Environment/EMC/EMI

Carnegie Mellon University
18-849b Dependable Embedded Systems
Spring 1999
Author: Eushiuan Tran

Abstract:

Embedded systems can exist in environments that are very harsh and noisy, which can lead to potential problems with electromagnetic interference (EMI). EMI consists of any unwanted, spurious, conducted, or radiated signals of electrical origin that can cause degradation in equipment performance. Because of these problems, all components must comply with specifications to ensure electromagnetic compatibility (EMC), and there are numerous design methods that can be used to prevent EMI. During the development life cycle of an embedded system, the product must be designed to comply with EMC standards, and the product must also be tested for EMC. In addition, other forms of environmental reliability testing must also be performed to ensure dependable system performance in its natural environment. Currently, there is still work to be done to harmonize various EMC standards to reduce trade barriers between countries and different use sectors, like Defense and Civilian. Therefore, care must be taken in developing embedded systems for compliance with the appropriate standards.

Contents:

Introduction

Embedded systems exist in a wide variety of environments. Because of this, special care must be taken in developing embedded systems that they can operate functionally in their intended environment. Many embedded systems exist in very harsh and noisy environments, which can lead to potential problems with electromagnetic interference (EMI). EMI consists of any unwanted, spurious, conducted, or radiated signals of electrical origin that can cause unacceptable degradation in system or equipment performance. Electromagnetic compatibility (EMC) is the ability of systems to function as designed, without malfunction or unacceptable degradation of performance due to EMI withing their operational environment. Any electrical, electromechanical, or electronic equipment must not adversely affect the performance of any other equipment or system as a result of EMI and vice versa. Examples of EMC problems include a computer interfering with FM radio reception, an operating vacuum cleaner causing "snow" on TV, a car radio buzzing when you drive under a power line, an airport radar interfering with laptop computer display, and a telephone being damaged by lightning-induced surges on phone line. [emclab99] While the effects of EMI are sometimes minor, like momentary interference on television, other times the effect may be more catastrophic. For example, a serious consequence can occur if a signal interferes with the operation of a medical equipment that was being used to monitor a patient in intensive care.

The origins of EMI are electrical, with the unwanted emissions being either conducted (voltages or currents) or radiated (electric or magnetic fields). For EMI to occur, 3 essential elements must exist: an electrical noise (EMI) source, a coupling path, and a victim receptor. The coupling path from a source to a receptor can be in 1 of 4 categories: conducted (electric current), inductively coupled (magnetic field), capacitively coupled (electric field), and radiated (electromagnetic field). [emclab99] More details will be given below about the sources and receptors. EMI can occur in 2 different situations: intersystem EMI and intrasystem EMI. Intersystem EMI occurs between 2 or more discrete systems while intrasystem EMI occurs between elements in the same system. [Violette87]

Key Concepts

Sources of EMI

An EMI source can be any device that transmits, distributes, processes, or utilizes any form of electrical energy where some aspect of its operation generates conducted or radiated signals that can cause equipment performance degradation. Figure 1 shows a taxonomy of the different sources of electromagnetic interference. A brief description of each category will be given below. [Violette87]

Figure 1: Taxonomy of EMI Sources [Violette87]

Receptors of EMI

Any EMI situation requires not only an emission source but also a receptor. A receptor is also called a "victim" source because it consists of any device, when exposed to conducted or radiated electromagnetic energy from emitting sources, will degrade or malfunction in performance. Many devices can be emission sources and receptors simultaneously. For example, most communication electronic systems can be emission and receptor sources because they contain transmitters and receivers. Figure 2 shows a taxonomy of different receptors that are susceptible to EMI. Similar to the emission source taxonomy, receptors can be divided into natural and man-made receptors. A brief description of each category will be given below. [Violette87]

Figure 2: Taxonomy of EMI receptors [Violette87]

EMC Design Considerations

During the design process, engineers must be certain that the system is designed to comply to EMC standards. There are many design considerations that need to be taken into account. While it is not the point of this paper to give detail explanations about EMC design techniques, brief descriptions will be given simply for an overview. Engineers needing more technical details can refer to any EMC handbook.

