Narrowband EMI consists of a single frequency or
a narrowband of interference frequencies. Narrowband
EMI usually has a minor effect on communications
or electronic equipment. It can be tuned out or filtered
out.
Broadband EMI is not a discrete frequency. It
occupies a relatively large part of the electromagnetic
spectrum. It causes the majority of EMI problems in
digital data equipment. It will be especially noticeable
data on Link 11.
CONTROL OF EMI. EMI can be controlled or
eliminated if some simple procedures are followed and
good installation practices adhered to. Lets look at
control and reduction for shipboard and shorebased
installations. Many of the problems are the same for
both installations.
Shipboard EMI Control. Shipboard EMI
control is greatly simplified for the typical digital data
installation.
Because of the ships steel hull and
construction, a great deal of shielding and isolation are
provided the typical shipboard computer room or digital
equipment space.
This blocks out the majority of
broadband interference generated both internally and
externally. Five major factors are considered in a
shipboard computer and digital equipment installation.
They are equipment location, equipment shielding,
system and equipment grounds, interconnection
cabling, and power source.
. Equipment location Computers should be
located in spaces that are free of sources of EMI. They
should not be located in spaces that contain radars, radio
transmitters, generators, or other rotating machinery.
. Equipment shielding Digital computers
should never be operated with drawers extended, cover
plates removed, or doors open. Modem computers
contain EMI-reducing gaskets and shields that enclose
the equipment. Always reinstall cover plates with all
the fasteners in place. If a cover plate or shield has to
be removed in the course of corrective maintenance,
ensure that the EMI reducing contacts or wire gaskets
on the equipment opening are in good condition before
the cover or shield is replaced.
l System and equipment grounds System and
equipment grounds are extremely important in digital
computer installations.
All cabinets should be
grounded together on a common system ground bus.
Each equipment cabinet is connected to the system
ground by a heavy ground cable. The system ground is
securely attached to the hull of the ship and provides a
good ground reference for the system. Paint on ground
straps or on the metal decks where the ground straps are
mechanically attached will result in poor electrical
connections.
l Interconnecting cables All interconnecting
cables used in a shipboard digital data system should be
shielded cables. They shoud be assembled correctly
according to installation drawings. The shield and
connector shell should be electrically connected and
properly secured at either end. The cables should never
be run in the same cableways as cables carrying rf
signals or high-power pulse cables. The shielding
protects the data cables from EMI to a great extent.
. Power source Power lines for digital
computers can provide a transmission path for EMI
from machinery spaces. The majority of input power
passes through noise elimination filters as it enters
computers.
Unusual random problems in digital
computers can sometimes be traced to defective line
filters.
Shorebased EMI Control. Control of EMI at a
shorebased installation requires the same consideration
or the same factors as a shipboard system with two
additions-site location and soil quality.
l Site location Shorebased digital data
equipment sites are sometimes built where the need
dictates or where a convenient building is available.
They are not always ideal sites. Sites built near a large
industrial complex such as a shipyard repair facility
(SRF) or a naval depot (NADEP) may be subjected to
EMI. They also can cause power line fluctuations if the
power source of the shore site and the SRF and NADEP
are the same. Additional line filters and regulators for
power lines may also be required to reduce EMI and
provide line power within the limits prescribed by
equipment manufacturers.
l Soil quality At a shore installation, a system
ground bus is usually attached to a grounding rod driven
into the soil. If the soil is dry, sandy, rocky soil as found
in the Southwestern United States and some places
overseas, you will have a poor ground. A suspected
system ground can be checked with an oscilloscope and
1:1 probe. Refer to Electromagnetic Compatibility,
NAVELEX 0967-LP-624-6010, for more information.
DIRECTIVES. Many directives provide
guidelines to follow for avoiding or reducing the
effects of EMI. The EIMB handbook entitled
Electromagnetic Interference Reduction, NAVSHIPS
0967-LP-000-0150, includes topics of shipboard EMI
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