Chapter 21. EMC Fundamentals

21.1. What Is EMC?

Electromagnetic interference (EMI) is a serious and increasing form of environmental pollution. Its effects range from minor annoyances due to crackles on broadcast reception, to potentially fatal accidents due to corruption of safety-critical control systems. Various forms of EMI may cause electrical and electronic malfunctions, can prevent the proper use of the radio frequency spectrum, can ignite flammable or other hazardous atmospheres, and may even have a direct effect on human tissue. As electronic systems penetrate more deeply into all aspects of society, so both the potential for interference effects and the potential for serious EMI-induced incidents increases.

Electromagnetic compatibility (EMC), then, is the absence of effects due to EMI. The definition of EMC, as it appears in the International Electrotechnical Vocabulary (IEC, 1990), is “The ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbance to anything in that environment.”

Some reported examples of electromagnetic incompatibility are

• In Norfolk, various makes of car would “go crazy” when they passed a particular air defense radar installation—dashboard indicators dropping to zero or maximum, lights and engines cutting out
• On one type of car, the central door locking and electric sunroof would operate when the car's mobile transmitter was used
• New electronic push-button telephones installed near the Brookmans Park medium wave transmitter in North London were constantly afflicted with BBC radio programs
• Interference to aeronautical safety communications at a U.S. airport was traced to an electronic cash register a mile away
• The instrument panel of a well-known airliner was said to carry the warning “ignore all instruments while transmitting HF”
• Electronic point-of-sale units used in shoe, clothing, and optician shops (where thick carpets and nylon-coated assistants were common) would experience lock-up, false data, and uncontrolled drawer openings
• When a piezo-electric cigarette lighter was lit near the cabinet of a car park barrier control box, the radiated pulse caused the barrier to open and drivers were able to park free of charge
• Lowering the pantographs of electric locomotives at British Rail's Liverpool Street station interfered with newly installed signaling control equipment, causing the signals to “fail safe” to red
• A digital TV set-top box initiated an air/sea rescue operation in Portsmouth harbor by creating an emission on the distress frequency
• Two Navy warships nearly collided when the radar transmissions of the frigate HMAS Anzac disabled the steering of the mine hunter HMAS Huon, Huon passing ahead of Anzac “at close range”

Many other examples have been collected over the years; the “Banana Skins” column in the EMC Journal, collated by Keith Armstrong, is a fruitful source, and the EMC group of the former U.K. Radiocommunications Agency commissioned an EMC Awareness (EMC, n.d.) Web page introducing the subject, which also contains a number of examples. Here are a few issues in more detail.

21.1.1. Portable Electronic Devices in Aircraft

Mobile cellular telephones are rapidly establishing themselves, through their sheer proliferation, as a serious EMC threat. Passengers boarding civil airliners are now familiar with the announcement that the use of such devices is not permitted on board. They may be less familiar with why this is regarded as necessary. The IFALPA International Quarterly Review reported 97 EMI-related events due to passenger “carry-on” electronic devices since 1983. To quote the Review, By 1990, the number of people boarding aeroplanes with electronic devices had grown significantly and the low-voltage operation of modern aircraft digital electronics were potentially more susceptible to EMI.A look at the data during the last 10 years indicates that the most likely time to experience EMI emissions is during cruise flight. This may be misleading, however. During the last three years, 43% of the reported events occurred in cruise flight while an almost equal percentage of events occurred in the climb and approach phases.Of particular note, during the last three years the number of events relating to computers, compact disc players, and phones has dramatically increased and these devices have been found to more likely cause interference with systems that control the flight of the aircraft.Recognizing an apparent instrument or autopilot malfunction to be EMI related may be difficult or impossible in many situations. In some reported events the aircraft was off course but indications in the cockpit displayed on course. Air traffic controllers had to bring the course deviations to the attention of the crews. It is believed that there are EMI events happening that are not recognized as related to EMI and therefore not reported.

