《电子工程师手册》学习资料（英文版）chapter 40 Compatibility.pdf

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