To guarantee that electronic circuits will perform as designed, they must be protected from electromagnetic interference (EMI). At the same time, the circuits themselves must not radiate emissions that can threaten or degrade the performance of other equipment. Because systems must share the electromagnetic spectrum, rules have been established to ensure a safe environment for all. These electromagnetic compatibility (EMC) standards guarantee that electronic equipment can move freely without any degradation in performance, and without interfering with other systems.
The large number of EMC regulatory bodies that have been established is an indication of how serious this issue has become (see the table). It is important to note that EMC regulations relate only to the operation of complete equipment--an empty enclosure cannot comply. Making sure that your system meets these standards can be costly, but it can also secure the economic success of the project.
Compliance with these EMC standards requires EMI protection at four levels: the component level, board level, system level, and overall system level. Most electronic OEM products are Level 3 systems, with the electronic circuitry, power sources, motherboard/backplane interconnect systems, and thermal management all housed in one enclosure. At this level, a well-designed enclosure and careful integration of the system with the enclosure provide sufficient shielding for both radiated and conducted electromagnetic emissions to ensure electromagnetic compatibility. This article deals with the challenges facing system designers who must provide EMI protection for these Level 3 systems.
The Nature Of EMI
Electromagnetic interference can be either radiated or conducted. Radiated interference travels in the form of radio waves, and is called radio-frequency interference (RFI). Conducted interference comes from the magnetic field generated by current flow in cables carrying signals and power.
Physical shielding provides signal attenuation (a weakening of radiated interference) through the reflection and absorption of electromagnetic waves (Fig. 1). Electromagnetic waves have both an electric (E) field and a magnetic (H) field. In the E field, attenuation by reflection improves with conductivity. It is adversely affected by increases in frequency, permeability, and distance from the signal source. In the H field, increasing conductivity, frequency, and distance from the source are beneficial, as is decreasing permeability.
These two fields oscillate at right angles to one another, with the ratio of E to H referred to as the wave impedance. When E-wave and H-wave components are of a fixed ratio, the product is a plane wave. When the current flow is high relative to the voltage, the wave impedance is low, so the result is predominantly an H field. When the voltage is high relative to the current flow, the wave impedance is high.
A metallic shield typically reflects E-wave energy and absorbs H-field waves. (The higher the magnetic permeability of the metallic shield, the greater the H-field absorption.) In modern electronics equipment, typical EMI emissions are high-frequency and high-impedance, so the major wave component is the E field.
In an ideal world, the perfect EMC enclosure would be manufactured from a heavy-gauge, dense material such as steel, and would have six solid, fully sealed sides, with absolutely no cables traveling in and out. However, we live in the real world, where EMC enclosures are much more interesting because they must provide effective EMI shielding while meeting some pretty inconvenient OEM system demands. These demands include slots and openings, heat management, power, I/O, data bus cables, and the ability to insert and remove single-board computers (SBCs) and line-replaceable units. Each of these requirements mandates special design considerations to understand the EMI problems presented and the solutions available.
Real-world enclosures have doors, panels, switches, fixing holes, ventilation grilles, and other features that penetrate the surface. The joints formed at the boundaries of these features and enclosures are opportunities for gaps and holes. In shielding terms, these openings are called slots, where a slot is a hole of any size or shape through which electromagnetic radiation can enter or exit the enclosure.
The EMC problems that slots cause are greater than one might imagine. Obviously, the number and size of slots are important in terms of diminished shielding. In addition, the effective length of the slot relative to the RFI frequency is also important, as is the orientation of the slot and its potential to behave as a waveguide "slot antenna."
The number and size of slots should be minimized. The higher the RFI frequency, the smaller the slot size should be for a given level of signal attenuation. The effective length of a slot is its major straight-line dimension. Effective lengths of slots in door or panel joints can be relatively long, and often need special attention. They are typically addressed with specialized gaskets. Many types of gaskets are available for such purposes, including metal-loaded polymers, metallic spiral gaskets, beryllium-copper (BeCu) fingers, and knitted wire mesh. Typically, these options are assessed on a cost/benefit basis.
VERO Electronics' EMC facility recently examined the practical effects of the relationship between slot size and aspect ratio on EMC performance. The test evaluated BeCu fingers used as a gasket on an enclosure door. Frequencies ranged from 100 to 1000 MHz, and the slot-length remained constant while its width was changed. Results showed that decreasing the aspect ratio of a slot adversely affects EMC performance (Fig. 2).
In practice, slot orientation is not reliable in blocking vertically or horizontally polarized RFI. Slot orientation can, in fact, compromise the enclosure's capacity to shield. Incident RFI causes current to flow in the shielding material. (These currents also act to oppose causative radiation.) When a slot in an enclosure wall interrupts these currents, electrical charges are set up along its edges, causing the slot to act as a waveguide slot antenna.