Particular points noted by the Review were that:

• Events are on the rise
• All phases of flight are exposed (not just cruise)
• Many devices may cause EMI (phones, computers, CD players, video cameras, stereos)
• Often there will be more than one device on a flight
• Passengers will turn on a device even after being told to turn it off
• Passengers will conceal usage of some devices (phones, computers)
• Passengers will turn devices on just after takeoff and just prior to landing
• Phones are a critical problem
• Specific device type and location should be recorded and reported by the crew
• When the emitting EMI device is shut off, the aircraft systems return to normal operation (in the case of positioning errors a course change may be necessary)
• Flight attendants should be briefed to recognize possible EMI devices

These are problems especially if passengers regard their need for personal communication as more important than a mere request from the crew. An article in the Electronic Times (“Interference from Mobiles,” 2000) reports that an aircraft carrying a German foreign minister had to make an emergency landing “after key cockpit equipment cut out.” It was claimed that mobile phone transmissions could be the only explanation and it was said that, “despite repeated requests from the crew, there were still a number of journalists and foreign office personnel using their phones.”

In 2000, the Civil Aviation Authority (CAA) carried out tests on two aircraft parked at Gatwick, which reinforces the ban on the use of mobile phones while the engine is running (“Interference from Mobiles,” 2000). The tests revealed that interference levels varied with relatively small changes in the phone's location and that the number of passengers on the flight could affect the level, since they absorbed some of the signal. Further testing has been done since, publicly reported by the CAA (2003), which showed that at the GSM mobile frequencies it was possible to create the following interference effects:

• Compass froze or overshot actual magnetic bearing
• Instability of indicators
• Digital VOR (VHF omnidirectional ranging, an aeronautical navigation aid using the VHF spectrum) navigation bearing display errors up to 5°
• VOR navigation To/From indicator reversal
• VOR and ILS (instrument landing system, an aeronautical navigation aid using the VHF spectrum) course deviation indicator errors with and without a failure flag
• Reduced sensitivity of the ILS localizer receiver
• Background noise on audio outputs

Nevertheless, there is considerable public pressure to allow use of cell phones on board, and the fact that more often than not interference does not actually create problems has led to the perception that there is no problem. To deal with both of these issues, some airlines are trialing a system which actually allows cell phones to be used via a picocell base station on the aircraft, communicating via satellite with the ground networks. The EMC implication of this is that, because the base station is very local, the phones are able to transmit at their minimum power, thus (hopefully) eliminating EMC interactions. Even so, there will almost certainly be restrictions on use, only above 10,000 ft and not during takeoff and landing.

21.1.2. Interference to Medical Devices

Another critical area with potentially life-threatening consequences is the EMC of electronic medical devices. A 1995 review article (Silberberg, 1995) described three incidents in detail and listed more than 100 EMI problems that were reported to the U.S. Food and Drug Administration between 1979 and 1993. It states bluntly that “EMI-related performance degradation in electronic medical devices has resulted in deaths, serious injuries, and the administration of inappropriate and possibly life-threatening treatment.”

The detailed case studies were as follows:

• Apnea monitors The essential function of an apnea monitor is to sound an alarm when breathing stops; the devices are used in hospitals and frequently prescribed for home use in the case of infants who either have exhibited or are at risk of experiencing prolonged apnea. After numerous reports of unexplained failure on the part of apnea monitors to alarm even upon death, their susceptibility to radiated RF was evaluated by the U.S. Center for Devices and Radiological Health (CDRH). Most commercial apnea monitors were found to erroneously detect respiration when exposed to relatively low field strengths, a situation that could result in failure to alarm during apnea. Most monitors were found to be susceptible above 1 V/m; one particular model was susceptible to pulsed fields above 0.05 V/m.
• Anesthetic gas monitor The CDRH received several reports of erroneous displays and latch-up of an anesthetic gas monitor during surgery. None of the reports mentioned EMI as a possible cause. FDA investigators found that the manufacturer had a list of 13 complaint sites, and his own investigations revealed that interference from certain types of electrosurgery units disrupted the communication link between the monitor and a central mass spectrometer, causing the monitor to fail to display the concentration of anesthetic gas in the operating room during surgery.
• Powered wheelchairs A QA manager at a large wheelchair manufacturer had received reports of powered wheelchairs spontaneously driving off curbs or piers when police or fire vehicles, harbor patrol boats, or CB or amateur radios were in the vicinity. Though CDRH databases showed reports of unintended motion—in several cases involving serious injury—none of these incidents had been attributed to EMI. When CDRH investigated the EMI susceptibility of the motion controllers on various makes of powered wheelchairs and scooters, they discovered susceptibilities in the range of 5 to 15 V/m. At the lower end of the range, the electric brakes would release, which could result in rolling if the chair happened to be stopped on an incline; as the field strength at a susceptible frequency was increased, the wheels would actually begin turning, with the speed being a function of field strength.