Cable wiring and harnessing is a significant EMI concern. Cables are required to distribute electrical power and transmit electrical signals for the operation of various systems. Since cables are usually routed to accomodate its function, it is often difficult to quantify its environment and it usually varies over both frequency and electric and magnetic field amplitudes. Cables can be EMI radiating sources if they act as radiating antennas, or be susceptible to EMI if they are receiving antennas. Cables can also be coupling paths. In addition, cables are sometimes harnessed together, so interference can also be between two cables that are close in proximity. Therefore, their performance is very difficult to predict. Many specifications classify wiring or cable types into four to six categories but these classifications are generally qualitative in nature. More quantitative classifications should look at levels of power transmitted, or susceptibility of termination. [Violette87]

Connectors are contacts that either link or separate two cables or other equipments. There may be anywhere from several to hundreds of individual wire-pins or coaxial sheaths making simultaneous cotact via a connector. EMI problems from connectors are usually related to poor contact which may result in arcing, or overheating that leads to arcing. Poor contact connections can also result in driven-circuit voltage variations from the contact impedence modulation of the driving-circuit source. Impedence coupling from outside sources can happen in connector grounding paths. Improperly shielded connectors or poor cable-connector-equipment- enclosure contact can cause radiated emission penetration or leaking through apertures. [Violette87]

Grounding is one of the least understood EMC subjects, despite the fact that it seems straightforward. Improper grounding is the source of many EMI problems. Grounding is necessary to prevent shock hazard, which occurs when a wiring or component insulation in an equipement frame or housing breaks down. Grounding also protects against lightning damage. Grounding is also necessary to reduce EMI due to electric field flux coupling, magnetic field flux coupling, and common impedance coupling. There are two reasons why grounding is not understood well. One reason is that shock and safety control requirements existed before the electronics and high frequency area, so traditional grounding techniques were developed to satisfy those requirements. A second reason is that sometimes a conflict occurs between requirements for safety grounds and EMI control. [Violette87]

Different considerations must be taken into account with shielding. Shielding is the use of conductive materials to reduce radiated EMI reflection or absorption. Usually, the theoretical attenuation offered by materials to electric, magnetic, and electromagnetic waves does not match that achieved in practice. This is because a shielded enclosure or housing is not completely sealed. Any shielding application has some kind of penetrations and apertures like meter windows, cover plates and access cover members, and push buttons. These apertures cause leakage and therefore compromises the integrity of the shielding material. Shielding integrity can be restored through the use of EMC gaskets, EMC sealants, and conductive grease. Gaskets provide either temporary or semipermanent sealing applications between joints and structures. Sealants include conductive epoxies which are used to join, bond, and seal two or more metallic metaling surfaces, and conductive caulking which is used to shield and seal two or more metallic mating members held together by other mechanical means. Conductive grease provides a low-resistivity contact path between mating members. [Violette87]

The previous design considerations dealt with topics that represent problems between sources and receptors. There are also EMI control techniques that are applied at the component, circuit, and equipment levels. The problem with resistors, inductors, and capacitors is that they do not behave at their stated values, especially at high frequencies due to the effects of parasitic inductance and capacitance. Under certain conditions, their performace degrades at frequencies as low as 1MHz. Inductive devices like transformers, solenoids, and relays produce low-impedence fields that are sources of EMI if they are uncontrolled. The main techniques available for controlling transient-producing devices involves using diodes and filters, and for controlling magnetic fields involves shielding. Surface tracking is an insulator problem that is a source of EMI. Surface tracking (or leakage) is a condition in which small currents creep across the insulator. It is caused by surface contamination of the insulation by moisture or solid conductive particles. The EMI control technique is to protect from contamination through the use of proper material and proper voltage design. Techniques used to minimize EMI in conductors include coating conductors with a high-permeability material and using hollow conductors at higher frequencies to minimize external fields. There are many techniques for different components, not all of which are listed here. This section only serves as a brief introduction. [Violette87]

Radio-frequency interference (RFI) is a serious EMI problem today due largely to the large number of radio transmitters that exist. Radio transmitters range from large, high-power transmitters such as broadcast, communications, and radar to small, low-power equipments such as handheld radios and cellular telephones. The problem with radio transmitters is twofold, as equipment can cause interference to nearby radio and television receivers, and equipment can be upset by nearby transmitters. Radio and television receivers can be very vulnerable to RFI pollution from nearby computers. Repetitive digital signals contain harmonics that can extend into the GHz range. This unwanted energy can be radiated through cables and wiring acting as antennas, or conducted through the ac power system. If the levels are high enough, the receivers can be damaged. It was this emissions problem that caused countries around the world to pass EMI regulations. In the U.S., complaints from consumers about interference with television disruption in the 70's drove the FCC to initiate mandatory EMI testing of personal and commercial computers in the 1980's. Digital circuits are usually the primary source of emissions, and analog circuits are more vulnerable to RFI than digital circuits. In protecting equipment against RFI, it is important to start at the circuit level. Filters can be used and sometimes multistage filters are needed. Slots and seams cause the most problem in RFI shielding, so high-quality shields and connectors are needed for adequate RFI protection. [Gerke94]

