Physics
Shielded Cable
A shielded cable is a type of electrical cable that has an additional layer of conductive material surrounding the insulated conductors. This shielding helps to protect the signal from external electromagnetic interference and reduces the emission of electromagnetic radiation from the cable. Shielded cables are commonly used in applications where signal integrity is crucial, such as in data transmission and telecommunications.
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11 Key excerpts on "Shielded Cable"
eBook - ePubGrounding and Shielding
Circuits and Interference
- Ralph Morrison(Author)
- 2016(Publication Date)
- Wiley-IEEE Press(Publisher)
CHAPTER 7 SHIELDING FROM RADIATIONOVERVIEW
Cable shields are often made of aluminum foil or tinned copper braid. Drain wires make it practical to connect to the foil. Coaxial cables have a smooth inner surface that allows for the circulation of current and provides control of characteristic impedance. Transfer impedance is a measure of shielding effectivity. Multiple shields, low-noise cable, and conduit each have merits that are discussed.The penetration of fields into enclosures is considered. This includes independent and dependent apertures, the wave penetration of conducting surfaces, and waveguides. The use of gaskets, honeycombs, and backshell connectors are described. Handling utility power, line filters, and signal lines at a hardware interface are discussed. Methods for limiting field penetration into and out of a screen room are offered.7.1 CABLES WITH SHIELDS
Decibels are used in many places in this chapter. Appendix A provides a review of this subject.In analog work, an aluminum foil is often used as a shield around a cable. The foil has a folded seam that runs the length of the cable. The inside of the aluminum foil is anodized to provide protection against corrosion. Because it is difficult to terminate the foil at the cable ends, a drain wire is provided on the outside of the cable foil. This drain wire is made of multistranded tinned copper wires that make contact with the foil along the length of the cable. If the foil should break, the drain wire connects the segments together. The drain wire is used as the shield connection at the cable ends.In audio work, where a cable carries a microphone signal a short distance, the cable can be a shielded single conductor. The shield is usually a woven braid, which is more durable than foil. Clip-on or handheld microphones that transmit voice on an rf carrier are rapidly taking over. In instrumentation, best practice requires that the signal common and the shield be separate conductors.
- Robert C. Scully, Mark A. Steffka, Clayton R. Paul(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
CHAPTER TEN Shielding This chapter addresses the concept of shielding of electronic circuits. The term shield usually refers to a metallic enclosure that completely encloses an electronic product or a portion of that product. There are two purposes of a shield, as illustrated in Fig. 10.1. The first, as shown in Fig. 10.1a, is to prevent the emissions of the electronics of the product or a portion of those electronics from radiating outside the boundaries of the product. The motivation here is to either prevent those emissions from causing the product to fail to comply with the radiated emissions limits or prevent the product from causing interference with other electronic products. The second purpose of a shield, as shown in Fig. 10.1b, is to prevent radiated emissions external to the product from coupling to the product’s electronics, which may cause interference in the product. As an example, shielding may be used to reduce the susceptibility to external signals such as high-power radars or radio and TV transmitters. A photograph of a shielded room that is used for EMC testing is shown in Fig. 10.1c. Therefore a shield is, conceptually, a barrier to the transmission of electromagnetic fields. We may view the effectiveness of a shield as being the ratio of the magnitude of the electric (magnetic) field that is incident on the barrier to the magnitude of the electric (magnetic) field that is transmitted through the barrier. Alternatively, we may view this as the ratio of the electric (magnetic) field inci- dent on the product’s electronics with the shield removed to that with the shield in place. In this latter sense, we may view the quantification of shielding effectiveness as being equivalent to an insertion loss, as was discussed for filters in Chapter 6. These notions give a qualitative idea about the meaning of the term shield and will be made more precise, quantitatively, in the following sections.
eBook - PDFGrounds for Grounding
A Handbook from Circuits to Systems
- Elya B. Joffe, Kai-Sang Lock(Authors)
- 2022(Publication Date)
- Wiley-IEEE Press(Publisher)
As sources, cables can conduct EMI emitted from the equipment and subsequently act as an unintentional antenna, trans- mitting EMI in the form or radiated energy. As a receptor, the cable can pick up incident-radiated EMI coupled from other radiators. The coupled signal appears as an additive signal on the lines as depicted in Figure 11.3. Table 11.1 lists types of cables used and their common applications, while Table 11.2 presents the type and origin of potential EMI sources in cables of the respec- tive configurations. As can be seen from Table 11.2 and Figure 11.3, a conductive sheath, or shield, placed over a wiring harness or cable, has two main purposes: to confine electromagnetic energy radiating from the enclosed wiring and to preclude external electromagnetic energy from coupling to the enclosed wiring. Shielding is thus the decoupling of radiated electromagnetic energy interactions between system wiring and its surroundings. 6 In addition to the benefits offered by the cable shield, twisted-pairs in differential signaling intercept coupled signals approx- imately equally, so the incident signals appear primarily as common-mode EMI. Twisted-pairs are said to be balanced if the impedances connected from each line to the local reference structure are identical. 7 11.3.2 Fundamental of Shielding Mechanisms Whenever an electromagnetic wave is incident upon a metal surface, attenuation of electromagnetic fields occurs due to two distinct mechanisms: (i) reflection (due to impedance mismatches) of the interference wave at the air–metal boundary as the incident electromagnetic wave strikes the metal surface and reflection at the metal–air boundary as the interference wave emerges from the metal shield and (ii) absorption of the interference wave in passing through the metal shield between the two boundaries. The remainder propagates beyond the metal barrier (transmission) (Figure 11.4).