Another issue is the effect on hearing aids: The problem of interference to hearing aids has been known for some time. Digital mobile phones use a form of radio transmission called Time Division Multiple Access (TDMA), which works by switching the radio frequency carrier rapidly on and off. If a hearing aid user is close to a digital mobile telephone, this switching of the radio frequency carrier may be picked up on the circuitry of the hearing aid. Where interference occurs, this results in a buzzing noise which varies from very faint to maximum volume of the aid … [A specialist standards panel] has determined that, although digital mobile telephones are being looked at as the source of likely interference, all radio systems using TDMA or similar transmissions are likely to cause some interference ( BSI News, December 1993).

These are all examples of the lack of a product's “fitness for purpose”; that is, to operate correctly and safely in its intended environment, which includes the electromagnetic environment. There are clear safety implications in the reports.

21.1.2.1. Hospital and Emergency Service Radio Management

Many types of hospital equipment are susceptible to RF radiation from hand-portable mobile radio transmitters—diagnostic equipment such as ECGs, EEGs, pulse oximeters, and other physiological monitoring equipment; and therapeutic equipment such as infusion pumps, ventilators, and defibrillators. Physiological (patient-coupled) monitoring equipment is very sensitive and hence very susceptible, although for every device type, some models consistently perform better than average (they exhibit good EMC design). The type of modulation employed by the mobile transmitter can be significant. For example, an external pacemaker withstood a GSM signal (modulated at 217 Hz) at 30-V/m field strength, but TETRA modulation (17 Hz) caused interference at 3 V/m.

This is of particular concern for ambulances, which in Europe are mandated to use the TETRA system for emergency communications, but that also carry an array of patient-coupled instrumentation for life support purposes. This has led to the U.K.'s Medicines and Healthcare Products Regulatory Agency (MHRA, 1999) recommending as follows:

• The use of portable handsets and cell phones inside ambulances should be restricted
• Special precautions are needed if a patient with an external pacemaker is being transported
• Displaying warning notices, providing staff training, and relocating parking bays are possible actions if risks of interference prove unacceptable when emergency vehicles are parked immediately outside patient treatment areas
• Caution should be exercised when treating patients with medical devices at the scene of an accident if an emergency vehicle is nearby
• Mobile data terminals should be subjected to any restrictions which are locally applied to cellphones

Various studies have tested medical devices and recommend that a distance of 1 to 1.5 m be maintained between typical hand-portable transmitters and medical equipment. The MHRA tested 178 different models of medical device using a wide range of radio handsets. Overall, in 23% of tests medical devices suffered electromagnetic interferencefrom handsets. Of these, interference incidents, 43% would have had a direct impact on patient care and were rated as serious. Only 4% exhibited effects with cell phones at 1-m distance, although at that distance emergency and security handsets had much greater effects (MHRA, 1997).

The difficulty with controlling the use of radio communications in hospitals and other medical situations is well illustrated in the MHRA's guidance document (MHRA, 2006), which itself refers to an ISO technical report on the subject (ISO, 2005): “Overly-restrictive policies may act as obstacles to beneficial technology and may not address the growing need for personal communication of patients, visitors and the workforce. At the other extreme, unmanaged use of mobile communications can place patients at risk.” The guidance stresses the need for an effective policy for health-care providers to manage the use of the radio frequency spectrum in their own sites. This includes considering areas where medical devices will not be affected and therefore no restrictions apply and other areas where authorized staff can use communication devices authorized by the hospital. Incidents should be reported when a medical device is suspected to have suffered electromagnetic interference.

21.1.2.2. Diathermy and Electrosurgery

As well as radio communications, medical diathermy and electrosurgery are well known as a source of significant interference problems that most surgeons simply learn to cope with. Medical diathermy (tissue heating) used for physiotherapy typically operates at 27 MHz with RF powers up to 400 W, although modern pulsed diathermy uses average RF powers around 40 W; but these levels are more than enough to interfere with many kinds of electro-medical equipment, particularly monitors.

