Not often in design work do you get a chance to make a product meet specification by bolting on a few basic parts. With electromagnetic compatibility (EMC), you have that option simply because many EMC failures concern a product's peripheral features such as cabling and apertures. Components such as ferrite cores and beads, feedthrough capacitors, connector shields, gaskets, and conductive tapes can all prevent unwanted signals from reaching the outside world. Conversely, the same components also offer a design some immunity from external interference. Not only do these basic components provide your design an important source of EMC first aid, but they can also throw a permanent lifeline to a finished product that would otherwise be an EMC casualty.

Applying first-aid fixes at the back end of a design cycle opposes almost everything EMC experts believe in. They advise, advocate, and even implore you to design in EMC from day one. That way, you suppress emissions at the source and prevent noncompliant noise levels from ever reaching the extremities of your product. Much of that preventive design work involves building in certain features at pc-board layout (Ref 1). While the advice is good—and appeals to the common-sense instincts of most designers—the fact remains that designers' priorities often lay elsewhere.

This situation prevails despite an increasing awareness of the importance of EMC and Europe's EMC Directive (Ref 2). That directive will ultimately enforce conformance by law; it applies equally to new products and products designed well before EMC became a hot topic. On older designs, applying first-aid measures is the only cost-effective compliance route and represents a genuine need for that approach.

So, for the time being, applying quick fixes remains a popular and necessary way—although not the purist's preferred method—to treat EMC-design problems. For a quick reference, see Table 1, which shows the range and variety of EMC components that can administer first aid to a finished design.

Broadly speaking, undesirable emissions occur either as radiations directly from your circuit board or indirectly, first by conduction along connecting cables and then by radiation. Directly radiated emissions leak through apertures or poor electrical joints in a product's shielding, and it's here that conductive adhesive tapes, wire mesh, and gaskets are effective remedies. Connector backshields, filtered connectors, cable shields, ferrites, and feedthrough capacitors all reduce emissions conducted onto connecting cables. For the purpose of meeting EMC specifications, assume conducted emissions concern signal frequencies of 150 kHz to 30 MHz, and radiated emissions cover 30 MHz to 1 GHz. At present, international standards for conducted emissions consider mainly effects on line cords, and, therefore, it's common practice to design in a line filter. By contrast, standard requirements overlook conducted emissions on I/O cables, and because I/O signals vary widely anyway, these lines are largely left unfiltered. Undesirable emissions on I/O cables give rise to the majority of EMC failures, and radiated-emission tests pick up those failures.

Whatever EMC tools you employ to make your tests (Ref 3), it's often most expedient to adopt a trial-and-error approach when solving EMC problems. It helps, though, to have some understanding of what each type of first-aid component can reasonably achieve. In most cases, however, your adding components will not upset the fundamental performance of the product. But in a few cases they can—for example, if you set about filtering a high-speed data bus.

The difference between series- and common-mode signals is a key distinction to appreciate in addressing EMC problems on cables. Series (or differential)-mode signals are legitimate data signals that consist of signal currents with a forward and return path. Common-mode signals are wholly illegitimate and occur mainly because of poor grounding somewhere in your overall system.

Both series- and common-mode currents can radiate noncompliant emissions, but series-mode problems are fewer simply because of a tendency of radiation from forward and return currents to cancel out. Although common-mode currents cause most cable-borne EMC failures, fortunately, it's this type of current that EMC first-aid measures readily suppress.

Your two options are to pass your I/O cable through a single ferrite loop or to insert common-mode chokes in series with each signal and return pair at the I/O interface. Vendors offer ferrite loops in an immense range of sizes, styles, and frequency specifications. To ease installation, loops are round, oblong, and either complete or in halves with a clamp. For both ferrite loops and common-mode chokes, you have to rely on trial-and-error. Most ferrite suppliers encourage this route by offering diagnostic kits containing an assortment of types. Calculations are impossible because you have no idea what value to assign to impedances around a common-mode loop. On that basis, it generally pays to install as large a component as space allows. In the case of ferrite loops, passing a cable through the loop more than once or adding more loops is also beneficial. The merit of ferrites is that they absorb, rather than reflect, radiation locally, and they dissipate the energy as heat.

