Introduction to EMC|电磁兼容问题分解|EMC的历史|电磁兼容的明天

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Introduction

A helicopter experiences a sudden, uncommanded flight surface movement that causes it to drop 2,500 feet before the pilots can regain control. A tramway car suddenly accelerates as it approaches the terminal slamming into a guard fence, shattering windows, and throwing passengers to the floor. A nuclear power plant goes on alert status when turbine control valves spontaneously close. Each of these actual events was a symptom of an electromagnetic compatibility problem.

Electromagnetic compatibility (EMC) is broadly defined as a state that exists when all devices in a system are able to function without error in their intended electromagnetic environment. In 1988, Army UH‑60 Black Hawk helicopters were found to exhibit a variety of problems when operating in the vicinity of licensed radio transmitters. The initial solution was to restrict the “intended electromagnetic environment.” Operator manuals were changed to advise pilots to avoid radio‑frequency emitters such as microwave antennas or shipboard radar. Flight crews were cautioned not to operate any electronic devices that were not part of the helicopter. The cost of “hardening” the army's 1000 existing helicopters to make them less susceptible to radiated emissions has been estimated to be as high as $450 million.

An EMC problem with a Roosevelt Island Tramway car was attributed to a speed control circuit that exhibited an unexpected sensitivity to transients on the tramway's power. This problem was easy to correct, but it could have been avoided altogether had EMC been a significant concern during the initial product design.

Spontaneous valve closures at the Niagara Mohawk Nine Mile Point #2 nuclear power plant were due to interference generated by workers’ wireless handsets. Despite the tremendous emphasis on safety and security that is placed on the design and construction of all nuclear power plants, the relatively weak emissions from common wireless handsets resulted in a major malfunction.

Unfortunately, these are not rare isolated occurrences. Electromagnetic compatibility problems result in many deaths and billions of dollars in lost revenue every year. The past decade has seen an explosive increase in the number and severity of EMC problems primarily due to the proliferation of microprocessor controlled devices, high‑frequency circuits and low‑power transmitters.

Elements of an EMC Problem

There are three essential elements to an EMC problem as illustrated in Figure 1. There must be a source of electromagnetic energy, a receptor (or victim) that cannot function properly due to the electromagnetic energy, and a path between them that couples the energy from the source to the receptor. Each of these three elements must be present although they may not be readily identified in every situation. Electromagnetic compatibility problems are generally solved by identifying at least two of these elements and eliminating (or attenuating) one of them.

Fig. 1.  The three essential elements of an EMC problem

For example, in the case of the nuclear power plant, the receptor was readily identified. The turbine control valves were malfunctioning. The source and the coupling path were originally unknown; however an investigation revealed that wireless handsets used by the plant employees were the source. Although at this point the coupling path was not known, the problem was solved by eliminating the source (e.g. restricting the use of low‑power radio transmitters in certain areas). A more thorough and perhaps more secure approach would be to identify the coupling path and take steps to eliminate it. For example, suppose it was determined that radiated emissions from a wireless handset were inducing currents on a cable that was connected to a printed circuit card that contained a circuit that controlled the turbine valves. If the operation of the circuit was found to be adversely affected by these induced currents, a possible coupling path would be identified. Shielding, filtering, or rerouting the cable, and filtering or redesigning the circuit would then be possible methods of attenuating the coupling path to the point where the problem is non‑existent.

The source of the tramway problem was thought to be transients on the tramway's power. The coupling path was presumably through the power supply to the speed control circuit, although investigators were unable to reproduce the failure so the source and coupling path were never identified conclusively. The receptor, on the other hand, was clearly shown to be the speed control circuit and this circuit was modified to keep it from becoming confused by unintentional random inputs. In other words, the solution was to eliminate the receptor by making the speed control circuit immune to the electromagnetic phenomenon produced by the source.

Potential sources of electromagnetic compatibility problems include radio transmitters, power lines, electronic circuits, lightning, lamp dimmers, electric motors, arc welders, solar flares and just about anything that utilizes or creates electromagnetic energy. Potential receptors include radio receivers, electronic circuits, appliances, people, and just about anything that utilizes or can detect electromagnetic energy.

