Hemming, L.H., Ungvichian, V, Roman, J M, Uman, M.A., Rubinstein, M Compatibility” The electrical Engineering Handbook Ed. Richard C. dorf Boca Raton CRC Press llc. 2000
Hemming, L.H., Ungvichian, V., Roman, J.M., Uman, M.A., Rubinstein, M. “Compatibility” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
40 Leland H Hemming Compatibility McDonnell douglas Helicopter Systems Vichate Ungvichian florida atlantic unit 40.1 Grounding, Shielding, and Filtering John M. roman Grounding. Shielding. Filtering Telematics 40.2 Spectrum, Specifications, and Measurement Techniques Electromagnetic Spectrum. Specifications. Measuremen Martin a. uman Procedures Marcos rubinstein Statistics. Electric and Magnetic Fields. Modeling of the Return Swiss PTT troke. Lightning-Overhead wire Interactions 40.1 Grounding Shielding, and filtering Leland H hemming Electromagnetic interference(EMI)is defined to exist when undesirable voltages or currents are present to influence adversely the performance of an electronic circuit or system. Interference can be within the system (intrasystem), or it can be between systems (intersystem). The system is the equipment or circuit over which one exercises design or management control. The cause of an EMI problem is an unplanned coupling between a source and a receptor by means of a transmission path Transmission paths may be conducted or radiated. Conducted interference occurs by means of metallic paths. Radiated interference occurs by means of near-and far-field coupling. These different paths illustrated in Fig. 40.1. The control of EMI is best achieved by applying good interference control principles during the desig process. These involve the selection of signal levels, impedance levels, frequencies, and circuit configurations that minimize conducted and radiated interference In addition, signal levels should be selected to be as low as possible, while being consistent with the required signal-to-noise ratio. Impedance levels should be chosen to minimize undesirable capacitive and inductive coupling The frequency spectral content should be designed for the specific needs of the circuit, minimizing interfer ence by constraining signals to desired paths, eliminating undesired paths, and separating signals from inter- ference Interference control is also achieved by physically separating leads carrying currents from different sources. For optimum control, the three major methods of EMI suppression-grounding, shielding, and filter ingshould be incorporated early in the design process. The control of EMI is first achieved by proper grounding, then by good shielding design, and finally by filtering Grounding is the process of electrically establishing a low impedance path between two or more points in a system. An ideal ground plane is a zero potential, zero impedance body that can be used as reference for all signals in the system. Associated with grounding is bonding, which is the establishment of a low impedance path between two metal surfaces. Shielding is the process of confining radiated energy to the bounds of a specific volume or preventing radiate energy from reaching a specific volume. Filtering is the process of eliminating conducted interference by controlling the spectral content of the conducted path. Filtering is the last step in the EMI design process c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 40 Compatibility 40.1 Grounding, Shielding, and Filtering Grounding • Shielding • Filtering 40.2 Spectrum, Specifications, and Measurement Techniques Electromagnetic Spectrum • Specifications • Measurement Procedures 40.3 Lightning Terminology and Physics • Lightning Occurrence Statistics • Electric and Magnetic Fields • Modeling of the Return Stroke • Lightning-Overhead Wire Interactions 40.1 Grounding, Shielding, and Filtering Leland H. Hemming Electromagnetic interference (EMI) is defined to exist when undesirable voltages or currents are present to influence adversely the performance of an electronic circuit or system. Interference can be within the system (intrasystem), or it can be between systems (intersystem). The system is the equipment or circuit over which one exercises design or management control. The cause of an EMI problem is an unplanned coupling between a source and a receptor by means of a transmission path. Transmission paths may be conducted or radiated. Conducted interference occurs by means of metallic paths. Radiated interference occurs by means of near- and far- field coupling. These different paths are illustrated in Fig. 40.1. The control of EMI is best achieved by applying good interference control principles during the design process. These involve the selection of signal levels, impedance levels, frequencies, and circuit configurations that minimize conducted and radiated interference. In addition, signal levels should be selected to be as low as possible, while being consistent with the required signal-to-noise ratio. Impedance levels should be chosen to minimize undesirable capacitive and inductive coupling. The frequency spectral content should be designed for the specific needs of the circuit, minimizing interference by constraining signals to desired paths, eliminating undesired paths, and separating signals from interference. Interference control is also achieved by physically