What is EMC?
Keeping the Peace:
Ensuring Compatibility Between the Electromagnetic
and Electric and Electrical Devices
by Kimball William
Former Chairman, IEEE EMC Education Committee
Table of Contents
- Electromagnetic Interference is a Two Sided Problem
- What it Effects
- The Search For Harmony
- EMC Engineering
- For More Information
Electrical noise has been a problem ever since Marconi put up his first antenna and found that mother nature was already 'on the air' with lightning storms. Since that time electrical engineers have been dealing with the problems of electrical interference in one form or another in every nook and cranny of electrical engineering. Almost every form of electrical or electronic device has had to live with the problem at one time or another. Today, electrical interference and the frequency with which it occurs is growing with the rapid spread of electrical and electronic devices.
Electromagnetic interference (EMI) can be demonstrated by tuning a radio receiver to a quiet portion of the AM band where there are no radio stations and placing a calculator with a light emitting diode (LED) display near the receiver. Turn on the radio, then turn on the calculator. You should hear a raucous buzzing from the receiver when you turn on the calculator. The noise you will hear is caused by the circuit which strobes the LED display, and each impulse sound is made by a small current pulse to a display segment. Try entering a number and listen to the tone change.
Today, we recognize that almost any device which operates on the principle of moving an electron from one point to another can be either a source or receiver of electrical noise. Since the problem first became obvious to early radio engineers, the term 'radio frequency interference' (RFI) was most often applied. As engineers spread the use of electronics beyond its beginnings in radio communication, it was found that electrical interference didn't confine itself to the radio spectrum. The newer term electromagnetic interference (EMI) acknowledges the fact that electrical interference encompasses the entire electromagnetic spectrum from the lowest magnetic frequencies through the highest microwave frequencies.
Electromagnetic Interference is a Two Sided Problem
Electromagnetic interference is a problem with two sides. On one hand is the concern that some sensitive electronic instrument or control system will suffer a loss of performance when an outside signal invades its circuitry. We say that the device is a victim, and that it is 'susceptible' to outside interference. The device, in effect, becomes a receiver of electromagnetic noise. On the other hand, if the device is electrically 'noisy', we say it is a 'culprit' and the main concern is due to its 'emission'. It is acting just like a small, unlicensed radio transmitter.
When two electrical or electronic devices must operate together in the same environment or in the same system, the potential for conflict between these unintended transmitters and receivers can present significant, and challenging problems. Some problems are obvious in the first prototype of a new device if it tends to 'self interfere'. This can happen when the design results in a strong emitter and a sensitive receiver in the same package.
Of more concern is the design which results in only one half of the problem. If a circuit is only a strong transmitter, or only a sensitive receiver, the potential for later problems is there, but may not be discovered until the design has left the engineering development laboratory, unless the device is tested for electromagnetic compatibility (EMC). Electromagnetic compatibility is the discipline of designing, analyzing and testing to ensure that electronic and electrical devices are compatible with their electromagnetic environment, being neither victims nor culprits.
What it Effects
In addition to the long standing concern for the safety and security of communications are concerns for the integrity of control, computing and measurement systems. While the possible disruption of communications to essential services such as fire and police departments remains, to this is added the possibility of interfering with such diverse electronics as computers, robotics control systems, missile firing systems, aircraft controls, microwave ovens, etc. The list goes on and on.
We now must concern ourselves with the need to ensure the safe operation of the controls of elevator systems in office buildings when operating near noisy air conditioner motors or with industrial robots operating near automatic welders or with braking systems in an automobile in the vicinity of the ignition. No one wants the computer in a banking institution to miscalculate a bank transaction due to the radar from a passing airplane or a night guard's walkie talkie or for a weighing system to overcharge a truck transport company for road usage when a nearby driver asks for a weather report using his CB radio. And certainly no one would want the lives of patients in a hospital emergency room threatened due to a malfunction in the electronic life support systems due to unintentional outside interference.
These, and other concerns, are typical of the potential for interference in our complex electromagnetic environment. And, the problem increases in magnitude and diversity every day as new electronic applications come into use. Each new device may turn out to be another unexpected transmitter or receiver or both.
