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APPENDIX. DEFINITIONS-continued - TM-11-490-50043
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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TM 11-490-5
Sparking--The sudden breakdown of the insulating
of the input frequencies to appear at the output. For
example, if fa is the frequency of the input signal, the
strength of the dielectric separating two electrodes,
output signal could contain signals at fa , 2fa , 3fa , 4fa ,
accompanied by a rush of electrical current across
and so forth at various amplitudes. When two or more
the "spark gap," and a flash of light indicating very
signals are simultanesouly inputted to a nonlinear
high temperature. Unlike the arc, the spark is of very
element, the output will contain signals at frequencies
short duration. Spurious emission-Any unwanted
that are the sums and differences of the fundamental
emission from a transmitter.
and harmonics of the frequencies of the input signals.
Spurious response--Any response of a receiver to
(See Intermodulation.)
frequencies  outside  the  designated  reception
Oscillator--A circuit that generates a periodic signal.
Parasitic oscillation--An undesirable oscillation occurring
Squelch--To automatically disable the audio output of a
in RF amplifiers or oscillators, usually caused by the
receiver when the incoming RF signal falls below a
change  in  impedance  of  circuit  elements,  at
preset level, or is absent.
frequencies remote from the normal operating
Susceptibility--The  degree  to  which  a  device,
frequencies.
equipment, or weapon system is open to effective
Pulse repetition frequency (PRF)--In pulsed radar or any
attack due to one or more inherent weaknesses.
other system employing periodic pulses, the number
Thermal agitation-Random movement of electrons
of pulses occurring each second, expressed in Hz.
within a conductor as a result of heat which may
Receiver response--The manner in which a receiver
cause unacceptable noise within the circuit.
Thermostatic device--A  device  which  operates  in
reacts to wanted and unwanted signals.
Rectification--Essentially, the process of converting ac
response to temperature variations. An example is a
thermostatically controlled switch.
to dc.
Time division multiplex (TDM)--A means by which two or
Refraction--The  change  of  direction,  or  apparent
more channels of information are transmitted and
bending experienced by an electromagnetic wave as
received simultaneously without mutual interference
it passes obliquely from one medium to another or
by virtue of assigning each channel specific time
through a medium having nonuniform density. An
intervals within the total unit time available.
example is the wave bending effect which occurs as
Transients--Momentary fluctuations of voltage and
an  electromagnetic  wave  passes  through  the
ionosphere. Saturation--The condition in any circuit
current in a circuit.  They may be the result of a
that exists when an increase in the input produces no
disturbance such as opening or closing a switch, a
further change in the output; for example, an electron
keying apparatus, or noise sources.
tube driven into th saturation region by too large an
Troposphere--A thermal atmospheric region, extending
input signal.
from  the  earth's  surface  to  the  ionosphere,
Shielding--The surrounding of electrical components
characterized by decreasing temperature with height,
with a conducting medium, such as aluminum, to
appreciable vertical wind motion, vapor content, and
reduce the capacitive and inductive coupling of
weather phenomena.
undesired signals from one circuit to another.
Vulnerability--The  characteristics  of  C-E  devices,
Shot effect--A troublesome phenomenon which causes
together with electromechanical devices, which
sputtering or popping noises in electronic equipment
cause them to suffer degradation of performance as
due to variations in the number of electrons emitted
a result of having been subjected to intentional
per second from the cathode of an electron tube and
electromagnetic interference in a manmade, hostile
instantaneous variation in the distribution of the
environment.
electrons among the tube electrodes.
White noise--A complex wave consisting of many radio
Single-conversion receiver--A receiver employing one
frequencies. Its energy content is relatively constant
local oscillator and one mixer stage for conversion to
per unit bandwidth across the RF spectrum.
a single intermediate frequency (IF) as opposed to a
double  conversion  receiver  which  uses  two
intermediate frequencies.
A-3


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