separating leads carrying currents from different sources. For optimum control, the three major methods of EMI suppression—grounding, shielding, and filtering—should be incorporated early in the design process. The control of EMI is first achieved by proper grounding, then by good shielding design, and finally by filtering. Grounding is the process of electrically establishing a low impedance path between two or more points in a system. An ideal ground plane is a zero potential, zero impedance body that can be used as reference for all signals in the system. Associated with grounding is bonding, which is the establishment of a low impedance path between two metal surfaces. Shielding is the process of confining radiated energy to the bounds of a specific volume or preventing radiated energy from reaching a specific volume. Filtering is the process of eliminating conducted interference by controlling the spectral content of the conducted path. Filtering is the last step in the EMI design process. Leland H. Hemming McDonnell Douglas Helicopter Systems Vichate Ungvichian Florida Atlantic University John M. Roman Telematics Martin A. Uman University of Florida, Gainesville Marcos Rubinstein Swiss PTT
(a)Floating Ground Radiated (b)Single-Point Ground Interferene OI Transmitter Computer Printer Conducted Interference Q interference FIGURE 40.1 Electromagnetic interference is caused FIGURE 40.2 The type of ground system used must by uncontrolled conductive paths and radiated near/far be selected carefully fields grounding Grounding Principles The three fundamental grounding techniques--floating, single-point, and multiple-point--are illustrated 40.2. 9 Floating grounds are used to isolate circuits or equipment from a common ground plane. Static charges are azard with this type of ground. Dangerous voltages may develop or a noise-producing discharge might occur Generally, bleeder resistors are used to control the static problem. Floating grounds are useful only at low frequencies where capacitive coupling paths are negligible The single-point ground is a single physical point in a circuit. By connecting all grounds to a common point, no interference will be produced in the equipment because the configuration does not result in potential differences across the equipment. At high frequencies care must be taken to prevent capacitive coupling, which will result in inter rence A multipoint ground system exists when each ground connection is made directly to the ground plane at the closest available point on it, thus minimizing ground lead lengths. A large conductive body is chosen for the ground. Care must be taken to avoid ground loops Circuit grounding design is dependent on the function of each type of circuit. In unbalanced systems,care must be taken to reduce the potential of common mode noise. Differential devices are commonly used to suppress this form of noise. The use of high circuit impedances should be minimized. Where it cannot be avoided, all interconnecting leads should be shielded, with the shield well grounded. Power supply grounding must be done properly to load inducted noise on a power supply bus. When electromechanical relay are used in a system, it is best that they be provided with their own power supplies. Cable shield grounding must be designed based upon the frequency range, impedance levels, balanced or unbalanced) and operating voltage and/or current Cross talk between cables is a major and must be carefully considered during the design process Building facility grounds must be provided for electrical faults, signal, and lightning. The fault protection (green wire)subsystem is for the protection of personnel and equipment from the hazards of electrical power faults and static charge buildup. The lightning protection system consists of air terminals (lightning rods), heavy duty down-conductors, and ground rods. The signal reference subsystem provides a ground for signal circuits to control static charges and noise and to establish a common reference between signals and loads. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Grounding Grounding Principles The three fundamental grounding techniques—floating, single-point, and multiple-point—are illustrated in Fig. 40.2. Floating grounds are used to isolate circuits or equipment from a common ground plane. Static charges are a hazard with this type of ground. Dangerous voltages may develop or a noise-producing discharge might occur. Generally, bleeder resistors are used to control the static problem. Floating grounds are useful only at low frequencies where capacitive coupling paths are negligible. The single-point ground is a single physical point in a circuit. By connecting all grounds to a common point, no interference will be produced in the equipment because the configuration does not result in potential differences across the equipment. At high frequencies care must be taken to prevent capacitive coupling, which will result in interference. A multipoint ground system exists when each ground connection is made directly to the ground plane at the closest available point on it, thus minimizing ground lead lengths. A large conductive body is chosen for the ground. Care must be taken to avoid ground loops. Circuit grounding design is dependent on the function of each