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Spurious response interference
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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CHAPTER 3. EMC PROBLEMS AS RELATED TO SIGNAL TYPES AND MULTIPLEXING TECHNIQUES


TM 11-490-5
servoamplifiers and dc amplifiers.  Since this type of
(2) Non-frequency selective equipment.  This
equipment  frequently  operates  by  controlling  the
term refers to all electrical and electronic devides other
amplitude of signals at the 60-Hz and 400-Hz powerline
than those which are tunable to, or fixed at, a selected
frequencies,  they  are  particularly  susceptible  to
band of frequencies.
In general, because of the
powerline transients and stray coupling to wiring and
complexity of types, arrangements, and operational
transformers.  Because they operate at relatively low
characteristics of equipments, and because of the many
signal levels, amplifiers of this type are troubled by
types of interference-causing signals, entry modes, and
common-mode  coupling  at  their  input  terminals.
interactions, the susceptibility characteristics of this
Rectification (removal of positive or negative-going
equipment  category  have  not  been  completely
voltage peaks) of high-frequency signals by nonlinear
documented or standardized. Susceptibility of individual
circuit elements can cause saturation (operation at the
equipments used for specific requirements is best
maximum limit of response regardless of input voltage
determined by test, either by simulation of interference
variations) or desensitization (lack of response to low-
signals or under actual operating conditions.  A brief
level signals) of gain characteristics, or parasitic
discussion of some susceptibility characteristics follows:
(unintended, self-sustaining) oscillations.
(a ) Digital computers. Since operation of digital
(c) Displays.  Cathode-ray tube displays are
computers depends upon pulses and levels of fixed
susceptible to emissions from radars, communication
amplitudes occurring at predetermined times, computers
transmissions, ignition systems, and other equipments.
are particularly susceptible to interference from pulsed
The main effect is the disruption of the visual
signals (such as from radars sweeping the area).
information presented to the equipment operator.
Experimental investigations of problems resulting from
Cathode-ray tubes are susceptible to stray magnetic
interference have demonstrated false switching in flip-
fields which deflect the electron beam. Interference in
flops (circuits designed to produce one of two possible
sweep circuits is characterized by distortion of the
outputs, depending on the last input signal received) and
display (by causing the sweep to be nonlinear), while
logic gates (circuits whose output depends on two or
undesired responses in the video sections may appear
more input signals), deteriorated signals, and erroneous
as intensity modulations. The circuits which process the
outputs. Computers are also susceptible to another type
video, or information signal, and those that generate the
of interference which is worthy of mention, even though
required sweep voltages are subject to powerline and
it is not electromagnetic. Computer memory banks are
common-mode coupling, as outlined in the previous
susceptible to magnetic fields and if these fields are of
section.
sufficient strength, data stored in the computer memory
(d) Test equipment. Test equipment is just as
may be lost.  Therefore, strong magnetic fields from
susceptible to unwanted electromagnetic energy as
either permanent magnets or electromagnets should not
many of the equipments under test.
Powerline
be allowed in the vicinity of computers.
transients and capacitive and inductive coupling may
(b ) Control devices. This equipment category
produce false indications when testing equipment.
includes  amplifiers  which  have  low-level,  high-
Noise may be introduced into the test equipment as a
impedance inputs and are used in the processing of
result of a grounding problem and thus increase the
sensor signals, control of electromechanical devices,
indicated noise level.
and  many  other  functions.
Examples  are
2-15


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