21.1.3. Thermostats

Thermostats and other automatic switching contacts of all sorts are a major source of noise complaints, particularly when they are faulty. The former U.K. Radiocommunications Agency dealt with many cases of interference caused by thermostats or the radio-suppression components fitted to them. In about 90% of these cases, the interference is attributable to thermostats in gas boilers. It seems that, as these operate in a heat-stressed environment, they are prone to more rapid deterioration than other domestic thermostats such as room thermostats, cylinder thermostats, and diverter switching valves. Sometimes the offending thermostat is found in the house that is suffering the interference, although there have been cases where the source of the interference has been found some distance away.

New domestic appliances are required to pass tests for “discontinuous disturbance” emissions (the current harmonized standard is EN 55014-1), but this does not guarantee that such products will remain noise-free after many years of operation. The limits for RF emissions are related in a complex way to the repetition rate and duration of the automatic switching event.

An example is the interference signal generated from a boiler gas control valve and its associated thermostat switching from standby to ON and vice versa. The low-power single-phase arc causes a short burst of radiation. When the thermostat is malfunctioning this burst of radiation can be heard as a rough rasping noise which typically lasts for a few seconds but may last for 20 s or more. It repeats typically every 10 min but, in some cases, a faulty thermostat may arc several times per minute. This kind of interference, which is intermittent in nature, is mostly noticed in relation to the reception of analog TV signals at 470 to 850 MHz and sometimes on FM radio at 88–108 MHz.

Replacing the faulty thermostat will normally resolve the problem, but a better solution is to fit suppression to all such switching contacts. This prevents the arc forming at the instant of switching and if properly designed has the side effect of lengthening the contact life, but the added cost is usually viewed unfavorably by manufacturers.

21.1.4. The Quacking Duck

In a lighter vein, probably the least critical EMC problem this author has encountered is the case of the quacking duck: There is a toy for the under-fives, which is a fluffy duck with a speech synthesizer that is programmed to quack various nursery rhyme tunes. It does this when a certain spot (hiding a sensor) on the duck is pressed, and it should not do it otherwise. While it was in its Christmas wrapping in our house, which is not electrically noisy, it was silent. But when it was taken to our daughter's house and left in the kitchen on top of the fridge, next to the microwave oven, the Christmas present quacked apparently at random and with no one going near it. Some disconcerting moments arose before it was eventually explained to the family that this was just another case of bad EMC and that they should not start to doubt their sanity!

21.2. Compatibility between and within Systems

The threat of EMI is controlled by adopting the practices of electromagnetic compatibility, as defined earlier. The concept of EMC has two complementary aspects:

• It describes the ability of electrical and electronic systems to operate without interfering with other systems
• It also describes the ability of such systems to operate as intended within a specified electromagnetic environment

Thus it is closely related to the environment within which the system operates. Effective EMC requires that the system is designed, manufactured, and tested with regard to its predicted operational electromagnetic environment; that is, the totality of electromagnetic phenomena existing at its location. Although the term electromagnetic tends to suggest an emphasis on high-frequency field-related phenomena, in practice the definition of EMC encompasses all frequencies and coupling paths, from DC through mains supply frequencies to radio frequencies and microwaves. And phenomena is not restricted to radio-based phenomena but also transient events and power-related disturbances.

21.2.1. Intrasystem EMC

There are two approaches to EMC. In one case the nature of the installation determines the approach. EMC is especially problematic when several electronic or electrical systems are packed into a very compact installation, such as on board aircraft, ships, satellites, or other vehicles. In these cases susceptible systems may be located very close to powerful emitters and special precautions are needed to maintain compatibility. To do this cost-effectively calls for a detailed knowledge of both the installation circumstances and the characteristics of the emitters and their potential victims. Military, aerospace, and vehicle EMC specifications have evolved to meet this need and are well established in their particular industry sectors.