type of circuit. In unbalanced systems, care must be taken to reduce the potential of common mode noise. Differential devices are commonly used to suppress this form of noise. The use of high circuit impedances should be minimized. Where it cannot be avoided, all interconnecting leads should be shielded, with the shield well grounded. Power supply grounding must be done properly to minimize load inducted noise on a power supply bus. When electromechanical relays are used in a system, it is best that they be provided with their own power supplies. Cable shield grounding must be designed based upon the frequency range, impedance levels, (whether balanced or unbalanced) and operating voltage and/or current. Cross talk between cables is a major problem and must be carefully considered during the design process. Building facility grounds must be provided for electrical faults, signal, and lightning. The fault protection (green wire) subsystem is for the protection of personnel and equipment from the hazards of electrical power faults and static charge buildup. The lightning protection system consists of air terminals (lightning rods), heavy duty down-conductors, and ground rods. The signal reference subsystem provides a ground for signal circuits to control static charges and noise and to establish a common reference between signals and loads. FIGURE 40.1 Electromagnetic interference is caused by uncontrolled conductive paths and radiated near/far fields. FIGURE 40.2 The type of ground system used must be selected carefully
Earth grounds may consist of vertical rods, horizontal grids or radials, plates, or incidental electrodes such as utility pipes or buried tanks. The latter must be constructed and tested to meet the design requirements of the Grounding Design Guidelines The following design guidelines represent good practice but should be applied subject to the detailed design b jectives of the system. Use single-point grounding for circuit dimensions less than 0.03A(wavelength) and multipoint ground ing for dimensions greater than 0.157. he type of grounding for circuit dimensions between 0.03 and 0. 15 i depends on the physical arrange ment of the ground leads as well as the conducted emission and conducted susceptibility limits of the ircuits to be grounded. Hybrid grounds may be needed for circuits that must handle a broad portion of the frequency spectrum. Apply floating ground isolation techniques (i. e, transformers)if ground loop problems occur Design ground reference planes so that they have high electrical conductivity and can be maintained Connect test equipment grounds directly to the grounds of the equipment being tested Make certain the ground connections can handle fault currents that might flow unexpectedly Circuit Grounding Maintain separate circuit ground systems for signal returns, signal shield returns, power system returns, and chassis or case grounds. These returns then can be tied together at a single ground reference point. For circuits that produce large, abrupt current variations, provide a separate grounding system, or provide a separate return lead to the ground to reduce transient coupling into other circuits. Isolate the grounds of low-level circuits from all other grounds. Where signal and power leads must cross, make the crossing so that the wires are perpendicular to each other. Use balanced differential circuitry to minimize the effects of ground circuit interference For circuits whose maximum dimension is significantly less than 2/4, use tightly twisted wires(either shielded or unshielded, depending on the application) that are single-point grounded to minimize equipment susceptibilit Cable Grounding Avoid pigtails when terminating cable shields When coaxial cable is needed for signal transmission, use the shield as the signal return and ground the generator end for low-frequency circuits Use multipoint grounding of the shield for high-frequency cIrcuits Provide multiple shields for low-level transmission lines. ding of each shield is ommended Shielding The control of near- and far-field coupling(radiation) is accomplished using shielding techniques. The first tep in the design of a shield is to determine what undesired field level may exist at a point with no shielding and what the tolerable field level is. The difference between the two then is the needed shielding effectiveness This section discusses the shielding effectiveness of various solid and nonsolid materials and their application to various shielding situations. Penetrations and their design are discussed so that the required shielding effectiveness is maintained. Finally, common shielding effectiveness testing methods are reviewed. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Earth grounds may consist of vertical rods, horizontal grids or radials, plates, or incidental electrodes such as utility pipes or buried tanks. The latter must be constructed and tested to meet the design requirements of the facility. Grounding Design Guidelines The following design guidelines represent good practice but should be applied subject to the detailed design objectives of the system. Fundamental Concepts • Use single-point grounding for circuit dimensions less than 0.03 l (wavelength) and multipoint grounding for dimensions greater than 0.15 l. • The type of grounding for circuit dimensions between 0.03 and 0.15 l depends on the physical arrangement of the ground leads as well as the conducted emission and conducted susceptibility limits of the circuits to be grounded. Hybrid grounds may be needed for circuits that must handle a broad portion of the frequency spectrum. • Apply floating ground isolation techniques (i.e., transformers) if ground loop problems occur. • Keep all ground leads as short as possible. • Design ground reference planes so that they have high electrical conductivity and can be maintained easily to retain good conductivity. Safety Considerations • Connect test equipment grounds directly to the grounds of the equipment being tested. • Make certain the ground connections can handle fault currents that might flow unexpectedly. Circuit Grounding • Maintain separate circuit ground systems for signal returns, signal shield returns, power system returns, and chassis or case grounds. These returns then can be tied together at a single ground reference point. • For circuits that produce large, abrupt current variations, provide a separate grounding system, or provide a separate return lead to the ground to reduce transient coupling into other circuits. • Isolate the grounds of low-level circuits from all other grounds. • Where signal and power leads must cross, make the crossing so that the wires are perpendicular to each other. • Use balanced differential circuitry to minimize the effects of ground circuit interference. • For circuits whose maximum dimension is significantly less than l/4, use tightly twisted wires (either shielded or unshielded, depending on the application) that are single-point grounded to minimize equipment susceptibility. Cable Grounding • Avoid pigtails when terminating cable shields. • When coaxial cable is needed for signal transmission, use the shield as the signal return and ground at the generator end for low-frequency circuits. Use multipoint grounding of the shield for high-frequency circuits. • Provide multiple shields for low-level transmission lines. Single-point grounding of each shield is recommended. Shielding The control of near- and far-field coupling (radiation) is accomplished using shielding techniques. The first step in the design of a shield is to determine what undesired field level may exist at a point with no shielding and what the tolerable field level is. The difference between the two then is the needed shielding effectiveness. This section discusses the shielding effectiveness of various solid and nonsolid materials and their application to various shielding situations. Penetrations and their design are discussed so that the required shielding effectiveness is maintained. Finally, common shielding effectiveness testing methods are reviewed
Inside of Enclosure Incident Wave Outside World nternal Reflected Wave FIGURE 40.3 Shielding effectiveness is the result of three loss mechanisms Enclosure Theory The attenuation provided by a shield results from three loss mechanisms as illustrated in Fig. 40.3. 1. Incident energy is reflected() by the surface of the shield because of the impedance discontinuity of the air-metal boundary. This mechanism does not require a particular material thickness but simply an impedance discontinuity. 2. Energy that does cross the boundary(not reflected) is attenuated(A)in passing through the shield 3. The energy that reaches the opposite face of the shield encounters another air-metal boundary and thus some of it is reflected(B) back into the shield. This term is only significant when a< 15 dB and is generally neglected because the barrier thickness is generally great enough to the 15-dB loss rule thumb Thus. s=R+a+b dB (40.1) Absorption loss is independent of the type of wave(electric/magnetic)and is given by A=1.314(fμ,o,)2ddB (402) where d is shield thickness in centimeters, H, is relative permeability, f is frequency in Hz, and o, is conductivity of metal relative to that of copper. Typical absorption loss is provided in Table 40.1 Reflection loss is a function of the intrinsic impedance of the metal boundary with respect to the wave impedance, and therefore, three conditions exist: near-field magnetic, near-field electric, and plane wave The relationship for low-impedance(magnetic field) source is R=20log10{1.173(,/fo)/D]+0.0535D(fo,/,)2+0.354dB}(40.3) where D is distance to source in meters. For a plane wave source the reflection loss is R=168-10log10(f,/o,)dB (40.4) For a high-impedance (electric field) source the reflection loss R is R=362-20log10[(μ,f3,)2DldB (405 c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Enclosure Theory The attenuation provided by a shield results from three loss mechanisms as illustrated in Fig. 40.3. 1. Incident energy is reflected (R) by the surface of the shield because of the impedance discontinuity of the air–metal boundary. This mechanism does not require a particular material thickness but simply an impedance discontinuity. 2. Energy that does cross the boundary (not reflected) is attenuated (A) in passing through the shield. 