Electrostatic discharge (ESD) is also an EMI problem. An ESD event starts with a very slow buildup of energy, followed by a very rapid breakdown. It is this fast breakdown that causes EMI problems in modern electronic systems. The energy discharge yields EMI frequencies in the hundreds of megahertz. The high speed and frequency of the ESD energy can damage circuits, bounce grounds, and cause upsets through electromagnetic coupling. The most common method of ESD generation is triboelectric charging which is caused by stripping electrons from one object and depositing electrons on another object. In an insulator, it may be a long time before charge recombination occurs, so a voltage builds. If the voltage becomes large enough, a rapid breakdown occurs in the air, creating the ESD arc or spark. Sources of triboelectric charging includes humans, furniture, and material or device movement. Humidity also affects ESD as the lower the humidity, the higher the likelihood of ESD problems. High humidity is helpful because the moisture reduces surface impedance and allows charges to recombine at a faster rate. However, high humidity leads leads to surface tracking or leakage at lower temperatures, so there is a tradeoff. ESD has several failure modes that are not completely independent of one another. These include direct hit to circuit, ground bounce, electromagnetic field coupling, and predischarge electric field. Like RFI problems, good protection against ESD problems start at the circuit level through the use of filters and multilayer boards. High-quality shields and connectors can be used for good ESD cable protection. The length-to-width ratio for grounds should be less than 3 to 1. Thin materials are adequate for shielding and special attention must be paid to slots and penetrations. [Gerke94]

In addition to the design considerations above, there are other EMC issues in the design of embedded systems. One is the compatibility among transmitters that are designed to work together. For example, when every car has a forward-pointing smart cruise control radar, and they are either next to each other or coming head-on at each other, the transmitters must be designed that there is not interference. Another problem to consider is what happens when a component is inserted in an intergrated system and causes EMI. For example, computer motherboards are designed with empty slots for different cards to be plugged into. In particular, video cards are FCC certified to ensure that they are compatible with whichever motherboard they are plugged into.

There are also issues concerning EMC when humans are the receptors. A scare that has not yet been proven deals with cellular phone emissions. A source cited that radiation emitted from cellular phones has shown to cause short-term memory loss and lapses in concentration. However, this is not a proven fact yet. There was also an early brain cancer scare with cell phones that actually led to the FCC limiting the transmitting power of handheld cell phones to 0.3 watts.

Environmental Reliability Testing

Environmental reliability testing is a systematic approach to the collection, analysis, and application of information regarding service use in environmental conditions. Environmental measurement and test became a growing concern during World War II and the Korean War and a discipline by the Space Race and the Vietnam War. It is very important that we understand the interactions of manufactured products and the environmental stresses they encounter when in use. Environmental reliability testing can be divided into three categories: development testing, verification testing, and production testing. Development testing refers to all tests performed generally before the design goes into production. Verification testing refers to testing performed on the production configuration. Production testing refers to the application of environmental stresses to large product populations to improve the manufacturing process and to verify that it remains under control. Questions to take into consideration for each type of testing are listed below. [Caruso96]

 A brief description of the various testing techniques will be presented below. It is important to note that most product verification tests are usually one-time tests peformed on a representative sample of the product. Some tests may degrade the equipment so it is unrealistic to run the test on every single unit. This raises the issue whether running tests on a representative sample of the product is a reliable prediction that all of the units will pass the test. Since you cannot run the test on every single unit, how many units should the test be performed on to get a good guarantee that all units will pass the test? Another issue is what kind or levels of design changes requires re-verification and re-certification? This is always a difficult issue to deal with, but it is certainly a very realistic concern.