The Search for Harmony
In the attempt to resolve the possible conflicts between sensitive electronics and a noisy environment, engineers seek to make circuits which are potential receivers more immune and those which are potential emitters less noisy. But, how much protection is enough, and are there practical limits?
Susceptibility, in general, tends to be an almost a self regulating phenomenon. This is because a product which tends to fail in service due to interference is usually called to the attention of the manufacturer by the disgruntled user. As a result, most producers of electronics are sooner or later aware of any serious susceptibility problem that they have built into their equipment. Emission, on the other hand, does not have this automatic "market feed back" characteristic. As a result, the Federal Communications Commission (FCC) has been given the authority to regulate emissions from products in the U.S.. And they do! The FCC has also been given the authority to regulate product susceptibility in the U.S. but, up to this point has chosen not to use this authority.
Since emitters and receiver circuits must coexist, there must be guide lines for emission and susceptibility limits. The guidelines can be found in formal standards and rules which are based upon the type of service the device will eventually see in its application. In these cases, the attempt is made to provide a a reasonable margin of safety between permissible emissions of unintentional transmitters (emitters) and the susceptibility levels of sensitive circuits. The most widely used limits for electromagnetic compatibility are spelled out in the Federal Communications Commissions (FCC) Part 15 and the Military Standards MIL-STD 461 and MIL-STD 462. These and other standards provide solid guidance based upon practical experience in the environment in which the system will operate.
In today's world of electronics we must face the necessity of dealing with standards for electronic emission and susceptibility levels. In the area of commercial and residential electronics, no digital design with a clock frequency equal to or greater than ten kilohertz can be sold, offered for sale or imported into the USA unless it can pass the emission requirements of FCC Part 15. No electronic device for the military will be accepted unless it is designed to pass MIL-STD 461/462 emission and susceptibility testing and successfully does so. Automotive electronics all must comply with SAE standards for emission (J551) and susceptibility (J1113). Industrial equipment must comply with NEMA, ANSI, UL, SAMA and IEEE standard tests to verify its behavior under EMI. Other countries have similar standards (e.g. CISPR, IEC and ISO in the European Economic Community).
These standards are not static. As technology changes, committees in various societies are working continuously to keep the standards which apply in their area of concern current for EMC design and testing.
In an ideal world, the solution to the problems of EMI would be in the hands of every electrical engineer. The design principles and techniques are well understood, and documented. Unfortunately, engineering curricula are so crowded with core materials that only a few universities can offer courses in EMC, even as an elective. In 1985, the IEEE EMC society through its EMC education committee conducted a survey of 350 engineering schools in North America and Europe requesting information on the availability of courses with at least 40 percent of the course material devoted to EMC. Only ten positive replies were received, although the majority of the schools which replied said that they thought that they should offer such material. A recent 'grass roots' pole indicated that little has changed in the educational system in this regard.
Electromagnetic compatibility as it is practiced today is a well defined technical discipline which is practiced by a relatively small number of engineers within the electrical engineering community. These engineers have, by and large, been those who recognized the problem and educated themselves inn the discipline. While thes dedicated few have been generally successful up till now, the explosive growth of electronics will soon out strip even their best efforts. At last count, the IEEE had 320,000 members, the IEEE EMC society had 4500. If this is a representative proportion of the engineering community mix, we are asking every EMC engineer to guide the work of over seventy of his co-workers and keep them out of trouble, and the ratio is getting worse.
It is not necessary for every electronic engineer to be an "expert" in EMC, any more than he is in packaging or thermal control or digital communications. What is desperately needed now, and more and more in the future, is for every electrical engineer to become AWARE of EMC and the general implications it has for his designs. If he needs help, he can call on the "expert" in his organization or contact an outside consultant for assistance. The one way he shouldn't become aware of EMC for the first time is when his best customer complains about an EMC caused field problem.
For More Information
If you would like to find out more about electromagnetic compatibility, what it is, what to do about it and what is being done today, check out our links to related sites, or better yet, come to one of our monthly meetings.