Biomedical Engineering Reference
In-Depth Information
impedance as well as lead to non-linear effects such
as rectification [2,18,19].
19.3.2 Filters and Shielding
Conducted emissions along power and signal trans-
mission lines are best dealt with by shielding and
filtering. Modern electronics produce high levels
of fast transients which are readily conducted
through the power mains. Standard EMI power
filters do an excellent job of attenuating these
unwanted signals and will work equally well against
most modest IEMI. High quality shielded cable
(preferably balanced line) with terminal ferrite EMI
filters will prevent common mode transients from
entering the equipment. Unused connectors should
be covered with conductive caps. Power and signal
cables should be unplugged when not in use. This
prevents high energy EMI such as that fromaHEMP
event from conducting into critical equipment. This
measure is especially critical during times of height-
ened vigilance. Small, hand-held equipment sealed
in conductive foil or polymer ESD bags will like-
wise have substantial shielding from high intensity
IEMI. External antennas, in addition to good light-
ning grounds, should be coupled to the inside trans-
mission line through a gas discharge tube. This will
rapidly shunt both lightning and HEMP impulses
to ground, rather than into sensitive radio equip-
ment. Cabinets should always be closed with their
EMI gaskets properly seated. Ventilation holes are
second only to cables in their ability to allow breach.
All perforations of the Faraday cage act as slot
antennas. They can be retrofittedwith copper screen,
bonded to the case to reduce this phenomenon.
Care must be taken to avoid reducing air flow
excessively.
Figure 19.6
Ionizing laser antenna for IEMI device.
Grounding
Testing
Shielding
Monitoring
Filtering
Physical security
Optical coupling
Figure 19.7
Effective strategies for EMI mitigation.
19.3.1 Grounding
Grounding is the most overlooked factor when
hardening against EMI. The avoidance of ground-
loops is critical to reducing interfering signals. A
ground is simply a current return path. The best
grounds are obtained using low impedance, single
point grounds, or a ground plane. At audio frequen-
cies, low resistance is the most important factor in
reducing impedance. At higher frequencies, capac-
itive and inductive reactance must also be consid-
ered. Wide, short (<1/20 wavelength) flat ribbons
are the preferred means of grounding at RF frequen-
cies. Ground-loop currents can also occur without
any direct contact, due to capacitive and induc-
tive coupling. Optical coupling of digital signals is
another way to prevent ground-loop currents from
flowing between devices. Even micro-amps across
micro-ohmground impedances will result in enough
signal to interfere with sensitive analog devices.
Consideration should be given to long-termbonding
of components. Corrosion of contacts can increase
19.3.3 Backup Systems
While an expensive option, thought should be
given to having backup systems for mission crit-
ical devices. Often, several members of a class
of device may be idle. Placing these in a physi-
cally secure, EMI-hardened state will ensure their
survival after an event. Uninterruptible power
supplies, which can filter both transients and
power sags will allow orderly shutdown during