3. The energy that reaches the opposite face of the shield encounters another air–metal boundary and thus some of it is reflected (B) back into the shield. This term is only significant when A < 15 dB and is generally neglected because the barrier thickness is generally great enough to exceed the 15-dB loss rule of thumb. Thus: S = R + A + B dB (40.1) Absorption loss is independent of the type of wave (electric/magnetic) and is given by A = 1.314( f mrsr)1/ 2d dB (40.2) where d is shield thickness in centimeters, mr is relative permeability, f is frequency in Hz, and sr is conductivity of metal relative to that of copper. Typical absorption loss is provided in Table 40.1. Reflection loss is a function of the intrinsic impedance of the metal boundary with respect to the wave impedance, and therefore, three conditions exist: near-field magnetic, near-field electric, and plane wave. The relationship for low-impedance (magnetic field) source is R = 20 log10{[1.173(mr /f sr)1/2/D] + 0.0535 D(f sr /mr)1/2 + 0.354 dB} (40.3) where D is distance to source in meters. For a plane wave source the reflection loss is R = 168 – 10 log10(f mr /sr) dB (40.4) For a high-impedance (electric field) source the reflection loss R is R = 362 – 20 log10[(mr f 3/sr)1/2D] dB (40.5) FIGURE 40.3 Shielding effectiveness is the result of three loss mechanisms
TABLE 40.1 Absorption Loss Is a Function of Type of Material and Frequency(Loss Shown is at 150 kHz) Relative Relative Absorption Conductivity Permeability Loss A, dB/mm Silver 105 Copper-hard drawn Cadmium Nickel 0.15 el, SAE1045 Lead ype Monel 0.04 ermelo 2500 el. stainless 0.02 SEssue ng that mater i t Pw 200 (el pw NSA-65-6 0000 DHz 30100Hz 300 IkHz 3 10kHz 30 100kHz 300 IMHz 3 10MHz 30 100MHz 300 1GHz 3 10GHz 30 Frequency FIGURE 40.4 The shielding effectiveness of common sheet metals, I m separation (a)26-gage steel;(b)3-oz copper foil; (c)0.030-in. aluminum sheet;(d)0.003-in Permalloy;(e)is a common specification for shielded enclosures. Figure 40.4 illustrates the shielding effectiveness of a variety of common materials versus various thicknesses for a source distance of 1 m. This is the shielding effectiveness of a six-sided enclosure. To be useful, the enclosure must be penetrated for various services or devices. This is illustrated in Fig. 40.5(a)for small enclosures and Fig. 40.5(b)for room-sized enclosures C 2000 by CRC Press LLC
© 2000 by CRC Press LLC Figure 40.4 illustrates the shielding effectiveness of a variety of common materials versus various thicknesses for a source distance of 1 m. This is the shielding effectiveness of a six-sided enclosure. To be useful, the enclosure must be penetrated for various services or devices. This is illustrated in Fig. 40.5(a) for small enclosures and Fig. 40.5(b) for room-sized enclosures. TABLE 40.1 Absorption Loss Is a Function of Type of Material and Frequency (Loss Shown is at 150 kHz) Relative Relative Absorption Metal Conductivity Permeability Loss A, dB/mm Silver 1.05 1 52 Copper—annealed 1.00 1 51 Copper—hard drawn 0.97 1 50 Gold 0.70 1 42 Aluminum 0.61 1 40 Magnesium 0.38 1 31 Zinc 0.29 1 28 Brass 0.26 1 26 Cadmium 0.23 1 24 Nickel 0.20 1 23 Phosphor–bronze 0.18 1 22 Iron 0.17 1000 650 Tin 0.15 1 20 Steel, SAE1045 0.10 1000 500 Beryllium 0.10 1 16 Lead 0.08 1 14 Hypernik 0.06 80000 3500a Monel 0.04 1 10 Mu-metal 0.03 80000 2500a Permalloy 0.03 80000 2500a Steel, stainless 0.02 1000 220a a Assuming that material is not saturated. Source: MIL-HB-419A. FIGURE 40.4 The shielding effectiveness of common sheet metals, 1 m separation. (a) 26-gage steel; (b) 3-oz. copper foil; (c) 0.030-in. aluminum sheet; (d) 0.003-in. Permalloy; (e) is a common specification for shielded enclosures. 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 Shielding Effectiveness, SdB, in dB Shielding Effectiveness, SdB, in dB 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 10Hz 30100Hz 300 1kHz 3 10kHz 30 100kHz 300 1MHz 3 10MHz 30 100MHz 300 1GHz 3 10GHz 30 Frequency H E E PW PW PW NSA-65-6 H (e) (e) (a) (b) (c) (c) (b) (d) (d) (a) (e)
Cover Plate Forced Air Gasket Lan口口口 Connectors Fuse FIGURE 40.5 Penetrations in small (a)and large(b)enclosures. Shielding penetrations Total tion of the basic shield and all of the leakages asso- ciated with the penetrations in the enclosure. the latter includes seams, doors, vents, control shafts, piping, filters, windows, screens, and fasteners The design of the seams is a function of the type Weld Materia of enclosure and the level and nature of the shielding Overiap Seam effectiveness required. For small instruments, com- uters,and similar equipment, the typical shielding ⑨d心 required is on the order of 60 dB for electric and C Fused Materiai Maximum Protection plane wave shielding. EMI gaskets are commonly sed to seal the openings in sheet metal construction In some high-performance applications the shielding Spot weld is achieved using very tight-fitting machined hous ings. Examples are IF strips and large dynamic range FIGURE 40.6 Methods of sealing enclosure seams. amplifier circuits. Various methods of sealin joints are illustrated in Fig. 40.6. EMI gasketing methods are shown in Fig. 40.7. For large room-sized enclosures, the performance requirements typically range from 60 to 120 dB. Conductive EMI shielding tape is used in the 60-dB realm, clamped seams for 80-100 dB, and continuous welded seams for 120-dB performance. These are illustrated in Fig. 40.8. good electromagnetic shielded door design must meet a variety of physical and electrical