Methods of coupling electromagnetic energy from a source to a receptor fall into one of four categories.

1. Conducted (electric current)
2. Inductively coupled (magnetic field)
3. Capacitively coupled (electric field)
4. Radiated (electromagnetic field)

Coupling paths often utilize 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.

A Brief History of EMC

In the late 1880's, the German physicist Heinrich Hertz performed experiments that demonstrated the phenomenon of radio wave propagation, thus confirming the theory published by James Clerk Maxwell two decades earlier. Hertz developed a spark in a small gap between two metal rods that were connected at the other end to metal plates as shown in Figure 2. The spark excitation created an oscillating current on the rods resulting in electromagnetic radiation near the resonant frequency of the antenna. The receiving antenna was a loop of wire with a very thin gap. A spark in the gap indicated the presence of a time‑varying field and the maximum spark gap length provided a measurement of the received field's strength.

Fig. 2.  Early antennas constructed by Heinrich Hertz

Guglielmo Marconi learned of Hertz's experiments and improved upon them. In 1895, he developed the wireless telegraph, the first communications device to convey information using radio waves. Although the significance of his invention was not initially appreciated, the U.S. Navy took an interest due to the potential of this device to enhance communication with ships at sea.

In 1899, the Navy initiated the first shipboard tests of the wireless telegraph. While the tests were successful in many ways, the Navy was unable to operate two transmitters simultaneously. The reason for this problem was that the operating frequency and bandwidth of the early wireless telegraph was primarily determined by the size, shape and construction of the antenna. Receiving antennas were always “tuned” (experimentally) to the same operating frequency as the transmitting antenna, however the bandwidth was difficult to control. Therefore when two transmitters were operating simultaneously, receivers detected the fields from both of them to some extent and the received signal was generally unintelligible. This early electromagnetic compatibility problem came to be referred to as Radio Frequency Interference (RFI). As the popularity of the wireless telegraph grew, so did the concern about RFI.

In 1904, Theodore Roosevelt signed an executive order empowering the Department of Commerce to regulate all private radio stations and the Navy to regulate all government stations (and all radio stations in times of war). Different types of radio transmitters were assigned different frequency allocations and often were only allowed to operate at certain times in order to reduce the potential for RFI.

By 1906, various spark‑quenching schemes and tuning circuits were being employed to reduce the bandwidth of wireless transmitters and receivers significantly. However, it was the invention of the vacuum tube oscillator in 1912 and the super heterodyne receiver in 1918 that made truly narrow band transmission and reception possible. These developments also made it possible to transmit reasonably clear human speech, which paved the way for commercial radio broadcasts.

The period from about 1925 to 1950 is known as the golden age of broadcasting. During this period the popularity of radio soared. As the number of radios proliferated, so did the electromagnetic compatibility problems. RFI was a common problem because the regulations governing intentional or unintentional interference with a commercial radio broadcast were lax and more people had access to radio equipment. In order to alleviate this problem, the Federal Communications Commission (FCC) was established in 1934 as an independent agency of the U.S. Government. It was empowered to regulate U.S. interstate and foreign communication by radio, wire, and cable. FCC regulations and licensing requirements significantly reduced the number of radio frequency interference problems.

However, due to the increasing number of radio receivers  being located in homes, the general public was introduced to a variety of new EMC problems. Unintentional electromagnetic radiation sources such as thunderstorms, gasoline engines, and electric appliances often created bigger interference problems than intentional radio transmitters.

Intrasystem interference was also a growing concern. Super heterodyne receivers contain their own local oscillator, which had to be isolated from other parts of the radio's own circuit. Radios and phonographs were lumped together in home entertainment systems. Radios were installed in automobiles, elevators, tractors, and airplanes. The developers and manufacturers of these systems found it necessary to develop better grounding, shielding, and filtering techniques in order to make their products function.

In the 1940's many new types of radio transmitters and receivers were developed for use during World War II. Radio signals were not only used for communication, but also to locate ships and planes (RADAR) and to jam enemy radio communications. Because of the immediate need, this equipment was hurriedly installed on ships and planes resulting in severe EMC problems.

