ferrite Beads 12..Use a Multilayer Instead of a Single-Layer Printed Circuit Boards hf-praxis 10/2016 67

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