requirements Figure 40.9 illustrates a number of ways this is accomplished. For electronic equipment, a variety of penetrations must be made to make the shielded volume functional. These include control shafts, windows, lights, filters, and displays. Careful design is required to maintain the required shielding integrit Shield Testing The most common specification used for shield evaluation is the procedure given in MIL-STD-285. This consists of establishing a reference level without the shield and then enclosing the receiver within the shield and determining the difference. The ratio is the shielding effectiveness. This applies regardless of materials used in the construction of the shield. Care must be taken in evaluating the results since the measured value is a function of a variety of factors, not all of which are definable c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Shielding Penetrations Total shielding effectiveness of an enclosure is a function of the basic shield and all of the leakages associated with the penetrations in the enclosure. The latter includes seams, doors, vents, control shafts, piping, filters, windows, screens, and fasteners. The design of the seams is a function of the type of enclosure and the level and nature of the shielding effectiveness required. For small instruments, computers, and similar equipment, the typical shielding required is on the order of 60 dB for electric and plane wave shielding. EMI gaskets are commonly used to seal the openings in sheet metal construction. In some high-performance applications the shielding is achieved using very tight-fitting machined housings. Examples are IF strips and large dynamic range log amplifier circuits. Various methods of sealing joints are illustrated in Fig. 40.6. EMI gasketing methods are shown in Fig. 40.7. For large room-sized enclosures, the performance requirements typically range from 60 to 120 dB. Conductive EMI shielding tape is used in the 60-dB realm, clamped seams for 80–100 dB, and continuous welded seams for 120-dB performance. These are illustrated in Fig. 40.8. A good electromagnetic shielded door design must meet a variety of physical and electrical requirements. Figure 40.9 illustrates a number of ways this is accomplished. For electronic equipment, a variety of penetrations must be made to make the shielded volume functional. These include control shafts, windows, lights, filters, and displays. Careful design is required to maintain the required shielding integrity. Shield Testing The most common specification used for shield evaluation is the procedure given in MIL-STD-285. This consists of establishing a reference level without the shield and then enclosing the receiver within the shield and determining the difference. The ratio is the shielding effectiveness. This applies regardless of materials used in the construction of the shield. Care must be taken in evaluating the results since the measured value is a function of a variety of factors, not all of which are definable. FIGURE 40.5 Penetrations in small (a) and large (b) enclosures. FIGURE 40.6 Methods of sealing enclosure seams
PAINT EMI GASKET EMI GASKET External Bolting Prevents EMI Leakage Insert Pressed-In and Flared Makes EMI Tight Joint(Alternate: Weld or Cement with Conductive Epoxy) FIGURE 40.7 Methods of constructing gasketed joints shielding 察条条条, ≡ FIGURE 40.8 Most common seams in large enclosures. (a) Foil and shielding tape;(b)clamped;(c)welded. Summary of Good Shielding Practice Shielding Effective Good conductors, such as copper and aluminum, should be used for electric field shields to obtain high reflection loss. A shielding material thick enough to support itself usually provides good electric shielding at all frequencies. magnetic materials, such as iron and special high-permeability alloys, should be used for magnetic field shields to obtain high absorbtion los In the plane wave region, the sealing of all apertures is critical to good shielding practice. Multiple Shields Multiple shields are quite useful where high degrees of shielding effectiveness are required Shield seams All openings or discontinuities should be addressed in the design process to ensure achievement of the required shielding effectiveness. Shield material should be selected not only from a shielding requirement, but also from electrochemical corrosion and strength considerations Whenever system design permits, use continuously overlapping welded seams. Obtain intimate contact between mating surfaces over as much of the seam as possible c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Summary of Good Shielding Practice Shielding Effectiveness • Good conductors, such as copper and aluminum, should be used for electric field shields to obtain high reflection loss. A shielding material thick enough to support itself usually provides good electric shielding at all frequencies. • Magnetic materials, such as iron and special high-permeability alloys, should be used for magnetic field shields to obtain high absorbtion loss. • In the plane wave region, the sealing of all apertures is critical to good shielding practice. Multiple Shields • Multiple shields are quite useful where high degrees of shielding effectiveness are required. Shield Seams • All openings or discontinuities should be addressed in the design process to ensure achievement of the required shielding effectiveness. Shield material should be selected not only from a shielding requirement, but also from electrochemical corrosion and strength considerations. • Whenever system design permits, use continuously overlapping welded seams. Obtain intimate contact between mating surfaces over as much of the seam as possible. FIGURE 40.7 Methods of constructing gasketed joints. FIGURE 40.8 Most common seams in large enclosures. (a) Foil and shielding tape; (b) clamped; (c) welded