Experiences with electromagnetic compatibility problems during the war prompted the development of the first joint Army‑Navy RFI standard, JAN‑I‑225, “Radio Interference Measurement,” published in 1945. Much more attention was devoted to RFI problems in general, and techniques for grounding, shielding and filtering in particular. Electromagnetic compatibility became an engineering specialization in a manner similar to antenna design or communications theory.

In 1954, the first Armour Research Foundation Conference on Radio Frequency Interference was held. This annual conference was sponsored by both government and industry. Three years later, the Professional Group on Radio Frequency Interference was established as the newest of several professional groups of the Institute of Radio Engineers. Today, this group is known as the Electromagnetic Compatibility Society of the Institute of Electrical and Electronics Engineers (IEEE).

During the 1960's, electronic devices and systems became an increasingly important part of our society and were crucial to our national defense. A typical aircraft carrier, for example, employed 35 radio transmitters, 56 radio receivers, 5 radars, 7 navigational aid systems, and well over 100 antennas [1]. During the Vietnam War, Navy ships were often forced to shut down critical systems in order to allow other systems to function. This alarming situation focused even more attention on the issue of electromagnetic compatibility. Outside the military, an increasing dependence on computers, satellites, telephones, radio and television made potential susceptibility to electromagnetic phenomena a very serious concern.

The 1970's witnessed the development of the microprocessor and the proliferation of small, low‑cost, low‑power semiconductor devices. Circuits utilizing these devices were much more sensitive to weak electromagnetic fields than the older vacuum tube circuits. As a result, more attention was directed toward solving an increasing number of electromagnetic susceptibility problems that occurred with these circuits.

In addition to traditional radiated electromagnetic susceptibility (RES) problems due to intentional and unintentional radio frequency transmitters, three additional classes of electromagnetic susceptibility problems gained prominence in the '70s. Perhaps the most familiar of these, outside the military, is electrostatic discharge (ESD). An electrostatic discharge occurs whenever two objects with a significantly different electric potential come together. The “shock” that is felt when a person reaches for a door knob after walking across a carpet on a dry day is a common example. Even discharges too weak to be felt however, are capable of destroying semiconductor devices.

Another electromagnetic susceptibility problem that gained notoriety during the '70s was referred to as EMP or ElectroMagnetic Pulse. The military realized that a high‑altitude detonation of a nuclear warhead would generate an extremely intense pulse of electromagnetic energy over a very wide area. This pulse could easily damage or disable critical electronic systems. To address this concern, a significant effort was initiated to develop shielding and surge protection techniques that would protect critical systems in this very severe environment.

The emergence of a third electromagnetic susceptibility problem, power line transient susceptibility (PLT), was also a direct consequence of the increased use of semiconductor devices. Vacuum tube circuits generally required huge power supplies that tended to isolate the electronics from noise on the power line. High‑speed, low‑power semiconductor devices on the other hand were much more sensitive to transients and their modest power requirements often resulted in the use of relatively small low‑cost supplies that did not provide much isolation from the power line. In addition, the low cost of these devices meant that more of them were being located in homes and offices where the power distribution is generally not well regulated and relatively noisy.

The emphasis on electromagnetic susceptibility during the 1970's is exemplified by the number of task groups, test procedures, and product standards dealing with susceptibility that emerged during this decade. One organization established in the late 70's known as the EOS/ESD Association (EOS stands for Electrical Over Stress) deals exclusively with the susceptibility problems mentioned above.

Another change that occurred during the 60's and 70's was the gradual displacement of the term RFI by the more general term EMI or Electromagnetic Interference. Since not all interference problems occurred at radio frequencies, this was considered to be a more descriptive nomenclature. EMI is often categorized as radiated EMI or conducted EMI depending on the coupling path.

Two events in the 1980's had significant, wide‑ranging effects on the field of electromagnetic compatibility.

• The introduction and proliferation of low priced personal computers and workstations.
• Revisions to Part 15 of the FCC Rules and Regulations that placed limits on the electromagnetic emissions from “computing devices.”

