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.
Compromises
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.
Slots
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.
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