Along with electrical issues, designers must also tangle with power source-generated heat. As most electronic circuitry is sensitive to heat, system packaging must provide for heat management.
Commercial thermal management techniques operate on the basis of forced-air convection. Air is drawn from the external environment, passed over the elements that dissipate heat, and then exhausted to the external environment. For purposes of EMI control, ventilating an enclosure in such a manner seems like the worst thing to do, yet it must be done. The designer must meet both the thermal considerations of the internal circuitry and the applicable EMC requirements.
In industrial embedded computers, the primary sources of heat are the power supply and signal-to-balance converters (SBCs). To protect the circuitry, the design must dissipate this heat so that the temperature within the enclosure does not exceed a predetermined allowable rise in temperature (T) above ambient. The goal is to do so while meeting specified levels of signal attenuation.
Before designing ventilation, engineers should consider several factors. How much ventilation is required? Is forced-air cooling necessary to increase airflow through restricted ventilation holes? Which frequencies are important?
Carefully designed grilles that balance the need for airflow with the ability to control EMI provide the ventilation. Several basic grille options are available. In all cases, the hole sizes are generally determined by the signal attenuation required, and the RFI frequency involved (Fig. 3). Keeping the number of holes needed to provide sufficient airflow, to a minimum, they must be spaced as close together as possible to minimize air turbulence. Field-cancellation effects may occur between adjacent holes. These effects could reduce the cumulative loss in signal attenuation that results from increasing the number of holes.
In sheet-metal grilles, ventilation ratios (the ratio of the area of the holes to the total surface area of the grille) can be as high as 75% if the holes are square. If the holes are round, the ventilation ratio ranges between 50% to 60%. The signal attenuation of round holes is better than that of dimensionally equal square holes.
Honeycomb vents, constructed of very thin sheet metal, have holes with good aspect ratios and signal attenuation properties, while allowing for good ventilation. On the downside, they're costly, fragile, and sometimes difficult to integrate reliably. The orientation of the holes may cause a polarized signal attenuation effect that can be overcome by using two layers of vents with the holes positioned orthogonally.
With ventilation ratios of greater than 80%, wire mesh, another enclosure alternative, sees marginally lower attenuation than the same hole dimensions in sheet metal. Wire mesh, too, can be difficult to integrate reliably into an enclosure.
The enclosed system generally requires power, and usually must communicate with other devices via I/O or data-bus cables. To accommodate these needs, the enclosure must have cable apertures, which, of course, degrade the ability of the enclosure to shield against radiated EMI. In addition, the cables themselves introduce conducted emission. This property is interference that is conducted through the cabling.
In high-performance, mission-critical defense equipment, I/O cabling uses a braided shield and passes through the enclosure wall via a bulkhead connector grounded to the chassis. The grounded cable braid forms a continuous shield with the bulkhead connector. This type of design provides high levels of EMI control, but at an economic cost.
In commercial applications, it is common simply to run a cable through an aperture in the enclosure wall, and use readily available products, such as ferrite beads, to minimize radiation to and from the cable. (Ferrites reduce radiation by suppressing conducted RF currents.) To use ferrites, slip the bead over the cable, where it can sit outside or inside the enclosure, as long as it is close to the shield.
The fitting of this bead to the cable produces an RF choke which has low impedance at low frequencies and relatively high impedance over a wide high-frequency band. The effectiveness of this impedance in reducing radiated or conducted interference depends on the relative magnitudes of the source, suppressor, and load impedance. For input power lines, filters are used to control radiated and conducted emissions. In the typical design cycle, engineers normally address these issues during the EMC testing phase.
SBCs And Replaceable Units
Embedded computers that operate on open platformsþsuch as VMS, VXI, or CPCIþare based on the principal that SBCs can be replaced and interchanged very easily by simply plugging them into the system's backplane. A VMEbus system can have up to 21 SBCs, resulting in 22 gaps between the SBCs and the enclosure. Card cages manufactured to IEC 297-3 standards typically use a U-shaped panel and a BeCu or stainless-steel spring to maintain the appropriate EMC performance. However, this approach may produce enough pressure to build between the SBCs' front panels that they must endure a large force to insert and remove them.
Newer enclosure specifications, such as IEEE-1101.10, typically call for extruded front panels with a near-zero insertion force. A metal-loaded polymer or metallic spring gasket inserted into the SBC panel extrusion answers that requirement.
Military and industrial process-control equipment and automated test equipment often require little down time when replacing a faulty power supply or fan unit. For these situations, each submodule is designed with suitable gasketing to ensure EMC integrity when a faulty unit is replaced by a spare.