Stop EMI from ruining your day and your system performance

How to control electromagnetic interference and ensure compatibility.

By Michel Mardiguian, author, and Donald L. Sweeney, D.L.S. Electronic Systems

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Electromagnetic interference (EMI) is the generic term describing a situation where an electrical disturbance generated by electrical or electronic equipment causes an undesirable response to another system. The origin could also be a natural phenomenon such as electrostatic discharge (ESD) or lightning strokes. It can also be man-made from radio transmitters and emissions from digital circuits.

The effect may range from a mere nuisance to a catastrophic failure, with associated financial losses and/or possibly human casualties. Electromagnetic compatibility (EMC) is just the opposite: EMI being the disease, and EMC is the cure. EMC is the discipline of analyzing, preventing or fixing interference problems.

EMI has existed ever since our modern societies started using electricity for transportation, domestic and industrial power, and transmitting intelligence—mostly using telecommunications and radio. In the early days of radio frequency (RF) engineering, many EMI aspects like receiver selectivity and spurious response, channel separation, high harmonic contents of transmitters and the like were unknown. But rapidly the tremendous growth of electronics and RF applications over the years has made the number of EMI incidents skyrocket in all areas: telephone and telecommunications, air navigation, radio and TV services, as well as mobile radio and cell phones.

In the major industrialized countries, RF regulating agencies and private industry decided that the days of empirical approaches and EMI gurus were numbered and that a logical strategy was badly needed. EMC was born, not as a new science, but as a multifaceted discipline, where EMI can be anticipated, simulated and prevented by proper design techniques. Progressively, the essential aspects such as source and victim dichotomy, coupling mechanisms and the like were set forth, along with specific components, instrumentation and measurement techniques.

A measure of this invisible EMI threat is given by the following examples of EMI incidents that occurred worldwide in the period where many EMC regulations were not yet applied. The full list of reported incidents, not to mention the unreported ones because they were not publicized, would amount to an impressive pile of reports.

EMI manifests itself in all degrees, from simple nuisances such as:

• 70,000 claims/year (United States) for jammed public radio and TV reception

• remote-controlled garage doors or car locking devices jammed by nearby airport radars

• surveillance cameras jammed by nearby variable speed motor drives

• cash registers and electronic weighing machines in food stores displaying wrong figures because of ESD

• pumps in gas stations displaying wrong price when drivers were using their radios

to catastrophic ones, such as:

• medical monitoring devices (ECG, EKG, blood analyzers, infusion pumps) delivering wrong values when hospital or emergency-medical-service personnel were using portable transmitters or paging devices nearby

• 30-m-high crane dropping its load in a harbor when dockworkers were using their walkie-talkies

• six ferries crashing into piers because the propeller pitch controls were interfered with by local radio-transmitters

• 134 deaths (seamen and pilots) and $72 million damage due to a radar beam unexpectedly firing a missile on the deck of an aircraft carrier, USS Forrestal, during the Vietnam War (Figure 1).

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Source-and-victim concept

Given the complexity of the interactions between the many elements and parameters when EMI is involved, a very clear and simple way for addressing the who-does-what of an EMI situation is the source-and-victim concept that sees an EMI problem as a theater act with three players:

• the source of EMI, which can be a natural event (ESD) or from a man-made device such as intentionally generated high frequency (authorized RF transmitters); it can also be a byproduct of its operation such as digital circuits and switch mode power supplies

• the victim of EMI, which can be any analog or digital circuit whose low-level input can be activated, damaged or disrupted by undesired signals (EMI)

• the coupling path between source and victim, which can be a conducted path through wires, a radiated path through air or something in-between such as cable-to-cable capacitive and magnetic crosstalk.

The combination of source, coupling path air or power wires and victim (radio receiver) is the basis for an overall understanding of EMI and is the key to EMI control in order to reach a satisfactory level of EMC (Figure 2). One can eliminate an interference problem by acting on one or several of the three actors, whichever is available for changes, at a reasonable cost.

Acting on the source: This of course is not feasible for natural events such as ESD and lightning, so in general only non-intentional, fortuitous RF sources can be modified to reduce their undesired RF emissions. Most common examples are digital circuits, switch-mode power supplies and even radio transmitters. Today this is handled through worldwide regulatory emission requirements.

Making the victim circuit less vulnerable: This carries the constraint that the essential characteristics of this circuit such as detection level, bandwidth or time constant remain unchanged. Therefore this option has often a very limited range.

Acting on the source-to-victim coupling path: This is where the designer has the largest choice of solutions: shielding, filtering, grounding, physical separation or orientation and loop area reduction . Thus, a sound approach will take EMC into account from the original concept to the design stage.

Another interesting side of the source-and-victim concept is that it is perfectly reversible (reciprocity). The mechanisms that could cause the circuit to be a victim are the same mechanisms that could make it a potential source of interference.

The importance of frequency

Practically all coupling mechanisms that are conveying the source emissions to the victim’s input are frequency dependent: that is, as frequency increases, the coupling coefficient increases. In some cases, it may even be aggravated at the frequency squared.

A quick example can give a measure of this aspect of frequency. Assume you have a long wire carrying a constant-value ac current of 1 A. At 10 cm distance from a 100-cm2 nearby loop, this 1-A current will induce, by magnetic coupling:

• at 50 Hz; 6.3 µV

• at 100 kHz; 12.6 mV

• at 10 MHz; 1.26 V.

4 parts of the EMC picture

Based on our source-and-victim principle, the reduction of EMI situations to a tolerable level will be secured by controlling the EMC performance of the equipment before it is put on the market, which can be verified by the following EMC tests.

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