The proliferation of low priced computers was important for two reasons. First, a large number of consumers and manufacturers were introduced to a product that was both a significant source and receptor of electromagnetic compatibility problems. Secondly, the availability of low cost, high speed computation spurred the development of a variety of numerical analysis techniques that have had an overwhelming influence on the ability of engineers to analyze and solve EMC problems.

The FCC regulations governing EMI from computing devices were phased in between 1980 and 1982. They required all electronic devices operating at frequencies of 10 kHz or greater and employing “digital techniques” to meet stringent limits regulating the electromagnetic emissions radiated by the device or coupled to the power lines. Virtually all computers and computer peripherals sold or advertised for sale in the U.S. have to meet these requirements. Many other countries established similar requirements.

The impact of these regulations was overwhelming because for the first time engineers, managers, and corporate presidents were made painfully aware of what it means to have an EMC problem. At a time when the market for computers was mushrooming, many of the latest, most advanced designs were being held back because they were unable to meet the FCC's EMI requirements. Companies formed EMC departments and advertised for EMC engineers. An entire industry emerged to supply shielding materials, ferrites, and filters to computer companies. EMC short courses, test labs, magazines, and consultants began appearing throughout the world. The international attention focused on EMC encouraged additional research. Significant progress was made toward the development of more comprehensive test procedures and meaningful standards.

In the 1990's, these trends are continuing. Computing devices are getting denser, faster, and more complex creating new challenges for the EMC engineer while advances in numerical electromagnetics are revolutionizing the state‑of‑the‑art in EMC analysis. Regulations limiting electromagnetic emissions continue to be upgraded and new regulations concerning the susceptibility of electronic devices are being developed and introduced.

The Future of Electromagnetic Compatibility

If you were to listen to a computer company executive explaining corporate strategy for dealing with EMC problems in the future, you would very likely hear something like “We are striving to make EMC an integral part of the product design process, rather than attempting to solve problems by 'patching' a design that is nearly complete.”  Of course this is not a new idea. Since the early RFI problems with the wireless telegraph, engineers have realized that it is cheaper, easier and more effective to design a product that is compatible than it is to “fix” an existing design that has an EMC problem. To some extent, early EMC involvement has been a goal all along and steady progress has been made.

For example, radio circuit designers are keenly aware of bandwidth requirements and out‑of‑band radiation is rarely a problem anymore even with prototype designs. When digital circuits first appeared, interference between the digital and analog portions of a device was common, however eventually this became less of a problem as circuit designers learned to isolate analog and digital grounds. Today, computers are routinely designed with some degree of shielding, filtering, and special grounding techniques.

The reason that “early EMC involvement” continues to be extolled as an idea whose time has come is that the scope and complexity of EMC problems is steadily increasing. New technologies create unique situations rendering existing EMC “fixes” and design rules obsolete. Engineers who are familiar with fundamental EMC concepts and analysis techniques can readily apply this knowledge to emerging technologies and anticipate potential EMC problems during a product's design phase. In the past however, the emphasis has been on communicating the design rules and fixes themselves rather than “burdening” the circuit designers with fundamental EMC concepts. As a result, EMC problems have kept one step ahead of the circuit designers and the call for “early EMC involvement” continues.

Fortunately, this situation is beginning to change. The tips and tricks that caused many engineers to view EMC as a black art are being examined more closely and used with greater caution. More significantly, the importance of many fundamental principles drawn from electromagnetics and circuit theory is being recognized. These principles are essential to an understanding of how a circuit interacts with it's electromagnetic environment.

In the years to come, as EMC continues to evolve from an engineering art to an engineering science, the need to make the principles of EMC part of the electrical engineering curriculum will become more apparent. Advances in computer hardware and numerical modeling techniques will enable the efficient application of these principles to the analysis of complex circuits and systems. Once circuit and system designers are familiar with these concepts and techniques, “early EMC involvement” will be the rule rather than just the goal.

References

 [1]    K.D. Wilson, “Communications, A Limiting Factor in Naval Warfare,” Naval Engineers Journal, February 1963, p. 53.