There are different points to remember when choosing environmental reliability tests for the development of any embedded system. First, there is no universal test procedure. No test will be completely effective in all environmental situations and each test has its strengths and weaknesses. The cheapest test is not always the best test as short-term and long-term tests are optimized for different purposes. There is no universal test purpose as each test has different goals. Test results are relative, not absolute. Therefore, test expectations should be realistic. Lessons from past experiences are also relevant, even if the testing techniques are obsolete. The purpose and goal of a test should be defined before the test is performed. Finally, no test can satisfy the goals of the overall testing process. Tests should be carefully chosen to eliminate nonproductive efforts, minimize redundancy, and maximize the likelihood of obtaining useful information. [Caruso96]

Standards Compliance

The ability to sell electronic products depends upon the products being able to meet the specifications contained in various regulations. Therefore, a brief study of the different emission standards is necessary. In the United States, the Federal Communications Commission (FCC) Rules and Regulations, Part 15 Subpart J deals with unintentional emissions from equipment that use digital techniques and generate or use timing signals or pulses of frequencies in excess of 10kHZ, or has a pulse rate of 10,000 pulses per second or higher. These specifications were first formed when users of television and other radio receivers complained about the interference of radiation from nearby operating digital devices. FCC 15J defines 2 classes of computing devices that must conform to emissions specifications. Computing devices refer to any computer peripheral including modems, printers, and other I/O devices. The 2 classes are defined as follows:

A device that passes Class B limits may be used in a Class A environment. There are different tests that are required for FCC compliance. There are 3 different types of FCC compliance as stated in the FCC Rules and Regulations. The International Special Committee on Radio Frequency Interference (CISPR) is an organization sponsored by the International Electrotechnical Commission (IEC). CISPR is composed of each of the national committees of the IEC, a United Nations commission, and other international unions, commissions, and committees. It is responsible for setting uniform limits on electromagnetic emissions from equipment so that trade would not be inhibited between member countries as a result of different emissions specifications. CISPR publications deal with inteference for the following items: Compliance with CISPR usually varies from country to country and each country has their own regulations regarding enforcement of the limits. [Violette87]

Currently, there is worldwide effort towards harmonizing various EMC standards to reduce the trade barriers between countries and various sectors like Defense and Civilian. The lack of harmonization of standards is a great burden on manufacturers of electrical and electronic systems because it increases the duration of the product development cycle and the compliance evaluation costs. The increased use of these products has made it absolutely necessary to harmonize various EMC standards. In the Defense sector, the success of military missions is dependent on the trouble-free field performance of the electronic and communication equipments used. In the Civilian sector, it is necessary to protect the radio frequency spectrum from the electromagnetic noise emission of electrical systems. CISPR has provided recommendations for the implementation of EMC. However, each country can choose its own set of test instrumentation, test procedures, and test limits in their own EMC standards. This causes any manufacturer wishing to supply electronic equipments to different countries and the Defense and Civilian sector having to deal with a plethora of EMC standards. [Sampath97]

In Europe, the market unification of 16 countries to form the European Union has affected the EMC standards scenario. The national EMC standards of these countries are being combined to form a harmonized EMC standard, called European Norms. This became known as the EMC Directive which went into effect January 1, 1996. All products that complied to the EMC Directive would bear a CE marking. This mark was required for any nation that wanted to sell electrical equipment in the European Community. Compliance can be completed by the following three ways:

In the United States, the MIL-STD-461D issued by the Department of Defense represents a harmonized standard for military equipments and subsystems.

Even with all the work that has been done to harmonize EMC standards, there are still more than 20 European Norms on EMC, various MIL-STD-461 documents, different FCC Rules and Regulations parts, and other EMC specifications from different industries. This still results in problems including different test requirements and different limits and units of measurements. Currently, 5 certification bodies in the world, North America's Underwriters Laboratories (UL), Germany's VDE, Italy's IMQ, the UK's BABT, and the TUV Product Services have teamed up to issue an international EMC mark to products that meet all of the standards followed by the major economies. Even though acquiring this international mark requires product testing to comply to each standard and a high bill toward testing charges, it is a good step toward achieving a global EMC compliance certificate for a product. [Sampath97]

Available tools, techniques, and metrics

EMI testing is needed because EMI predictions and analysis alone are inadequate to assure compliance with EMI regulations. It is also necessary due to the complexity of design for EMC.

There are a number of methods and nomenclature used in measuring EMI emission and susceptibility characteristics of equipment and subsystems. There are three levels of testings that exist and generally, the higher the level of testing, the more difficult and expensive it becomes.

EMI testing can also be divided into three categories - compliance, engineering, and audit testing.

Relationship to other topics

Conclusions

There are several important conclusions to take away from this section. Mainly, they are the following: EMC is a very important issue that embedded systems designers have to deal with. Even though it is a very difficult topic, there are may practical design techniques that can be used to design for EMC. This will greatly assist designers who are unfamiliar with EM theory to be confident in their design for EMC.

Annotated Reference List

Further Reading

Loose Ends

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