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