In the case of series-mod e signals, you can estimate circuit source, transmission, and load impedances. Therefore, you can design a passive lowpass filter to bandlimit I/O lines (Ref 1). The range of filter components is wide, but a common characteristic is definable low inductance. Again, the simplest component is a ferrite core, although in this case the ferrite needs to surround each signal path and needs careful selection. Also available are ferrite plates with holes to match popular connectors' pinout configurations. Other possible filter components include 3-terminal capacitors with and without ferrite beads, chip inductors and capacitors, feedthrough capacitors, and ready-made encapsulated filters.

The easiest and neatest way to apply in-line filtering is either to swap a regular connector for a filtered type or to insert a filtered adapter between your present plug and socket. Filtered connectors exist mainly as replacements for D-type connectors, although you have a wide array of filter-component combinations within the range. With feedthrough capacitor values ranging from 50 to 2000 pF and using or discarding ferrite plate inductors (impedance 35 Ohm at 100 MHz), you can tailor lowpass bandwidth to your application.

Cable shielding is the next step, and again, your options are many. Your main decisions are how thick to make the shield and whether to make a ground connection at one or both ends of the cable. If your knowledge of shielding theory is rusty, there's no shortage of revision notes (Ref 1). Whatever method you choose, a low impedance connection from shield to system ground is essential. The sight of a pigtail connection—alias, common-mode impedance—appalls EMC experts, and it's the primary function of connector back shields to eliminate these offensive joints. Connector backshields, like filtered connectors, generally suit D-type connectors.

If you're using an unshielded ribbon cable, as a first step, try ribbon with a single-sided aluminum-shield backing. Beyond that, you'll need to use a complete braid of zip-on sheath.

If you've applied all the first-aid fixes to your I/O cables, but EMC problems persist, it's time to consider enclosing your main circuit components. Here's where adhesive conductive foils excel. Foils form the most adaptable EMC diagnostic material. Foil material is either copper or aluminum, with a choice of conductive or nonconductive adhesive. Foil surface is either smooth or embossed, the embossed version providing lower contact resistance (approximately 1 m Ohm/in.²) to a supporting surface and other layers of foil.

You can readily fit foil screens to individual components or to whole sections of a circuit as an experiment. You can also use foils to improve contact along enclosure seams, which is a useful way of testing the need for a proper EMC gasket (Ref 5). Additionally, where an enclosure is already sealed except for essential display windows or cooling vents, foil temporarily placed across these apertures will also test the likely benefit of installing EMC mesh. Another experimental use of foils is as an alternative to conductive spray paints for lining the inside of plastic product enclosures.

If you reach the stage of installing gaskets, mesh, or EMC windows, then your work transmutes from low-cost first aid to high-cost intensive care. But, this is the penalty you pay for ignoring experts' words of caution. Even so, applying exotic modifications this late to a design may still be the most profitable way forward. For, as well as providing permanent low-cost solutions, EMC first-aid components will sustain a product at higher cost until it's convenient or worthwhile to conduct major surgery.

1. Williams, Tim, EMC for Product Designers, Butterworth- Heinemann, 1992.

2. Kerridge, Brian, "Europe Lays down EMC Law," EDN, September 16, 1991, pg 57.

3. Kerridge, Brian, "EMC Bench Tools," EDN, October 1, 1992, pg 78.

4. Anderson, Mark, "EMI Countermeasures," Euro-EMC conference proceedings, October, 1993, Reed Exhibitions, Richmond, UK.

5. Bates, Ron, "EMC Gaskets—Traps to Avoid," Euro-EMC conference proceedings, Ibid.