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Recognition, Isolation, and Remedy of EMC Problems
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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Table 4-1. Audio Output Responses Characteristic of Various Types of Interference


TM 11-490-5
oscilloscope.  A portable radio or high impedance
analyzer, if available, to see how it compares with a
earphones are also useful tools for an experienced
normal receive signal.
operator to determine the presence and nature of RF
3. If necessary, have the distant desired
interference.
transmission disabled, to allow a better visual and aural
examination of the receiver output to see whether
(e) If it is determined that cochannel RF
cochannel interference is occurring.
interference is present, the possibility of deliberate
interference or jamming by unfriendly forces must be
4. If necessary, change the receiver
investigated.
antenna and/or alter the antenna azimuth slightly (if
practicable) to observe any effects on the extent of the
(f) If it is suspected that the interference is
interference.
jamming, do no use the orderwire or any other form of
clean-text transmission to advise another station of the
5. If the above steps are inconclusive,
severity of the interference, or even that interference is
disconnect the receiver antenna from the receiver and
present, until it has been determined that jamming is not
connect in its place the carrier output of a suitable signal
involved or the proper authority has been informed and
generator adjusted to the receive frequency, at an
a go-ahead has been obtained.
appropriate level. (If the receiver has a beat frequency
oscillator, it should be disabled.)
(g) If the interference is not being introduced
through the RF medium, rapid isolation: , to either
6. Apply a suitable degree of modulation
baseband (audio) or the multiplex carrier will assist in
to the signal generator and observe the receiver output.
recognition of the type of interference.
If the
This is to ensure that the receiver is not introducing any
interference was introduced into the baseband audio
interference due to nonlinearity.)
signal or the multiplex system prior to transmission, it
7. The next step is to arrange an RF
may be difficult to recognize at the receiver unless the
"back to back" test, keeping the receiver isolated from
operator has experienced this type of interference
its antenna, as follows:
previously. Therefore, the origin of the interference to
(a) Retain the connection from the
either the baseband audio or multiplex system can
signal generator to the receiver input, but switch off the
normally be traced more rapidly by checking the
modulation.
multiplex system at both receiver and transmitter sites.
(b) Connect the composite transmit
Detailed procedures for isolating the entry point into the
output of a multiplex/submultiplex system to the
system are contained in (2) below; however, a check of
"external modulation" -jacks or terminals of the signal
the input and output signals of the multiplex equipments
generator. (Insure that the depth of modulation is such
can provide a rapid determination of the multiplex
as to provide linear operation.
performance. If this performance is determined to be
(c) Provide keying on the channels
normal, then further isolation to cabling and switching
which were suffering interference.
circuits as detailed in c (3) (a ) and c (3) (b ) below may
(d) Adjust the receiver output to
be required.
normal receive multiplex/submultiplex system levels. In
(2)
Determine nature of interference.
this way a "back to back" arrangement is achieved
(a) Monitor the channel or channels in
which incorporates all receiving equipment except the
question with high impedance earphones or other
antenna system. If the interference is still present, it
available test equipment to determine the nature of the
originated within the station. If it is no longer present, it
interference. Table 4-1 provides a listing of the audible
was introduced by RF means.  Remember, however,
output to be expected from a receiver under various
that it could be originating at the desired distant station
interference conditions and may be used to assist in the
due to a fault condition there.
recognition of interference sources.  Note that the
(c) In making the above determination, it must be
conditions listed in this table will be in addition to the
remembered that even if interference is introduced by
signal normally heard, which itself may be in the form of
RF means, it could be locally generated. However, the
tones, recognizable to experienced operating personnel,
determination greatly facilitates isolation procedures.
but from which it is impossible to extract the intelligence
of the signal by merely listening.  That is, other end
(d) If the interference is entering the system by RF
instrumentation would be required for signals such as
means, a check must be made to establish the source.
teletype, digital, and facsimile, in order that the manner
Cochannel interference might require the distant wanted
in which the intelligence of these signals is affected can
station to disable its transmitter output to allow the local
be determined.  As an alternative, the desired signal
station to demodulate and print (if possible) the
could be disconnected at the far station so that the
interfering signal as a means of identification. Adjacent
interference alone could be heard. Figure 4-1 shows
channel interference is more easily discernible and is
usually easy to see on a spectrum analyzer or
4-4


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