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Page Title: Recognition, Isolation, and Remedy of EMC Problems
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TM 11-490-5
operations. If available at the site, a battery powered
4-3.  Recognition, Isolation, and Remedy of EMC
multiband radio becomes an. effective diagnostic tool
providing aural indications which, when used in
a. Introduction.
conjunction with an improvised directional antenna, can
(1) The most important factors in resolving
aid in determining the direction of the interference
EMC problems are the knowledge operators have of
source.  Aural indications can often be correlated to
their stations and the ability to speedily detect any
aural noise at a patch panel or output of equipment. If a
operational abnormalities.
A knowledge of normal
frequency is noted, at which the type of interference is
meter and indicator readings, signal "sounds" at audio
heard, the radio should be taken outside of the area or
frequency, and visual representation of signals on a
away from the shielding influence of equipment so that
cathode ray oscilloscope display or spectrum analyzer
an attempt can be made to identify the source of EMI. If
are all good tools in detecting abnormalities and
the EMI is weaker outside, the radio should be brought
effecting rapid corrective action.
This should be
inside as the source is probably in the building.  An
coupled with a thorough familiarity with all aspects of
improvised loop antenna used with either the radio or a
station configuration.  The station log, together with
frequency selective voltmeter may be used to determine
performance records for channels and systems, is an
the direction of the EMI source or to make a specific
invaluable record of problems and their causes. Use of
determination of the generating equipments and leads
these logs may facilitate and even expedite resolution of
carrying the noise. It may be necessary to obtain the
EMC problems.  A thorough knowledge of routine
assistance of maintenance personnel in fabrication of a
maintenance checks will facilitate the development of
suitable loop antenna.  Being battery-powered, and
procedures designed to remedy abnormalities causes by
therefore free of the station's primary power source, the
EMC problems.
radio can be used to determine whether the interference
(2) An interference problem can be isolated
is entering the system by RF radiation or through the
to  a  particular  element  or  equipment  within  a
primary power lines.  Poor grounds, cracked or dirty
communications site by signal tracing, if adequate
insulators, or defective transformers in the ac power
equipment monitoring or test instruments are available,
system can, under certain conditions, be located by
or by substitution when instrumentation is limited.
moving the radio closer to the suspected interference
However, in some cases, it can be extremely difficult to
determine  precisely  at  what  point  (component,
b. Recognition Procedure.
subsystem, or system) interference is being introduced.
(1) Determine origin of the interference.
Frequently, resolution of a problem is achieved not by a
(a)The effective clearance of a
study of what is affected, but rather on determination of
the source and the coupling medium, then controlling,
suspected  EMC  problem  requires  a  rapid  initial
reducing, or eliminating either the source or the coupling
assessment of the origin of the interference. That is,
medium. In any case, the important factor is to proceed
was it generated within the local station, or was it
methodically rather than skip from equipment to
generated by a distant station? In the latter case, the
equipment in a random manner. For instance, if trouble
source must be determined.
To allow effective
shows up on a channel, and it has been determined that
application of isolation procedures (para 4-3c ) this
the transmitting station is not at fault. the trouble could
determination must be made without delay.
lie within circuit modules at the channel level, such as
(b)  Although interference can be
line amplifiers; in channel, group, and supergroup
introduced into a station from an external source by
demultiplexer circuits: in baseband amplifier modules;
means of cables and transmission lines, the most
within the receiver; or at the antenna. Probing of these
common avenue of entry in systems employing radio
circuits, in this order, at the system test jacks and use of
equipment  is  through  antenna/receiver  systems.
appropriate test equipment should isolate the problem.
Although interfering RF emissions may be locally
Substitution, in the event that test equipment is not
generated,  the  majority  are  caused  by  distant
available, involves the placing in service of alternate
transmitting stations.  Steps which can be taken to
channels, modules, and equipments by means of
determine whether the interference is entering the
patching  and  special  cabling,  if  necessary  in
system by RF means are as follows:
The substitution method, although
1. Check whether the signal level has
effective in isolating interference to a particular
increased significantly at the receiver output. If it has,
equipment or its associated cabling, is generally slow
this may indicate an interfering signal on or close to the
and has the disadvantage of requiring interruption of
receive frequency.
service during substitution, unless parallel circuits can
2. Display the output on a spectrum
be used to advantage without disturbing normal

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