This, then, can be characterized as an intrasystem approach: The EMC interactions occur between parts of the overall system, the whole of which is amenable to characterization. It may not be necessary or desirable to draw a boundary around individual products in the system but rather to consider how they affect or are affected by other parts of the same system. Mitigation measures can be applied as easily, and sometimes more easily, at the system level as at the equipment level.

21.2.2. Intersystem EMC

The second approach assumes that the system will operate in an environment which is electromagnetically benign within certain limits and that its proximity to other sensitive equipment will also be controlled within limits. This approach is appropriate for most electronics used in homes, offices, and industry and similar environments. So, for example, most of the time a personal computer will not be operated in the vicinity of a high-power radar transmitter nor will it be put right next to a mobile radio receiving antenna. This allows a very broad set of limits to be placed on both the permissible emissions from a device and on the levels of disturbance within which the device should reasonably be expected to continue operating. These limits are directly related to the class of environment—domestic, commercial, industrial, and so forth—for which the device is marketed. The limits and the methods of demonstrating that they have been met form the basis for a set of standards, some aimed at emissions and some at immunity, for the EMC performance of any given product in isolation. This makes it an intersystem approach rather than an intrasystem approach and means that a necessary part of the process is defining the boundary of the product—easy for typical commercial electronic devices, harder when it comes to installations.

Note that compliance with such standards will not guarantee electromagnetic compatibility under all conditions. Rather, it establishes a probability (hopefully very high) that equipment will not cause interference nor be susceptible to it when operated under typical conditions. There will inevitably be some special circumstances under which proper EMC will not be attained—such as operating a computer within the near field of a powerful transmitter—and extra protection measures must be accepted.

21.2.3. When Intrasystem Meets Intersystem

Difficulty arises when these two approaches are confused one with the other, or at the interface where they meet. This can happen when commercial equipment is used in other environments, for instance on vehicles or in aircraft, and we get the issues referred to earlier, for instance with passenger electronic devices; the PED might be compliant with its normal requirements but that is not necessarily relevant to its use in these different surroundings. Military projects might require commercial-off-the-shelf products to be procured, but their EMC performance requirements are substantially mismatched to military needs. Grounding and bonding techniques that are necessary and appropriate for intrasystem requirements can be misapplied to attempt to meet the EMC directive.

From the product designer's point of view, many of the necessary techniques are similar or common to both approaches, but there are instances where they diverge and so we need to be clear about which approach is being considered in any given case.

References

CAA 2003 CAA Effects of Interference from Cellular Telephones on Aircraft Avionic Equipment Safety Regulation Group, Civil Aviation Authority, CAA Paper 2003/3, (April 30) 2003 available from www.caa.co.uk

IEC 1990 IEC Glossary of Electrotechnical, Power, Telecommunication, Electronics, Lighting and Colour Terms: Electromagnetic Compatibility IEC 61000: Electromagnetic Compatibility. Geneva, Switzerland: International Electrotechnical Commission 1990 available from www.iec.ch

“Interference from Mobiles.” 2000 “Interference from Mobiles.” Electronics Times 200047- (October 23)

ISO 2005 ISO Health Informatics—Use of Mobile Wireless Communication and Computing Technology in Healthcare Facilities—Recommendations for the Management of Electromagnetic Interference with Medical Devices 2005 International Standards Organization Geneva, Switzerland

MHRA 1997 MHRA Electromagnetic Compatibility of Medical Devices with Mobile Communications Medicines and Healthcare Products Regulatory Agency, DB 9702 1997 available from www.mhra.gov.uk

MHRA 1999 MHRA Emergency Service Radios and Mobile Data Terminals: Compatibility Problems with Medical Devices Medicines and Healthcare Products Regulatory Agency, DB 1999(02) 1999 available from www.mhra.gov.uk

MHRA 2006 MHRAMobile Communications Interference Medicines and Healthcare Products Regulatory Agency 2006 available from www.mhra.gov.uk

Silberberg 1995 J.L. Silberberg, Electronic Medical Devices and EMI Compliance Engineering, Annual Reference Guide 1995 European edition CDRH

U.K. Radiocommunications Agency U.K. Radiocommunications Agency (n.d.) EMC Awareness. Web pages available at the time of writing from www.ofcom.org.uk/static/archive/ra/topics/research/RAwebPages/Radiocomms/index.htm