eBook - PDF- NORMAN ELLIS(Author)
- 1998(Publication Date)
- Newnes(Publisher)
Chapter 8 Conducted interference So far, we have been considering radiated interference, both from the viewpoint of emission and immunity. However, as was indicated in Chapter 1, equipment can be upset by unwanted signals entering or leaving an equipment by power wires, communication cables or remote monitoring wiring. Protection of conductors Apart from minimizing loop areas, maximizing distance from the source and generally taking care in installation, the simplest way to protect conductors from radiating or being subjected to interfer-ence is to shield them. Having decided to shield, we immediately encounter two problems: • What sort of shield is required? • Do we earth the shield and if so where? The most effective shield is a solid metal one, but the penalty is lack of flexibility and probable increase in weight. To improve flexibility a helical metalized tape or braided shield can be used, but how will this affect the shielding effectiveness and how can this be measured? CONDUCTED INTERFERENCE 117 I--+-I ri~1 ..f : {): I I , , , I I J Zr = Y-where L V = Voltsover unit length LX I 8.1 Leaky shields. In the case of a coaxial cable the wanted signal is carried on the centre conductor and on the inside of the shield, but if the shield is imperfect, external fields will cause some potential to occur along the inside of the shield and interfere with the wanted signal by causing a current to flow through the load; the greater the potential V on the inside of the shield, the lower the shielding effectiveness (see Fig. 8.1), The value of V II gives the surface transfer impedance, ZT measured in 'lIm and the smaller the value of ZT' the greater the shielding effectiveness. Figure 8.2 shows a plot of ZT versus frequency for various types of cable shield.
eBook - ePubEMC for Product Designers
Meeting the European EMC Directive
- Tim Williams(Author)
- 2014(Publication Date)
- Newnes(Publisher)
Chapter 6Interfaces, filtering and shielding
Publisher Summary
This chapter provides an overview on interfaces, filtering, and shielding. Cables, and the connectors that form the interface to the equipment, must be carefully specified. The main purpose is to ensure that differential-mode signals are prevented from radiating from the cables, and that common-mode cable currents are neither impressed on the cable by the signal circuit nor are coupled into the signal circuit from external fields via the cable. The effectiveness of the filter configuration depends on the impedances seen at either end of the filter network. Shielding involves placing a conductive surface around the critical parts of the circuit so that the electromagnetic field that couples to it is attenuated by a combination of reflection and absorption. Shielding is often an expensive and difficult-to-implement design decision because of many other factors—aesthetic, tooling, and accessibility—work against it.6.1 Cables and connectors
The most important sources of radiation from a system, or of coupling into a system, are the external cables. Due to their length these are more efficient at interacting with the electromagnetic environment than enclosures, pcbs or other mechanical structures. Cables, and the connectors which form the interface to the equipment, must be carefully specified. The main purpose of this is to ensure that differential-mode signals are prevented from radiating from the cables, and that common-mode cable currents are neither impressed on the cable by the signal circuit nor are coupled into the signal circuit from external fields via the cable.In many cases you will have to use screened cables. Exceptions are the mains power cable (provided a mains filter is fitted), and low-frequency interfaces which can be properly filtered to provide transient and RF immunity. An unfiltered, unscreened interface will provide a path for external emissions and for undesired inward coupling. The way that the cable screen is terminated at the connector interface is critical in maintaining the screening properties of the cable.
