5.14 Electromagnetic Environment.pdf

Electromagnetic Environment (5.14) Learning Objectives 5.14 Understand how to minimise or prevent EMI/RFI from being generated by devices (S). 5.14.1 Explain the influence of the following phenomena on maintenance practices for electronic systems: EMC - electromagnetic compatibility (...

Electromagnetic Environment (5.14) Learning Objectives 5.14 Understand how to minimise or prevent EMI/RFI from being generated by devices (S). 5.14.1 Explain the influence of the following phenomena on maintenance practices for electronic systems: EMC - electromagnetic compatibility (Level 2). 5.14.2 Explain the influence of the following phenomena on maintenance practices for electronic systems: EMI - electromagnetic interference (Level 2). 5.14.3 Explain the influence of the following phenomena on maintenance practices for electronic systems: HIRF - high intensity radiated field (Level 2). 5.14.4 Describe the influence of the following phenomena on maintenance practices for electronic systems: lightning and lightning protection (Level 2). 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 61 of 172 CASA Part 66 - Training Materials Only  Electromagnetic Interference in Electrical Systems Electromagnetic Environment Terminology Throughout this topic a number of definitions, terms and acronyms will be referenced. Key Definitions Terms Definition Electromagnetic Environment The totality of electromagnetic phenomena existing at a given location. (EME) Electromagnetic Capability of equipment or systems to be operated in the intended operational Compatibility electromagnetic environment at designed levels of efficiency without (EMC) degradation due to electromagnetic interference. Electromagnetic Defined by NATO as any electromagnetic disturbance that interrupts, Interference obstructs or otherwise degrades or limits the effective performance of (EMI) electronics/electrical equipment. High-Intensity Radiated Field Man-made sources of electromagnetic radiation generated external to aircraft. (HIRF) Radio- Electromagnetic interference (EMI), also called radio-frequency interference Frequency (RFI) when in the radio frequency spectrum, is a disturbance generated by an Interference external source that affects an electrical circuit by electromagnetic induction, (RFI) electrostatic coupling, or conduction. Avionic Frequency Bands The frequency bands used by avionics systems span the electromagnetic spectrum from a few kilohertz to several gigahertz. VHF Omnidirectional Range (VOR) is a radio beacon used in point-to-point navigation. It operates from 108 to 118 MHz. Glideslope Systems used during landings operate in the 328 to 335 MHz range. Distance-Measuring Equipment (DME), which gauges the distance between the aircraft and ground- based transponders, operates at just over 1 GHz. Also in the spectrum above 1 GHz are global positioning, collision avoidance and cockpit weather radar systems. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 62 of 172 CASA Part 66 - Training Materials Only  PED Frequency Bands Personal Electronic Devices (PEDs) operate at frequencies from 10 to 15 KHz for AM radios and up to 400 MHz for laptop computers. When the higher harmonics of these signals are taken into account, the emitted frequencies cover almost the entire range of navigation and communication frequencies used on the aircraft – and PEDs are just a single class of EMI emitters. When the full spectrum of other radiated and conducted EMI emitters is considered, it becomes clear that the entire system of electronic equipment aboard aircraft is at risk of EMI. But the fact that all avionics equipment and cabling which are critical to the functioning of aircraft are shielded against EMI raises an interesting question: How exactly does EMI, such as Radio Frequency Interference (RFI) from a passenger radio or laptop, permeate the system? EMI Permeation In many cases, the cause is simply inadequate shielding, or shielding which has been damaged during servicing or has degraded due to corrosion, thus increasing the resistance of the electrical connection to ground. As effective shielding depends on good grounding, any additional resistance in the system – for example, at a corroded backshell or a poorly installed shield termination crimp ring – can enable the wires to pick up interfering signals directly. Aircraft with navigation and communication antennas located outside the skin can also pick up EMI radiated through passenger windows and other unshielded openings in the plane. The pathway for RFI from a passenger PED would, in this example, be out the window, back into the plane via an unprotected or RFI-sensitive antenna, and then directly into a navigation receiver, autopilot computer or other avionics device. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 63 of 172 CASA Part 66 - Training Materials Only  Electromagnetic Interference In the early 1980s, a British Harrier Jump Jet landed for the very first time on a U.S. aircraft carrier. On the flight deck, the carrier's yellow-shirted deck crew stood ready to secure the jet and roll it to the hangar deck elevator. On the bridge, a dozen pairs of eyes watched intently as the Harrier made its signature vertical landing. But as the powerful vectored thrust turbofan engine brought the aircraft down in its measured descent to the flight deck, a terrible accident occurred: Electromagnetic interference radiating from the carrier's massive island of electronic equipment disrupted the Harrier's electronic controls and triggered the pilot's emergency ejector seat. The pilot was propelled through the canopy of the jet with explosive force, killing him instantly. On the flight deck, emergency crews worked rapidly to control the now pilotless plane. But the damage was done. Electromagnetic Interference (EMI), defined by the North Atlantic Treaty Organisation (NATO) as an electromagnetic disturbance which interrupts, obstructs or otherwise degrades the effective performance of electronic or electrical equipment, had claimed another victim. Aircraft are designed and built to withstand interference from a broad range of electromagnetic fields. In fact, the outer shell of the plane as well as its internal electronic equipment and interconnect cabling are designed to prevent penetration of disruptive electromagnetic signals – both those generated internally and those emanating from external sources. Electromagnetic interference Effects of EMI Electromagnetic interference can jam sensitive equipment and burn out electric circuits. In aircraft, EMI can affect everything from fly-by-wire flight control systems to a cockpit fuel gauge, and in extreme cases it can send a plane into an uncommanded dive or shut down a critical avionics system. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 64 of 172 CASA Part 66 - Training Materials Only  Electromagnetic Compatibility EMC is the ability of equipment to operate satisfactorily in its EM environment without introducing intolerable EM disturbances to other electrical devices in that environment. Aviation Australia Electromagnetic compatibility 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 65 of 172 CASA Part 66 - Training Materials Only  Elements of an EMC Problem There are three essential elements to any Electromagnetic Compatibility (EMC) problem: A source of an electromagnetic phenomenon A receptor (or target) that cannot function properly due to the electromagnetic phenomenon A path between them that allows the source to interfere with the receptor. Each of these three elements must be present, although they may not be readily identified in every situation. EMC problems are generally solved by identifying at least two of these elements and eliminating (or attenuating) one of them. Aviation Australia Three elements of an EMC problem Potential Sources Potential sources of EMC problems include radio transmitters, power lines, electronic circuits, lightning, lamp dimmers, electric motors, arc welders, solar flares and just about anything that utilises or creates electromagnetic energy. Potential Receptors Potential receptors include radio receivers, electronic circuits, appliances, people and nearly any object that utilises or can detect electromagnetic energy. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 66 of 172 CASA Part 66 - Training Materials Only  Coupling Path Methods of coupling electromagnetic energy from a source to a receptor fall into four categories: Conducted (electric current) Inductively coupled (magnetic field) Capacitively coupled (electric field) Radiated (electromagnetic field). Coupling paths often utilise a complex combination of these methods, making the path difficult to identify even when the source and receptor are known. There may be multiple coupling paths, and steps taken to attenuate one path may enhance another. EMI coupling paths 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 67 of 172 CASA Part 66 - Training Materials Only  Electric and Magnetic Fields An electric field has the ability to exist with only one pole. All magnetic material exists with two poles. In this way, an electric field is not like a magnetic field. The lines of force of a magnetic field go from north to south in a curved manner. Electric fields do the same with opposite charges present. In this case, electric force naturally travels in straight lines from the centre of its point of origin outward, no matter its size. Aviation Australia Electric force Electric (E) fields are created by voltages on conductor areas, and magnetic (M) fields are created by currents flowing (in loops, as they always do). All electrical signals create both types of fields with their conductors, so all conductors leak their signals to their external environment and allow external fields to leak into their signals. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 68 of 172 CASA Part 66 - Training Materials Only  Leakage and Antenna Effect of Conductors The frequencies in common use in daily life range from AC power lines through audio frequencies; long, medium, and short-wave radio; FM and TV broadcast; to 900 MHz and 1.8 GHz for mobile phones. The real spectrum is busier than this – all of the range above 9 kHz is used for something by someone. Mains rectifiers emit switching noise at harmonics of the fundamental to considerable frequencies, depending on their power. Aviation Australia Frequencies in common use in daily life A 5-kVA or so power supply (whether linear or switch-mode) can fail conducted emissions limits up to several megahertz due to the switching noise of its 50- or 60-Hz bridge rectifier. Thyristor-based DC motor drives and phase-angle AC power control will have similar emissions. These emissions can easily interfere with long- and medium-wave broadcasting and with part of the short-wave band. Switch-mode power converters can operate at fundamental frequencies between 2 and 500 kHz. It is not unusual for a switch-mode convertor to have significant levels of emissions at 1000 times its switching frequency. An overlay of decibels (dB) versus megahertz shows the emissions from a 70- kHz switching power supply of a personal computer. These emissions can interfere with radio communications up to and including the FM broadcast band. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 69 of 172 CASA Part 66 - Training Materials Only  Aviation Australia Emissions from a 70 kHz switching power supply of a personal computer It is not unusual for these commonplace items to exceed emissions limits at frequencies of 200 MHz or more. As personal computers are now using 400-MHz clocks and heading for 1 GHz, it is obvious that digital technology is capable of interfering with (and being interfered with) all of the upper range of our spectrum. The reason for mentioning this is that all conductors are antennas. They all convert conducted electricity into electromagnetic fields, which can then leak out into the wider environment. They all convert electromagnetic fields in their locality to conducted electrical signals. There are no exceptions to this rule in our universe. Conductors are thus the principal means by which signals cause radiated emissions and by which external fields contaminate signals (susceptibility and immunity). At distances greater than one-sixth of the wavelength (l) of the frequencies of concern, E and M fields develop into full electromagnetic (EM) fields with both electric and magnetic components. For example, the transition to full EM fields occurs at approximately 1.5 m for 30 MHz, 150 mm for 300 MHz, and 50 mm for 900 MHz. So, as frequencies increase, treating conductors as merely electric or magnetic field emitters and receivers becomes inadequate. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 70 of 172 CASA Part 66 - Training Materials Only  Frequencies increase, treating conductors as merely electric or magnetic field emitters and receivers becomes inadequate Another effect of increasing frequencies is that when wavelength is comparable with conductor length, resonances occur. At some of these frequencies, the conversion of signals to fields (and vice versa) can reach almost 100%. For example, a standard whip antenna is merely a length of wire and is a perfect converter of signals to fields when its length equals one fourth of its wavelength. This is a simplistic description, but as far as the user of cables and connectors is concerned, the important thing is that all conductors can behave as resonant antennas. Obviously we want them to be very poor antennas, and assuming that a conductor is like a whip antenna (good enough for our purposes), we can use a conductor length versus antenna efficiency table as a guide. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 71 of 172 CASA Part 66 - Training Materials Only  Aviation Australia Conductors can behave as resonant antennae The vertical axis shows metres of conductor length, and the spectrum is retained as a visual guide. The rightmost diagonal shows conductor length versus frequency for a perfect antenna. Obviously, at frequencies in common use, even very short conductors can cause emissions and immunity problems. A signal or field at 100 MHz finds a 1-m-long conductor to be a very efficient antenna, and at 1 GHz, 100-mm conductors make good antennae. This simple fact is responsible for a large number of ‘black magic’ EMC problems. The middle diagonal shows conductor lengths which do not make very efficient antennas, but can still cause problems. The left diagonal shows lengths which are so short that (for all except the most critical products) their antenna effects can usually be neglected. How many times have you heard someone say, ‘It’s OK, I’ve earthed it’? All earth conductors are antennas, too. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 72 of 172 CASA Part 66 - Training Materials Only  Natural Sources of EMI Historically, EMI has been a factor in aircraft construction since the 1930s, when brass conduit was first used to shield cabling for newly introduced electronic communication systems from reciprocating engines and magneto ignitions. But such man-made electromagnetic ‘noise’ generated incidentally by motors, generators and other machinery turned out to be just one of the classes of EMI which would affect the safe operation of aircraft. Interference Naturally occurring radio noise originating from atmospheric disturbances (including lightning) and extraterrestrial sources (such as sunspots) can also degrade the performance of electronic equipment. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 73 of 172 CASA Part 66 - Training Materials Only  Lightning Strikes and Lightning Protection Protection Against the Damaging Effects of Lightning The heating and magnetic forces produced by the high currents of a lightning strike can cause structural damage (direct effects), and the associated electric and magnetic fields can induce transients which may damage or disrupt electrical equipment (indirect effects). Aircraft lightning strike Operators of systems which are critical to safety need to know that these functions will not be jeopardised by lightning effects. Lightning damage may also lead to expensive downtime and repairs. The current pulse which flows during a lightning strike generates strong local magnetic fields. These fields generate induced voltage and current transients in nearby wiring, which can cause damage or disruption to the electrical or electronic equipment linked by the wiring. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 74 of 172 CASA Part 66 - Training Materials Only  Electrical Bonding Electrical bonding is important for lightning protection of aircraft and systems, especially to facilitate safe conduction of lightning currents through the airframe. Most bonding is achieved through normal airframe riveted or bolted joints, but some externally mounted parts, such as control surfaces, engine nacelles and antennas, may require additional bonding provisions. Bonding between airframe and flight control surface Antenna and air data probes that are mounted on exterior surfaces within lightning strike zones should be provided with a means to safely transfer lightning currents to the airframe and to prevent hazardous surges from being conducted into the airframe via antenna cables or wire harnesses. Usually the antenna mounting bolts provide adequate lightning current paths. Screen capture from an A320 AMM – Installation of the VHF antenna 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 75 of 172 CASA Part 66 - Training Materials Only  Man-Made Sources of EMI RF Emitters Communications signals may also interfere with the operation of sensitive electronic equipment. To protect avionics systems from this class of interference, intentional radio frequency (RF) emitters like CB radios, remote-controlled toys and walkie-talkies are banned outright on commercial airline flights. Most, but not all, airlines extend the ban to portable radios and television receivers. Handheld radio can be a source of EMI 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 76 of 172 CASA Part 66 - Training Materials Only  Personal Electronic Devices Personal Electronic Devices (PED) such as laptop computers, handheld scanners and game players, while not intentional emitters, can produce signals in the 1-MHz range and can therefore affect the performance of avionics equipment. As navigation cabling and other critical wiring runs along the fuselage inside the aircraft skin, it is no wonder that passengers sitting just a few feet away are warned about the indiscriminate use of such devices. Since the thin sheet of dielectric material that forms the inside of the passenger compartment – typically fibreglass – offers no shielding whatsoever and since commercial passenger jets contain up to 150 mi of electrical wiring, it is extremely important for passengers to heed regulations for the use of disruptive electronic equipment. Obviously, such internal sources of EMI are particularly dangerous to aircraft because they are so close to the systems they might affect. Personal electronic devices can be a source of EMI 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 77 of 172 CASA Part 66 - Training Materials Only  External RF Sources External sources, such as radio and radar transmitters on the ground, or radar from a passing military plane, can be even more disruptive due to their high power and frequency. Helicopter radar system components 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 78 of 172 CASA Part 66 - Training Materials Only  Effect of Airframe As if the many external and internal sources of EMI were not enough of a concern, a major EMI co- conspirator in aircraft is the aluminium airframe itself, which in certain circumstances can act as a resonant cavity. Behaving much like a satellite dish, the airframe can compound the effects of both internal and external EMI by concentrating transient signals and broadcasting the interference into nearby equipment. Photo by Pexels Airframe can compound effects of both internal and external EMI 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 79 of 172 CASA Part 66 - Training Materials Only  High-Intensity Radiated Field High-Intensity Radiated Emissions (HIRF), also known as High-Intensity Radio Emissions, are emissions from radar, microwave, radio and television transmitters; high-power AM/FM radio broadcast systems; TV transmitters; airport and weather radar, both ground-based and airborne, such as surveillance radar; and other powerful communications systems. HIRF encompasses man-made sources of electromagnetic radiation generated external to the aircraft and considered as possibly interfering with safe flight. The easiest way to distinguish HIRF from other types of EMI is to state what it is not. HIRF does not include interference among on-board systems; this type of interference is referred to as an EMC issue. HIRF also does not include EMI effects caused by PEDs carried by passengers, such as smartphones, laptop computers and portable radios. However, a rapid increase in the technology of personal communications causes concern about the potential EMI threat posed by PEDs. HIRF does not include the effects of lightning or of static electricity generated on the aeroplane; this is called Electrostatic Discharge (ESD). The effect of lightning on aircraft and avionics systems is similar to that produced by low-frequency HIRF (kHz frequency range). HIRF sources are only those emitters that intentionally generate emissions. Pixabay Public Licence Ground radio towers can be a source of HIRF The frequency spectrum between 10 kHz and 18 GHz divides into two distinctly different parts around 400 MHz. Below this frequency most electro­magnetic use of the spectrum is for communications and navigation devices which are generally weakly directional, continuously on the air, and radiate signals that are modulated so that peak and average power are very close to each other. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 80 of 172 CASA Part 66 - Training Materials Only  Aviation Australia High Intensity Radiated Field (HIRF) Systems operating above 400 MHz are generally narrow in beam width and often pulsed. Their peak signal is much higher than the average radiated signal. Applications in this range are surveillance devices such as radar, high speed data signals, and satellite command and control and telemetry signals. They also include telemetry and command and control signals associated with modern weapons systems such as missiles. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 81 of 172 CASA Part 66 - Training Materials Only  EMI Management Addressing EMI Problems When electromagnetic interference is suspected, the first step towards resolving the problem is to determine the mechanism used for energy transfer to the affected device(s): Radiation Conduction Induction. Radiated Electromagnetic Interference Radiated electromagnetic energy entering an adjacent device is one of the most difficult to identify and control. The existence of this condition implies that there is insufficient control of ingress energy in an affected adjacent device (target), and of emissions from the radiating device (source). In general, when dealing with radiated EMI, the facility is faced with three options: Remove (or reduce) the source ‘Harden’ the target Separate the devices (remove the path). Aviation Australia Addressing EMI problems - EMI management Effective shielding of avionic devices must anticipate: Radiated susceptibility – the degree to which outside interference affects the reliable functioning of equipment Radiated emissions – the extent to which the device itself creates electromagnetic waves which can affect its function. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 82 of 172 CASA Part 66 - Training Materials Only  In both cases, the techniques for managing interference include reflecting the signals outright, reducing entry points in equipment and cable shields, absorbing the interference in permeable material and dissipating it as heat, or conducting the EMI along the skin of the device/cable and shunting it to ground. EMI Permeation In many cases the cause of EMI permeation is simply inadequate shielding, or shielding which has been damaged during servicing or has degraded due to corrosion, thus increasing the resistance of the electrical connection to ground. As effective shielding is dependent on good grounding, any additional resistance in the system - for example at a corroded backshell or a poorly installed shield termination crimp ring - can enable the wires to pick up interfering signals directly. Aircraft with navigation and communication antennas located outside the skin of the aircraft can also pick up EMI radiated through passenger windows and other unshielded openings in the plane. The pathway for RFI from a passenger PED would, in this example, be out the window, back into the plane via an unprotected or RFI sensitive antenna, and then directly into a navigation receiver, autopilot computer or other avionic device. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 83 of 172 CASA Part 66 - Training Materials Only  EMI Shielding Shielding from EMI Types Radio Frequency Interference (RFI) – This is given off from anything that operates within the radio wave spectrum range, typically frequencies greater than 10 kHz. Some of these devices include antennas, electronic and communications equipment, wireless network devices, and radar and navigation aids. An effective shield to contain or protect equipment from the effects of RFI is generally a foil wrap shield followed by a high-coverage braided shield. Crosstalk – Crosstalk occurs when cables feeding different components are bundled together, which happens when they are operating at different frequencies and/or voltages. This is prevented by isolating the wires for individual components with a shield or altering the lay length of the twisted pairs. Electromagnetic Interference (EMI) – This often occurs when cables are run in close proximity to components which produce an electromagnetic field. Industrial motors, welding equipment and elevators are a few examples of devices that produce a low-frequency magnetic or electrical field. This can be prevented by employing a ferrous metal braid and a quality grounding technique. Electronic Environment – Multitudes of electrical components emit noise, including high-voltage AC power cables, relays, coil -driven solenoids and power supplies. A shared ground with a shielded noisy component often results in intermittent noise. This can be controlled by shielding the signal cable with a non-ferrous braid with moderate coverage. High Voltage Transients – High-voltage transient spikes and ESD can be extremely detrimental to sensitive equipment. Shielding with non-ferrous shield and proper isolation is the best way to eliminate them. Summary In practical terms, EMI management is accomplished by plating the skins of cases and cable shields, building up the density (thickness) of shield material, or eliminating line-of-sight entry points through which electromagnetic waves can penetrate or escape. The frequency of the interfering signal is a critical concern when designing an effective shielding solution. Low-frequency magnetic waves in the 1 to 30 kHz range, for example, are most effectively shielded by absorbing the signals in highly permeable material. High-frequency signals (30 kHz and above) are most effectively shielded by reducing entry windows and by ensuring adequate surface conductivity to ground. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 84 of 172 CASA Part 66 - Training Materials Only  Cable Shielding In interconnected applications, wires and cables are typically shielded by placing a conductive material between the cable conductor and its outer jacket, or by covering individual conductors within a cable with shielding material. The shield can act on EMI in two ways. First, it can reflect the energy. Second, it can pick up the noise and conduct it to ground. In either case, the EMI does not reach the conductors. Some energy still passes through the shield, but it is so highly attenuated that it does not cause interference. Aviation Australia Cable shielding Shields must also be effectively terminated to the connector backshell to prevent radiation entering the system at the backshell/connector/shield interface and defeating the purpose of the shield. Cable shielding is manufactured in a wide range of designs and configurations. Each type of shielding has advantages which must be considered when selecting the best and most cost-effective option for a given application. Braided Shields Braided shields provide exceptional structural integrity while maintaining good flexibility and flex life. Ferrous braided shields are effective at minimising low-frequency EMI at audio and RF ranges. In use, the EMI reduction depends on the signal amplitude and frequency in relation to the many combinations of mesh count, wire diameter and braid material. Generally, the higher the percentage of braid coverage, the more effective the shield is against high-frequency emissions. Materials include tin-plated copper, nickel-plated copper and tin-plated iron/copper as well as hybrid materials such as metallised Kevlar (Aracon®). 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 85 of 172 CASA Part 66 - Training Materials Only  Foil Shields Foil shields are made from aluminium foil laminated to a polyester or polypropylene film. Foil shields provide 100% cable or component coverage, improving protection against radiated emission and ingress at audio and radio frequencies. Because of their small size, foil shields are commonly used to shield individual pairs in multi-conductor cable to reduce crosstalk. Foil shields may also be bonded to a coaxial cable insulation or cable jacket with a layer of adhesive, allowing for faster, easier and more reliable termination. Used in combination with braided shield, foil can provide maximum shield efficiency across the frequency spectrum. Aviation Australia Coaxial cable with braided and foil shielding 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 86 of 172 CASA Part 66 - Training Materials Only  Multi-Shielding Combining multiple shielding types is known as multi-shielding. Multi-shielding provides superior attenuation that is effective from the kilohertz to the gigahertz frequency range. It is effective for sensitive conductors and power wiring exposed to a wide frequency range, interference environment, and internal cross-coupling. Shielding offers the following benefits: Provides electrostatic protection Attenuates electrical field interference Protects conductors from external magnetic fields Provides metal armour protection for conductors Contains sources of EMI Minimises the requirements for segregation and special routing of power and signal wiring and interconnecting cables Reduces the effects of Electromagnetic Pulses (EMPs). Shield Construction A cable whose conductors are to be protected from an external source of EMI has the outer braiding constructed of a low-permeability material laid over the inner braiding, which is constructed of a high-permeability material. First Boundary Reflection Loss The incident radiation of external EMI strikes the outside surface of the outer shield (the first boundary reflection loss area) where some of the radiation is reflected. The radiation then penetrates into the material, where absorption takes place. Second Reflection Loss The second reflection loss occurs at the inside surface of the outer shield (the second boundary reflection loss area). Third Boundary Reflection Loss If the radiation is strong enough, it will proceed through this outer shield to the outside surface of the inner shield (the third boundary reflection loss area), where more of the reduced radiation is reflected. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 87 of 172 CASA Part 66 - Training Materials Only  Fourth Boundary Reflection Loss From there it proceeds to the fourth boundary reflection loss area (the inside surface of the inner shield), during which time some of the radiation becomes absorbed into the inner shield material. To prevent electromagnetic interference being radiated from signals present in the wires of a cable, the inner layer of braiding should be of low-permeability material and the outer layer constructed of high-permeability material. Shields can also be configured with a high-permeability shield sandwiched between two low- permeability shields. In any event, the low-permeability shield should always be closest to the source of interference. Low-permeability shield braided over a high-permeability shield 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 88 of 172 CASA Part 66 - Training Materials Only  Effect of Multi-Shielding Multi-shielding protects signals susceptible to magnetic field induction as well as to electrical fields. The input signal is interference-free until it comes into contact with the power line magnetic field (or other magnetic field), which then causes a distorted signal in the equipment. The effect of multi-shielding Copper has very little EMI shielding effect at low frequencies. Multi-shielding’s low-permeability braid provides moderate shielding at low frequencies and good shielding at high frequencies. The high-permeability braid, although furnishing less shielding at high frequencies, provides excellent shielding at low frequencies. The end result is a shielding combination that is effective throughout a broad frequency range. Aviation Australia End result is a shielding combination that is effective throughout a broad frequency range 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 89 of 172 CASA Part 66 - Training Materials Only  EMI Management A shielded system is only as good as its weakest component. A high-quality cable is defeated by a low- quality connector. Similarly, a great connector cannot do anything to improve a poor cable. EMI To prevent EMI: Braid shields most effective For very lowest frequencies, only conduit is effective Resistance of shield is critical Foil shield resistance is too high (foil is thin). RFI To prevent RFI: Foil shields most effective Braid shields become ‘wavelength dependent’ Holes in braid let high frequencies in or out. Broadband Coverage Braid for low frequencies Foil for high frequencies. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 90 of 172 CASA Part 66 - Training Materials Only  EMC/EMI Problems Several approaches to mitigate EMC/EMI problems can be employed, although many of the solutions listed below are more directly controlled by the manufacturer of the affected and radiating devices. EMI Caused by Radiation When radiation is the suspected method of propagation, an increase in the shielding of the major units, both internal and external, might be attempted. The addition of Radio Frequency (RF) filters to wiring entering and exiting the equipment, from both the radiating source and the susceptible devices, may reduce the intensity or the effect of the received radiation. The effectiveness of grounding (ohmic values as well as lengths) should be explored. The length of the connection between the radiating device and an effective radio frequency ground can be critical. An increase in separation between the devices may alleviate the problem. EMI Caused by Conduction Interference by conduction can usually be reduced or eliminated by a careful analysis of wiring routing and connections, including potential loops existing between electrical busses or other devices and the affected equipment. Adding filters to inputs and power sources is another possible solution. EMI Caused by Induction When induction is suspected, there are several approaches to mitigation. Cabling can be replaced with more effectively shielded types. When the substitution of special cabling with a shielded type is not possible, rerouting or increasing the distance between cable bundles may offer a solution. EMI Reduction Options Electromagnetic interference can be reduced or eliminated by using various suppression techniques. The amount of EMI produced by a radio transmitter can be controlled by cutting transmitting antennas to the correct frequency, limiting bandwidth, and using electronic filtering networks and metallic shielding. Radiated EMI during transmission can be controlled by physically separating the transmitting and receiving antennas, using directional antennas and limiting antenna bandwidth. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 91 of 172 CASA Part 66 - Training Materials Only  EMI Minimisation Balanced Circuits Twisted Wires It is usual to use twisted wires and the current is said to be balanced because the impedance from each pair of connecting wires to earth are equal. The centre taps of the balanced sides of the coupling transformer may also be earthed. Twisted wires The balanced circuit overcomes the three disadvantages of the unbalanced circuit. Inductive (L) and capacitive (C) pick up are eliminated since equal and opposite currents are induced in each wire of the balanced circuit and thus cancel out. The same applies to interfering currents in the common earth in the case where the centre taps of the transformers are earthed. Advantages Electrical noise going into or coming from the cable can be prevented. Cross-talk is minimised. Disadvantages Twisted pair’s susceptibility to electromagnetic interference greatly depends on the pair twisting schemes staying intact during the installation. Thus, twisted pair cables usually have tight requirements for maximum pulling tension as well as minimum bend radius. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 92 of 172 CASA Part 66 - Training Materials Only  Wiring for EMI minimisation PCB Continuous Ground Plane A continuous ground plane under all high-speed signal lines on PCBs will help reduce the production of EMI. A ground plane layer printed over sensitive or noisy circuitry provides shielding from EMI and reduces emissions and crosstalk. Aviation Australia PCB ground plane 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 93 of 172 CASA Part 66 - Training Materials Only  Structure Shielding Structure shielding may be used in electronic assemblies to cover sensitive components or prevent EMI from damaging certain components. Structure shielding is a method of protecting susceptible circuits inside the aircraft from lightning, HIRF and EMI. Metal structures provide a low impedance path for currents generated by EMI so that these currents will be minimised on systems and wiring. In addition, enclosed structures, such as the fuselage, provide some shielding for radiated fields. The principle of shielding is derived from the fact that the total charge completely enclosed by a conductive surface will be zero, regardless of electromagnetic fields external to the surface. The completely closed conductive surface is often called a Faraday cage, named after Michael Faraday, the English physicist and chemist who provided experimental data proving the concept. Faraday cage as possible with respect to external EMI that may disrupt internal circuits of the aircraft Of course, no aircraft can be a perfect Faraday cage since there must be openings, doors, windows, vents, etc. The goal of the structure shielding is to seal the cracks or holes in the fuselage and make it as close to a Faraday cage as possible with respect to external EMI that may disrupt internal circuits of the aircraft. It is becoming more common to see aircraft structure shielding in new aircraft designs. 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 94 of 172 CASA Part 66 - Training Materials Only  This is due to several factors: The increase in power levels of internal and external sources (lightning and HIRF) The increased sensitivity and number of electronic components within the aircraft Aircraft designs that incorporate composite materials in the structure, which are not as conductive as metal and do not provide the same level of shielding, particularly against lightning. Static Discharging Rather than allow any static build up to discharge randomly points on the airframe, low resistance paths to atmosphere are created by placing static wick dischargers at the airframe extremities, such as the wing and fin tip areas. The interference produced is thus reduced and takes place well away from most avionic and electrical equipment. However, deterioration of the dischargers with time can cause a gradual increase in static interference and must be guarded against. Discharge wicks 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 95 of 172 CASA Part 66 - Training Materials Only  General Precautions Capacitor Filters The inclusion of capacitors across potentially noisy components such as relays and motors is often incorporated to reduce interference. Failure of these capacitors will often cause noise to be present, particularly in audio systems, which may originate in systems quite divorced from the avionic equipment. The brushes and commuters of rotating machines must be kept clean and smooth to prevent arcing, the primary cause of interference from these devices. Compass Safe Some LRUs may produce EMI which would have a detrimental effect on the aircraft compass system. Such items have the “compass safe distance” marked on them, and must be located in the aircraft in excess of this distance from the compass installation. Aviation Australia General precautions for reducing EMI and RFI 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 96 of 172 CASA Part 66 - Training Materials Only  Antennas Electromagnetic Interference can be reduced or eliminated by using various suppression techniques. The amount of EMI that is produced by a radio transmitter can be controlled by cutting transmitting antennas to the correct frequency, limiting bandwidth, and using electronic filtering networks and metallic shielding. Radiated EMI during transmission can be controlled by the physical separation of the transmitting and receiving antennas, the use of directional antennas, and limiting antenna bandwidth. Aviation Australia Location of aerials to reduce EMI 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 97 of 172 CASA Part 66 - Training Materials Only  Fibre Optics Among their many other virtues, fibre optics are completely immune to EMI. This means the evolution of fly-by-wire systems to fly-by-light systems not only expands data-carrying capacity and reduces the weight of interconnect cabling, but also eliminates the many risks and problems of EMI. For this reason, among others, many retrofit projects are currently underway to replace traditional copper conductors with fibre. Fibre optics immune to EMI 2022-07-22 B1-05b Digital Techniques / Electronic Instrument Systems Page 98 of 172 CASA Part 66 - Training Materials Only