APPENDIX. DEFINITIONS-continued

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.

bandwidth.

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.

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