eBook - ePub- Peter Wilson(Author)
- 2017(Publication Date)
- Newnes(Publisher)
Fig. 1.18 shows the options.Electrostatic Screening
When you are using Shielded Cable to prevent electrostatic radiation from output or interunit lines, ground-loop induction is usually not a problem because the signals are not susceptible, and the cable shield is best connected to ground at both ends. The important point is that each conductor has a distributed (and measurable) capacitance to the shield, so that currents on the shield will flow as long as there are ac signals propagating within it (see Fig. 1.19 ). These shield currents must be provided with a low-impedance ground return path so that the shield voltages do not become substantial. The same applies in reverse when you consider coupling of noise induced on the shield into the conductors.Figure 1.19 Conductor to Shield Coupling Capacitance.Surface Transfer Impedance
At high frequencies, the notion of surface transfer impedance becomes useful as a measure of shielding effectiveness. This is the ratio of voltage developed between the inner and outer conductors of Shielded Cable due to interference current flowing in the shield, expressed in milliohms per unit length. It should not be confused with characteristic impedance, with which it has no connection. A typical single braid screen will be 10 mΩ/m or so below 1 MHz, rising at a rate of 20 dB/decade with increasing frequency. The common aluminum/mylar foil screens are around 20 dB worse. Unhappily, surface transfer impedance is rarely specified by cable manufacturers.1.1.12. The Safety Earth
A brief word is in order about the need to ensure a mains earth connection, since it is obvious from the preceding discussion that this requirement is frequently at odds with antiinterference grounding practice. Most countries now have electrical standards which require that equipment powered from dangerous voltages should have a means of protecting the user from the consequences of component failure. The main hazard is deemed to be inadvertent connection of the live mains voltage to parts of the equipment with which the user could come into contact directly, such as a metal case or a ground terminal.
eBook - PDF- Tim Williams(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
This will preserve dc and low-frequency continuity while blocking the flow of large induced high-frequency currents along the shield. The shield should never be grounded at the opposite end to the signal. Figure 1.18 shows the options. Electrostatic screening When you are using Shielded Cable to prevent electrostatic radiation from output or The Circuit Designer's Companion 19 Figure 1.18 Cable shield connection options inter-unit lines, ground loop induction is usually not a problem because the signals are not susceptible, and the cable shield is best connected to ground at both ends. The important point is that each conductor has a distributed (and measurable) capacitance to the shield, so that currents on the shield will flow as long as there are ac signals propagating within it. These shield currents must be provided with a low-impedance ground return path so that the shield voltages do not become substantial. The same applies in reverse when you consider coupling of noise induced on the shield into the conductors. ^ ^ % ^ ^ ^ x ^ 2 % Y ^ ^ 3 — = ic ac =c J L ^ j ^ x x x x x x x x x x x x x x x x x x x T x x x x x x x x ^ ^ x x x x x x x ^ ^ x x x x x x x x x x x x x x x x 3 V v x x ^ Figure 1.19 Conductor-to-shield coupling capacitance Surface transfer impedance At high frequencies, the notion of surface transfer impedance becomes useful as a measure of shielding effectiveness. This is the ratio of voltage developed between the inner and outer conductors of Shielded Cable due to interference current flowing in the shield, expressed in milliohms per unit length. It should not be confused with characteristic impedance, with which it has no connection. A typical single braid screen will be ten milliohms/m or so below 1MHz, rising at a rate of 20dB/decade with increasing frequency. The common aluminium/mylar foil screens are around 20dB worse. Unhappily, surface transfer impedance is rarely specified by cable manufacturers.
eBook - ePub- Peter Wilson(Author)
- 2011(Publication Date)
- Newnes(Publisher)
The important point is that each conductor has a distributed (and measurable) capacitance to the shield, so that currents on the shield will flow as long as there are AC signals propagating within it. See Figure 1.19. These shield currents must be provided with a low-impedance ground return path so that the shield voltages do not become substantial. The same applies in reverse when you consider coupling of noise induced on the shield into the conductors. FIGURE 1.19 Conductor to shield coupling capacitance Surface transfer impedance At high frequencies, the notion of surface transfer impedance becomes useful as a measure of shielding effectiveness. This is the ratio of voltage developed between the inner and outer conductors of Shielded Cable due to interference current flowing in the shield, expressed in milliohms per unit length. It should not be confused with characteristic impedance, with which it has no connection. A typical single braid screen will be 10 milliohms/m or so below 1 MHz, rising at a rate of 20 dB/decade with increasing frequency. The common aluminum/Mylar foil screens are around 20 dB worse. Unhappily, surface transfer impedance is rarely specified by cable manufacturers. 1.1.12 The safety earth A brief word is in order about the need to ensure a mains earth connection, since it is obvious from the preceding discussion that this requirement is frequently at odds with anti-interference grounding practice. Most countries now have electrical standards which require that equipment powered from dangerous voltages should have a means of protecting the user from the consequences of component failure. The main hazard is deemed to be inadvertent connection of the live mains voltage to parts of the equipment with which the user could come into contact directly, such as a metal case or a ground terminal. Imagine that the fault is such that it makes a short circuit between live and case, as shown in Figure 1.20
- Junqi Zheng(Author)
- 2019(Publication Date)
- Wiley(Publisher)
Magnetic field is induced by high current and low voltage signal in circuit. The propagation of the magnetic field can be regarded as the coupling due to mutual inductance. Magnetic shielding is mainly achieved by the low reluctance of high permeability magnetic material, which can shunt the magnetic flux, and significantly reduces the internal magnetic field strength of the protected area by shield. For shield design, these approaches may decrease the shield’s effectiveness:- Generally selecting a high permeability magnetic material, such as permalloy
- Increasing the thickness of the shield
- Placing the shielded objects not too close to the shield, in order to reduce the magnetic flux passing through the shielded objects
- Paying attention to the mechanical design of the shield, including the seams and vents
The electromagnetic field is the electromagnetic wave generated by the alternating propagation of electric field and magnetic field. Electromagnetic shielding is a method to prevent electromagnetic field propagation in space by the shield. When the electromagnetic wave arrives at the surface of the shield, due to the discontinuity of the interface impedance between air and metal, the incident wave will be reflected. This kind of reflection does not require very thick shielding material, as long as the impedance discontinuity exists on the interface. The energy not reflected by the shield is attenuated or absorbed by the shield. The remaining energy not absorbed by the shield; while propagating to the other metal surface, it meets the metal‐air impedance discontinuity and is reflected again. There is multi‐reflection between the two metal interfaces.The electrical field around circuits is generated by signals with high voltage and low current, which can be regarded as the coupling caused by parasitic capacitance. Electric field shielding is to change the original coupling path; hence, the electric field cannot reach the other end.