Recessed Frame Phosphor Bronze Key hole anger Stock Mesh Phosphor Bronze nger Stock Phosphor Bron Access Pai Finger Stock Door Leaf 区 RF Gasket Section Through Door and Jamb Bronze knife edge on Door Leat FIGURE 40.9 Methods of sealing seams in RF enclosure small (a) and large(b)doors. Surfaces to be mated must be clean and free from nonconducting finishes, unless the bonding process ositively and effectively cuts through the finish. When electromagnetic compatibility(EMc)and finish specifications conflict, the finishing requirements must be modified Case Construction Case material should have good shielding properties. Seams should be welded or overlapped. Panels and cover plates should be attached using conductive gasket material with closely spaced fasteners Mating surfaces should be cleaned just before assembly to ensure good electrical contact and to minimize corrosion A variety of special devices are available for sealing around doors, vents, and windows. isolation is achieved by circuit design; physical isolation may be achieved by proper shieding Ctrical Internal interference generating circuits must be isolated both electrically and physically. Electrical For components external to the case, use EMI boots on toggle switches, EMI rotary shaft seals on rotary shafts, and screening and shielding on meters and other indicator faces. Cable shields Cabling that penetrates a case should be shielded and the shield should be terminated in a peripheral bond at the point of entry. This peripheral bond should be made to the connector or adaptor shell Filtering An electrical filter is a combination of lumped or distributed circuit elements arranged so that it has a frequency haracteristic that passes some frequencies and blocks others c 2000 by CRC Press LLC
© 2000 by CRC Press LLC • Surfaces to be mated must be clean and free from nonconducting finishes, unless the bonding process positively and effectively cuts through the finish. When electromagnetic compatibility (EMC) and finish specifications conflict, the finishing requirements must be modified. Case Construction • Case material should have good shielding properties. • Seams should be welded or overlapped. • Panels and cover plates should be attached using conductive gasket material with closely spaced fasteners. • Mating surfaces should be cleaned just before assembly to ensure good electrical contact and to minimize corrosion. • A variety of special devices are available for sealing around doors, vents, and windows. • Internal interference generating circuits must be isolated both electrically and physically. Electrical isolation is achieved by circuit design; physical isolation may be achieved by proper shielding. • For components external to the case, use EMI boots on toggle switches, EMI rotary shaft seals on rotary shafts, and screening and shielding on meters and other indicator faces. Cable Shields • Cabling that penetrates a case should be shielded and the shield should be terminated in a peripheral bond at the point of entry. This peripheral bond should be made to the connector or adaptor shell. Filtering An electrical filter is a combination of lumped or distributed circuit elements arranged so that it has a frequency characteristic that passes some frequencies and blocks others. FIGURE 40.9 Methods of sealing seams in RF enclosure small (a) and large (b) doors
Filters provide an effective means for the reduction and suppression of electromagnetic interference as they control the spectral content of signal paths. The application of filtering requires careful consideration of an extensive list of factors including insertion loss, impedance, power handling capability, signal distortion, tun ability, cost, weight, size, and rejection of undesired signals. Often they are used as stopgap measures, but if The types of filters are classified according to the band of frequencies to be transmitted or attenuated. The basic types illustrated in Fig. 40. 10 include low-pass, high-pass, bandpass, and bandstop(reject) Filters can be composed of lumped, distributed, or dissipative elements; the type used is mainly a function Filtering guidance It is best to filter at the interference source Suppress all spurious signa Ensure that all filter elements interface properly with other EMC elements, ie, proper mounting of a filter in a shielded enclosure Filter design Filters using lumped and distributive elements generally are reflective, in that the various component combi nations are designed for high series impedance and low shunt impedance in the stopband while providing low series impedance and high shunt impedance in the passband. The impedance mismatches associated with the use of reflective filters can result in an increase of interference. In