Electrical Engineering Dictionary - part 66

vector, with

m ≤ n (n denotes number of de-

grees of freedom). Using minimal represen-
tation of the orientation matrix, direct kine-
matics equation can be written in the follow-

ing form:

x =

= k(q) where φ is a set

of vectors for minimal representation and

denotes three position coordinates.

external stability

stability concepts re-

lated to the input–output behavior of the sys-
tem.

externally vented machine

classification

describing an electrical machine constructed
with an open frame in which ventilation air
is forced through the machine by blower(s)
mounted outside the machine enclosure.

extinction angle

time in electrical degrees

from the instant the current in a valve reaches
zero (end of conduction) to the time the valve
voltage changes sign and becomes positive.

extinction cross section

the sum of the

scattering and the absorption cross sections.

extrapolation

one of several methods to

estimate the values of a sequence

r(k) for

lags

|k| > p from the given values of r(k)

for

|k| ≤ p.

extrinsic

associated with the outside or

exterior. In devices and device modeling,
extrinsic refers to that part of the device or
model associated with the passive structures
that provide interconnects and contacts to
other components, but are still considered a
part of the device.

extrinsic fiber optic sensor

a fiber optic

sensor where the fiber delivers light to and
from a sensing element external to the fiber.
Chemical sensors are an example where the
sensing element exhibits a change in opti-
cal property such as absorption, fluorescence
or phosphorescence upon detection of the
species to be measured.

2000 by CRC Press LLC

common notation for higher band edge

frequency in hertz.

common notation for lower band edge

frequency in hertz.

Fabry–Perot etalon

interferometer con-

sisting of two highly reflecting flat or spher-
ical mirrors; only resonant frequencies are
transmitted.

Fabry–Perot laser

a laser source where

the gain medium is placed within a Fabry–
Perot cavity, which provides feedback into
the laser medium. Several simultaneous las-
ing modes are supported in such cavities.

Fabry–Perot resonator

any open (as op-

posed to a cavity) resonator, usually the as-
sembly of two parallel plates resembling the
optical Fabry–Perot interferometer. See also

standing-wave resonator

Fabry–Perot structure

Fabry–Perot

etalon or interferometer that has an optically
nonlinear medium in its cavity.

facsimile

the process of making an ex-

act copy of a document through scanning of
the subject copy, electronic transmission of
the resultant signals modulated by the subject
copy, and making a record copy at a remote
location.

facsimile encoding

a bilevel coding meth-

od applied to the encoding and transmission
of documents. Facsimile systems may in-
clude support for grayscale image coding too,
which is described under still image coding.

FACTS

See

flexible AC transmission

system

fading channel

signal fluctuation caused

by multiple propagation paths over a radio
channel is called fading. May be categorized
as fast fading or slow fading, depending on
the rate of fading with respect to the informa-
tion symbol rate. May also be categorized as
frequency selective or frequency nonselec-
tive, depending on the transfer function of
the radio channel. See also

fading margin

fading Rayleigh

fading Rician

fading margin

the margin by which the

average signal-to-noise ratio (SNR) in a ra-
dio communications link is over-designed,
in order to compensate for variations in the
short-term SNR that occur due to fading of
the signal. The signal fading is typically due
to multipath propagation, which arises due
to the presence of multiple reflectors in the
radio link. Utilizing diversity schemes and
transmitter power control are typically em-
ployed in order to reduce the fading margin
required in a radio system.

fading rate

the rate at which the received

signal level crosses the median signal level
in a downward direction (i.e., with a negative
slope). It is usually expressed in fades per
second or fades per minute, depending on the
actual rate of fading.

fading Rayleigh

in mobile wireless com-

munications, wide fluctuations in received
signal strength (e.g., swings of 30 – 40 dB)
and phase caused by scattering of the trans-
mitted signal off of surrounding objects. The
scattering induces a Gaussian distribution on
the in-phase and quadrature signal compo-
nents, so that the received signal envelope has
a Rayleigh distribution. See also

Rayleigh

distribution

fading Rician

similar to Rayleigh fading,

the only difference being that a direct line-
of-sight component is present in the received
signal in addition to the scattered signal. The
received signal envelope has a Rician distri-
bution. See also

Rice distribution

2000 by CRC Press LLC

fail safe

pertaining to a circuit, for a set of

faults, if and only if for any fault in this set and
for every valid input code either the output is
correct or assumes some defined safe state.

fail-stop processor

a processor that does

not perform incorrect computation in the
event of a fault. Self-checking logic is of-
ten used to approximate fail-stop processing.

failure

manifestation of an error at system

level. It relates to execution of wrong actions,
nonexecution of correct actions, performance
degradation, etc.

failure mechanism

a physical or chem-

ical defect that results in partial degradation
or complete failure of a product.

fairness

(1) the concept of providing

equivalent or near-equivalent access to a
shared resource for all requestors.

(2) a fair policy requires that tasks,

threads, or processes are allowed access to
a resource for which they compete.