eBook - ePubElectromagnetic Compatibility
Principles and Applications, Second Edition, Revised and Expanded
- David Weston(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
When a Shielded Cable cannot be used, any spare conductors in the cable should be connected to a “clean” ground. If the cable enters a metal enclosure, current flow on the outside of the enclosure is likely to be minimal, whereas current flow on the inside of the enclosure may be considerable. Thus connection of the spare conductors or the shield of the cable to the outside of the enclosure is likely to be effective, whereas connection of spare conductors or the shield of a cable to the inside of the enclosure or to a noisy signal ground may increase the level of radiation!7.8 Shielded Connectors, Backshells, and other Shield Termination Techniques
The effectiveness of a Shielded Cable is often compromised by use of a connector that exhibits a high transfer impedance or low shielding effectiveness (i.e., high leakage).Connectors specifically designed for use in harsh EM environments usually include metal fingers that make electrical contact between the mating halves of the connector and reduce the high transfer impedance inherent in a purely metal-to-metal interface that is not under pressure.In addition, the bulkhead mounting section of the connector often contains a groove for the inclusion of an O-ring type of EMI gasket. Where the surface is flat, a rectangular gasket may be used of the correct material, as discussed in Chapter 6.5. Use of the wrong material, as previously noted, may decrease the shielding effectiveness. EMI backshells are designed to connect the shields concentrically around the backshell with a low-impedance connection, with another low impedance at the backshell-to-connector interface. Backshells designed for less harsh EM environments but that provide some EM protection, for example, to meet FCC levels of emission, are often constructed of nonconductive material coated with a thin layer of deposited conductive material. It should be emphasized that such backshells are not suitable for the conduction of high-level transient current such as that indirectly induced in cables by EMP or a lightning strike. The shielding effectiveness of various types of backshells designed for D-type connectors is plotted in Figures 7.52 and 7.53 , from Ref. 20 . The test setup is shown in Figure 7.54
eBook - ePubEMC for Installers
Electromagnetic Compatibility of Systems and Installations
- Mark Van Helvoort, Mathieu Melenhorst(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
The symmetry is even further disturbed when cables are being pulled (too hard) instead of being laid. The unbalance in twisted-pair networks is the most important reason for disturbances. This unbalance is called transverse conversion loss (TCL) (IEC 11801 2009). If the TCL is too large the induced disturbance can be reduced by adding a shield (see Section 3.2.3). For high data rate connections, which means high frequencies, great care should be taken when connecting the cable shield to ground (see Section 3.2.4). Under laboratory conditions it has been shown that improper connection can lead to higher radiation of an FTP cable compared to a UTP cable. From history it seems that at the introduction of higher speeds network equipment Shielded Cables are preferred, while when this new technology has matured unShielded Cables can be used. One of the arguments regularly used against using shielded twisted-pair cables is that the shield causes additional damping of the transmitted signal. Theoretically, this is correct; however, in practical installations, the same argument can be used against unshielded cabling, because cabling is typically routed in metal conduits in which case damping also occurs but in an undefined manner. 3.4 EMC characterization A variety of properties are used to characterize the EMC performance of cables and cable assemblies, such as transfer impedance, shielding effectiveness, screening attenuation, and coupling attenuation, which will be discussed in the next paragraphs. In the past also optical braid coverage was used, but has proven to be not useful. 3.4.1 Transfer impedance The transfer impedance (Schelkunoff 1934), sometimes also called surface transfer impedance, is the most robust method to quantify EMC performance. Its value can be determined by sending a CM current over the cable assembly and measure the induced differential voltage
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