such cases, the use of dissipative elements is found to be useful. A broad range of ferrite components are available in the form of beads, tubes, connector shells, and pins. A very effective method of low-pass filtering is to form the ferrite into a coaxial geometry, the properties of which are proportional to the length of the ferrite, as shown in Fig. 40.11. Application of filtering takes many forms. A common problem is transient suppression as illustrated in Fig 40.12. All sources of transient interference should be treated at the source. Power line filtering is recommended to eliminate conducted interference from reaching the powerline and adjacent equipment. Active filtering is very useful in that it can be built in as part of the circuit design and can be effective in passing only the design signals. A variety of noise blankers, cancelers, and limiter circuits are available for active cancellation of interference pecial Filter Type filters are used in the design of electronic equipment. Transmitters require a variety of filters to achieve a noise-free output. Receive preselectors play a useful role in interference rejection. Both distributed (cavity )and lumped element components are used. IF filters control the selectivity of a receiving system and use a variety of mechanical and electrical filtering Testing The general requirements for electromagnetic filters are detailed in MIL-F-15733, MIL-F-18327, and MIL-F- 25880. Insertion loss is measured in accordance with MIL-STD-220 Defining Terms Earth electrode system: A network of electrically interconnected rods, plates, mats, or grids, installed for the purpose of establishing a low-resistance contact with earth. The design objective for resistance to earth of this subsystem should not exceed 10 Q2. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Filters provide an effective means for the reduction and suppression of electromagnetic interference as they control the spectral content of signal paths. The application of filtering requires careful consideration of an extensive list of factors including insertion loss, impedance, power handling capability, signal distortion, tunability, cost, weight, size, and rejection of undesired signals. Often they are used as stopgap measures, but if suppression techniques are used early in the design process, then the complexity and cost of interference fixes can be minimized. There are many textbooks on filtering, which should be used for specific applications. The types of filters are classified according to the band of frequencies to be transmitted or attenuated. The basic types illustrated in Fig. 40.10 include low-pass, high-pass, bandpass, and bandstop (reject). Filters can be composed of lumped, distributed, or dissipative elements; the type used is mainly a function of frequency. Filtering Guidance • It is best to filter at the interference source. • Suppress all spurious signals. • Design nonsusceptible circuits. • Ensure that all filter elements interface properly with other EMC elements, i.e., proper mounting of a filter in a shielded enclosure. Filter Design Filters using lumped and distributive elements generally are reflective, in that the various component combinations are designed for high series impedance and low shunt impedance in the stopband while providing low series impedance and high shunt impedance in the passband. The impedance mismatches associated with the use of reflective filters can result in an increase of interference. In such cases, the use of dissipative elements is found to be useful. A broad range of ferrite components are available in the form of beads, tubes, connector shells, and pins. A very effective method of low-pass filtering is to form the ferrite into a coaxial geometry, the properties of which are proportional to the length of the ferrite, as shown in Fig. 40.11. Application of filtering takes many forms. A common problem is transient suppression as illustrated in Fig. 40.12. All sources of transient interference should be treated at the source. Power line filtering is recommended to eliminate conducted interference from reaching the powerline and adjacent equipment. Active filtering is very useful in that it can be built in as part of the circuit design and can be effective in passing only the design signals. A variety of noise blankers, cancelers, and limiter circuits are available for active cancellation of interference. Special Filter Types A variety of special-purpose filters are used in the design of electronic equipment. Transmitters require a variety of filters to achieve a noise-free output. Receive preselectors play a useful role in interference rejection. Both distributed (cavity) and lumped element components are used. IF filters control the selectivity of a receiving system and use a variety of mechanical and electrical filtering components. Testing The general requirements for electromagnetic filters are detailed in MIL-F-15733, MIL-F-18327, and MIL-F- 25880. Insertion loss is measured in accordance with MIL-STD-220. Defining Terms Earth electrode system: A network of electrically interconnected rods, plates, mats, or grids, installed for the purpose of establishing a low-resistance contact with earth. The design objective for resistance to earth of this subsystem should not exceed 10 W