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APPENDIX. DEFINITIONS
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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APPENDIX. DEFINITIONS-continued - TM-11-490-50044


TM 11-490-5
frequencies which are the sum and difference of the
the use of electromagnetic energy to determine, exploit,
two original frequencies.
reduce, or prevent hostile use of the electromagnetic
Image frequency--The frequency which differs from the
spectrum, and action which retains friendly use of the
electromagnetic spectrum.  There are three divisions
tuned frequency of a receiver by an amount equal to
within EW:
twice  the  inermediate  frequency  (the  first
Electronic Warfare Support Measures (ESM): That
intermediate  frequency  of  a  dual  conversion
receiver) of the receiver. The image frequency can
division of EW involving actions taken to search for,
be above or below the receive tuned frequency
intercept, locate, and immediately identify radiated
depending on whether the local oscillator is set
electromagnetic energy for the purpose of immediate
above or below the receiver tuned frequency.  An
threat recognition.
unwanted signal at the image frequency may
Electronic Countermeasures (ECM): That division of
become an interfering signal.
EW involving actions taken to prevent or reduce an
Inductive coupling--The type of coupling in which the
enemy's effective use of the electromagnetic spectrum.
mechanism is mutual inductance between the
ECM includes:
interference source and the signal system; that is,
1. Jamming. The deliberate radiation, radiation, or
the interference is induced in the signal system by a
reflection of electromagnetic energy with one object of
magnetic field produced by the interference source.
impairing the use of electronic devices, equipment, or
Intermodulation--The production of frequencies in the
systems being used by an enemy.
output of a nonlinear device equal to the sums and
2. Deception. The deliberate radiation, radiation,
differences of the frequencies present at the input of
alteration, absorption, or reflection of electromagnetic
the  device,  plus  the  sumsand  differences  of
energy in a manner intended to mislead an enemy in the
harmonics of the input frequencies. For example, if f
interpretation or use of information received by his
a and f b are the frequencies of the two input signals,
electronic systems. There are two categories of
the output signal could contain signals at frequencies
deception:
of fa , fb , 2fa , 3fa , 4fa. . . . , 2fb, 3fb, 4fb. . . . , fa fb, fa
a. Manipulative. The alteration or simulation
2fb , fa 3fb. . . . , fb 2fa , fb 3fa , fb 4fb , and so
of friendly electromagnetic radiations to accomplish
forth; in general; nfa mfb , where either n or m could
deception.
be 0, 1, 2, 3 and so forth. The signal components
b. Imitative.
Introducing radiations into
thus
generated
are
sometimes
called
enemy channels which imitate his own emissions.
"Intermodulation products" The "order" of the product
Electronic Counter-Countermeasures (ECCM): That
is n + m; for example, fa + 2fb is a third order
division of electronic warfare (EW) involving actions
Intermodulation product.
taken  to  insure  friendly  effective  use  of  the
Ionization--The process which results in the formation of
electromagnetic spectrum despite the enemy's use of
an atom, or molecularly bound group of atoms, in
EW.
which one or more electrons have been gained or
Emission  spectrum--The  power  versus  frequency
lost, resulting in an ion or ions, each having a net
distribution of an emitted signal, including the
negative or positive charge.
fundamental frequency, associated sidebands, and
Jamming--See Electronic Warfare Local oscillator-An
all spurious emissions.
oscillator, incorporated within the equipment, whose
Frequency division multiplex (FDM)--A means by which
output is mixed with another frequency waveform for
two or more channels of information are transmitted
frequency conversion.
and  received  simultaneously  without  mutual
Magnetic  field  coupling--Identical  with  Inductive
interference by virtue of assigning each channel a
Coupling.
separate  frequency  within  the  total  available
Nonlinear  Element--An  element  in  which  the
frequency band.
relationships between the output and input are not
Group loop--In an electrical system, a potentially
linear; that is, the element alters the input (voltage
detrimental
circuit
path
formed
by
the
and/or current) waveform and produces an altered or
interconnection.  of two or more points that are
distorted version of the input waveform.  Some
nonimally at ground potential.
elements or devices are designed to be non-linear
Harmonic  frequency--An  integral  multiple  of  the
(such as mixers, detectors, and clippers) while some
fundamental frequency; that is, the frequencies of 2,
are linear under some conditions and nonlinear under
3, 4, 5, ... times the fundamental frequency.
other conditions (for example, a linear amplifier
Heterodyne-The combining (beating) together in an
operating  under  designed  conditions  versus
electrical circuit of two frequencies to produce new
overdriven  conditions  resulting  in  waveform
distortion). Distortion causes harmonics
A-2


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