(3) the degree to which a scheduling or al-

location policy is equitable and nondiscrimi-
natory in granting requests among processes
competing for access to limited system re-
sources such as memory, CPU, or network
bandwidth.

fall time

(1) in digital electronics, the pe-

riod of duration of the transition of a digital
signal from a stable high-voltage level to a
stable low-voltage level.

(2) in optics, the time interval for the

falling edge of an optical pulse to transition
from 90% to 10% of the pulse amplitude. Al-
ternatively, values of 80% and 20% may be
used.

falling edge

(1) the region of a waveform

when the wave goes from its high state to its
low state.

(2) the high-to-low transition in voltage of

a time-varying digital signal.

false color

the replacement of a color in

a colored image by a different color, usually
not present in the original image. Used to
highlight regions or distinguish pixels of sim-
ilar colors. See also

pseudo color

false sharing

the situation when more

than one processor accesses different parts of
the same line in their caches but not the same
data words within the line. This can cause
significant performance degradation because
cache coherence protocols consider the line
as the smallest unit to be transferred or inval-
idated.

fan beam reconstruction

reconstruction

of a computed tomography image from pro-
jections created by a point source that emits
a fan- or wedge-shaped beam of radiation.
Fan beam reconstruction enables data to be
gathered much more quickly than by using
a linear beam to produce parallel projec-
tions. See also

fan-in

(1) the number of inputs to a mod-

ule. This is usually used in connection with
logic gates.

(2) multiple inputs of a channel. If the

channel is a bus, only an input is allowed
at a time. When

N light beams are com-

bined with an optical element, only 1

/N of

the power of each beam finds its way into the
combined beam. Large fan-in is quite im-
practical for VLSI, because it uses a unique
discrete channel for each input and current
flows are limited by transistor capacitance.

fan-out

(1) the limit to the number of load-

ing inputs that can be reliably driven by a
driving device’s output.

(2) multiple outputs of a bus. A signal

is distributed into multiple channels. With a
fan-out of

N, each channel receives only 1/N

of the light power. Large fan-out is quite im-
practical for VLSI, because it uses a unique
discrete channel for each output and current
flows are limited by transistor capacitance.

2000 by CRC Press LLC

Fano algorithm

a sequential decoding al-

gorithm for decoding of trellis codes.

Fano mode

a bound nonradiative surface

mode that propagates along an interface and
decays in a nonoscillatory manner in a direc-
tion perpendicular to the propagation. Such
a mode occurs when one of the media is a
plasma medium and has a negative dielectric
function. A metal at optical frequency is an
example of such a medium.

Fano’s inequality

information theoretic

inequality bounding the probability of incor-
rectly guessing the value of one random vari-
able based on observation of another. If

is the probability of incorrectly guessing a
random variable

X ∈ X , based upon obser-

vation of the random variable

Y , then

H (P

) + P

log

(|X − 1) ≥ H (X|Y )

Named after its discoverer, R. M. Fano
(1952). Used in proving the weak converse to
the channel coding theorem (Shannon’s sec-
ond theorem).

Fano’s limit

theoretical limit relating the

achievable gain and bandwidth of a given
passive lossless matching network when ter-
minated in an arbitrary load impedance.

far field

(1) that region of space in which

the electric field and magnetic field compo-
nents of an electromagnetic wave are related
by the impedance of free space. The far field
is generally considered to begin no closer
to the NIER source than a distance of sev-
eral wavelengths or several times the antenna
aperture.

(2) that region of the field of a certain

source where the angular field distribution
depends in a known way from the distance
of the source. Generally, in free space, if we
consider as a source an antenna with max-
imum overall dimension

D, assumed large

compared to the wavelength, the far-field re-
gion is commonly taken to exist at distances
greater than 2

/λ from the antenna, λ being

the wavelength. Also called the Fraunhofer
region.

far pointer

a pointer to a far segment. In

× 86 architecture, a far pointer specifies

the segment address and the offset.

far-field pattern

graph or chart repre-

senting the absolute or normalized antenna
gain as a function of angle (typically azimuth
or elevation) and used to describe the direc-
tional properties of an antenna in the far field
(Fraunhofer region).

far-infrared (FIR)

spectral region often

considered to range from about 10 to 100 mi-
crometers.

farad

the basic unit of measure in capac-

itors. A capacitor charged to 1 volt with a
charge of 1 coulomb (1 ampere flowing for
1 second) has a capacitance of 1 farad.

Faraday effect

the rotation of the plane of

polarization of a high-frequency signal (mi-
crowave RF, optical field) in the presence of
a magnetic field.

Faraday rotation

(1) rotation in the di-

rection of polarization experienced by a wave
traveling through an anisotropic medium.
Important examples of media in which the
phenomenon occurs include the earth’s iono-
sphere and ferrites biased by a static magnetic
field.

(2) depolarization caused in a plasma

(e.g., the ionosphere) resulting from interac-
tion between the ions of the plasma and the
magnetic field of the wave.

(3) rotation in the polarization vector

experienced by a wave after it propagates
through a gyromagnetic medium.

Faraday rotator

a magneto-optical de-

vice that changes the orientation plane of
polarized light when it passes parallel to a
magnetic field through a substance with pro-
nounced absorption lines.

2000 by CRC Press LLC

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