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 MIL-HDBK-419A
CHAPTER 6
INTERFERENCE COUPLING AND REDUCTION
6.1 INTRODUCTION.
A large number of diverse equipments are usually present in an electronics complex. The various systems and
subsystems making up the complex may be concentrated in a small area such as a single room or they may be
distributed over a wide geographical area and be located in several buildings. Whether the distances between
individual equipments are large or small, the entire system must function as an integral unit. Each equipment
must supply its designated output -- whether it be audio or rf, or analog or digital -- to some terminal point
such as an antenna, land line, or another piece of equipment. Both primary and backup power must be supplied.
Critical points in the system must be monitored both locally and remotely. To perform all the required tasks
and functions, many control, power distribution, and signal transmission networks are necessary.
Within the interconnected complex, many potentially incompatible signals are present. For example, at one
extreme are the large power sources (primarily dc and 60 Hz) supplying the various subsystems. At the other
extreme, low level dc and very low frequency signals from monitors, indicators and other specialized devices
are present. Also in the low frequency range are audio signals used for communications and control functions.
In the higher frequency region of the spectrum. are the rf signals ranging from hf to microwaves used for
communications, surveillance and tracking, and other functions. These signals range in amplitude from the
microwatt levels typical at communications receiver inputs to the kilowatt and megawatt levels transmitted by
some radar systems. Ranging from audio frequencies into the rf region are the broadband display and
communications systems, both analog and digital, which may span from a few hertz to several megahertz in
frequency and may range in amplitude from a few millivolts to a few volts, Falling in overlapping frequency
ranges, these various signals present within the complex may interact in an undesirable manner to cause
interference (generally manifested as annoying "noise").
Interference is any extraneous electrical or electromagnetic disturbance that tends to interfere with the
reception of desired signals or that produces undesirable responses in electronic systems. Interference can be
produced by both natural and man-made sources either external or internal to the electronic system. The major
objective of interference reduction in modern electronic equipments and facilities is to minimize and, if
possible, prevent degradation in the performance of the various electronic systems by the interactions of
undesired signals, both internal and external.
The correct operation of complex electronic equipments and facilities is inherently dependent on the
frequencies and amplitudes of both the signals utilized in the system and the interference signals present in the
facility. If the frequency of an undesired signal is within the operating frequency range of the system, errors in
the system response may be obtained. The extent of the system response is a function of the amplitude of the
undesired signal relative to that of the desired signal. For example, in systems operating with high level
signals, undesired signals with amplitudes on the order of volts may be tolerable, while in low level systems a
few microvolts may produce intolerable errors in the response of the system. An important element in the
control of unwanted interactions between signals is the proper grounding of the system.
6-1
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