Recognition, Isolation, and Remedy of EMC Problems

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

Problems

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

source.

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:

emergencies.

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

4-3