Electromagnetic Compatibility (EMC) and Related Concerns

The need for electro-magnetic compatibility (EMC) can cut two ways: the need to make electrical equipment immune to electro-magnetic interference (EMI) from outside (protecting the sensitive contents from accidentally receiving any externally generated radiation that could disrupt the correct function of the circuit); and the need to keep it from emitting any electro-magnetic interference itself (protecting the outside world from accidental emissions of internally generated radiation that could waste energy, leak secret information, or disrupt other equipment and be illegal).

The two sides, immunity and containment, might also be encountered by individual modules of a single system (with the need to protect one part of the system from interference from another part).

Electromagnetic interference (EMI), includes radio-frequency interference (RFI) and microwave interference. Its successful avoidance is referred to as electro-magnetic compatibility (EMC).

The 25 years of EMC components article in the "Conformity" magazine provides a good introduction, summarising the history of the various standards that are encountered in the literature:

• FTZ/VDE-0875/0871 in West Germany and FCC part 15 (class-A and class-B) in USA (1979)
• CISPR-22 (EN-55022) in the EU
• IEC-61000-x (1990), IEC/EN-61000-4-3
• IEEE-STD-299 (1991), successor of MIL-STD-285 (1956), for SE tests
• IEEE-STD-1302, to replace SAE-ARP-1705, MIL-G-38528, DEF-STD-59-103
• ASTM-D-4935
• EMC Directive (20 July 2007)

EMI Filters

Vulnerable points in an electronic circuit include: the mains cable (since this is a long conductor, and hence a very good antenna); external signal cables (for the same reasons); data, address and control buses (because they are long lengths of conductor at the exposed back of the equipment, operating at high frequencies).

Long leads can have an inductor (choke) inserted in them. Alternatively, they can just have a ferrite sleeve put around them, making the cable itself behave as an inductor. Since there are two (or more) electrical connectors inside the screening, it forms a common-mode inductor.

EMI Shielding

On the circuit board or integrated circuit, sharp bends and corners in high frequency lines causes a problem: the sharp bend or corner can appear to the electromagnetic wave like a badly matched transmission line, and energy can escape (or enter) directly at the sharp edge.

Careful PCB design is one useful counter-measure. This might involve the use of CAD software that is specifically aimed at EMI avoidance.

The usual counter-measure, though, is just to screen the circuit in an EMI-proof shield. A metal box is the simplest possibility, though metal-coatings on plastic, or electrically conductive plastic, are also possible.

The box will still leak radiation (either in or out) via any holes that have been cut for external leads, access hatchways, display windows, ventilator openings. Various EMI gaskets and grills are available for combating these.

The shielding effectiveness (SE) is measured in decibels.

Test Chambers

In order to protect the contents from externally generated radiation, test chambers need to be screened from external radiation.

In the other direction, it is not just the outside world that needs to be protected from internally generated radiation. They also need to be made anechoic (without RF echoes) of internal radiation, so that the equipment behaves as if it were out in the open. Just as an acoustic chamber has sound-absorbing foam cones across its entire inner surface, so an anechoic chamber is covered with electrically or magnetically conductive foam cones.

Related Topics

Electromagnetic shielding generally means shielding against microwave and radio frequency radiation. Shielding against infrared, light, ultraviolet and X-ray radiation technically ought to be included, and often is, albeit inadvertently (inasmuch that a metal box designed to shield against the first two will probably shield against all the others as well, with the possible exception of X-rays).

Similarly, the metal box can also be made to shield against electric fields on their own, and depending on the choice of metal for the box, against magnetic fields, too.

These topics are briefly discussed, next.

Electric Field Shielding

A conductive layer to shield against EMI will also greatly protect against electrostatic fields (acting as a Faraday cage) and hence against the concerns of electrostatic discharge (ESD), but it needs to be connected to ground, which EMI protection does not necessarily need. Also, the operatives on the production line might need to be earthed (using mats and wrist-straps), and to use equipment, materials and clothing that do not generate static electric fields. Indeed, one major difference is that EMI is a concern during the running of the equipment, while ESD tends to be a concern during its fabrication.

Transient Voltage Surge (TVS) and Over-Voltage Protection (OVP)

Equipment that is designed to work at 12V might need protection on its power supply leads, and data lines, from any voltages that go well above this voltage. This would protect it against inadvertent connection of faulty power supplies (over voltage protection); it might also provide protection against transient voltage surges (TVS), and from static electricity and lightning strikes. Thus, this, too, overlaps with the concerns for ESD protection.

Magnetic Field Shielding

This includes the concerns of protecting the outside world from internally generated magnetic fields: from motor windings, for example. It also includes the concerns of protecting the sensitive contents from externally generated magnetic fields: magnetic "short-circuits" to protect sensitive equipment and data-storage media.

Just as a metal box provides a short circuit for an electrostatic field, so a box made of a suitable metal (notably iron, with nickel and cobalt as more exotic alternatives) provides a short circuit for a magnetic field. The field lines strike the box at the given incident angle, bend to follow the contours of the box, and leave on the other side of the box at roughly the same angle as they had entered. By following the contours of the box, the inside of the box is turned into a protected island, free from external field lines.

Visible Light, UV, X-ray and Gamma-ray Shielding

This includes the concerns of protecting the outside world from internally generated radiation (from switch-contacts, motor commutators and arc-welders that each generate UV, cathode-ray tubes that generate X-rays, and nuclear sources that generate gamma-rays). It also includes the concerns of protecting the sensitive contents from externally generated radiation (such as from arc-welding, a nearby medical scanner, solar exposure and cosmic rays).

In general, the amount of matter in the shield is important (or rather the thickness and the density), for the given wavelength of radiation. A box with a metal coating should be enough to block the passage of visible light, and near ultraviolet, but a solid metal box might be needed for the even shorter wavelengths. For X-rays, the box would need to be thick, and made of lead (Pb), say, in preference to aluminium (Al).

For cosmic rays, and radiation from nuclear sources, the incident particles are so energetic, and able to knock out new particles, that the complexity of the atoms in the shielding starts to become important, too. It is all very well that lead would be good at stopping the primary radiation, but it would generate so many secondary rays that it often cannot be used (in the case of nuclear sources, the gamma rays might be accompanied by fast moving neutrons, for which this is even more relevent). The ideal substance then becomes the lightest, namely hydrogen, and hence needs to be deployed in extremely thick walls, in enormous quantities. Consequently, hydrogen-rich substances, such as water, become the shielding materials of choice.

IR Shielding

As before, this includes the dual concerns of protecting the outside world from internally generated radiation, and protecting the sensitive contents from externally generated radiation. There is also the problem of protecting against heat conduction and convection, which, of course, overlaps with the issues of thermal shielding and thermal insulation.