Non-frequency selective equipment

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

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