Electrical Engineering Dictionary - part 55

dispatch

the determination of the power

output of each plant in an electric power sys-
tem.

dispersed generation problem

in eco-

nomic dispatch calculations, the task of ac-
counting for the production capacity of co-
generation or dispersed sources such as pho-
tovoltaic or wind generation plants.

dispersion

(1) a characteristic of a

medium in which the propagation velocity
of a wave varies as a function of signal fre-
quency.

As a pulse propagates through an optical

fiber, its chromatic components will spread
out or “disperse” in time. This phenomenon
limits the distance between optical regenera-
tors in fiber communication systems. There
are four sources of dispersion: modal disper-
sion, material dispersion, waveguide disper-
sion, and nonlinear dispersion.

(2) the variation of the index of refraction

of a material as a function of wavelength.

dispersion compensator

a device that

compensates for the accumulated chromatic
dispersion in a fiber optic transmission sys-
tem. Three main schemes exist: fiber grating
dispersion compensators, dispersion com-
pensating fiber, and phase conjugation or
mid-span spectral inversion.

dispersion diagram

a plot of propagation

constant versus frequency.

dispersion of authority

a situation in

which the decisions, for example, decisions
concerned with the manipulated inputs to the
controlled process, are distributed between
several (control) decision units; dispersion
of authority may result either from natural,
legislative, or other reasons, or may appear
due to design of the controller; decentralized
control, in particular, is based on the disper-
sion of authority between the local decision
units. Also known as dispersion of control.

dispersion of control

See

dispersion of

authority

dispersion shifted fiber

single mode opti-

cal fibers with zero dispersion in the 1550 nm
telecommunications window. Prior to 1985,
single mode optical fibers were designed to
have zero dispersion in the 1310 nm telecom-
munications window.

dispersive medium

(1) a medium for

which the permittivity or the permeability (or
both) are frequency dependent.

(2) in optics, medium in which the in-

dex of refraction varies significantly with fre-
quency.

displaced frame difference (DFD)

the

difference between a given digital image
frame and its estimate obtained by using the
motion compensation technique. It is useful
in image (sequence) data compression and
motion estimation.

displacement current

a field quantity that

describes the completion of a circuit when a
conducting path is not present.

displacement parameter

complex pa-

rameter representing the displacement of the
amplitude and phase centers of a Gaussian
beam from the axis of an optical system.

display

a device that provides a visual

non-permanent display of system input and
or output. Common display technology in-
cludes CRT (cathode ray tube), LED (light
emitting diode), PDP (plasma display panel),
EL (electroluminescense) and LCD (liquid
crystal display).

dissipated power

the power dissipated as

a heat, which is defined by subtracting an RF
output power from a DC input power.

dissipation

the phenomenon associated

with the attenuation of a propagating wave in
a medium with material losses.

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dissipation factor (DF)

the ratio of the

effective series resistance of a capacitor to its
reactance at a specified frequency measured
in percentage. Also known as loss tangent.

dissipation power

(1) ratio of real power

(in phase power) to reaction power (shifted
90

◦

out of phase).

(2) the ratio of the imaginary to real parts

of the complex permittivity, expressed as a
dimensionless ratio.

dissipative half-space

used for analy-

sis of complex systems, it is constructed by
dividing the infinite space in two by some
convenient fashion. The resulting two half-
spaces can both be infinite, or one finite and
one infinite. The resulting half-space is then
filled with a dissipative material (other than
a perfect insulator or conductor).

dissipator

a form of air terminal which is

meant to prevent lightning strikes to a struc-
ture by reducing the surface charge on the
earths’s surface in the immediate region of
the structure.

distance

See

distance between symbol strings

a mea-

sure of the difference between two symbol
strings. The most frequently used distance
measures between two strings include Ham-
ming distance, edit or Levenshtein distance,
and maximum posterior probability distance.
See also

Hamming distance

edit distance

maximum posterior probability distance

distance measure

a function

d(x, y) de-

fined on a metric space, such that

d(x, y) ≥ 0, where = holds iff x = y,
d(x, y) = d(y, x), and
d(x, y) ≤ d(s, z) + d(z, y).

See also

similarity measure

distance profile

for convolutional codes,

the minimum Hamming weight of all se-
quences of a specific length emerging from
the zero state. A distance profile for one code
is superior to that of another if

1. all values of the distance profiles of the

two codes up to a certain depth

p (lower than

the constraint length) are equal; and

2. the superior distance profile code has

higher values of the distance profile for all
depths above the given depth

distance protection

relaying principle

based upon estimating fault location (dis-
tance) and providing a response based upon
the distance to the fault.

distance relay

See

impedance relay

distance resolution

delay resolution

mapped into the spatial domain, measured in
units of distance (meters). Each unit of time
delay corresponds to a unit of distance d trav-
eled by the radio wave through the equation

d = cτ,

where

τ is the delay and c is the speed of

light. Distance resolution gives the smallest
difference in path length resolved by a signal.
Is also a measure of the spatial resolution ca-
pability of a signal (or measurement system).

distance transform

a map of all the pixels

in a shape showing the closest distance of
each point in the shape from the background;
also, an image in which the distance maps of
all the shapes in the image are indicated. See
also

distance

distortion

(1) addition of an unwanted

component to an electronic signal.

(2) undesired change in a signal’s ampli-

tude and/or phase as the result of it passing
through an active nonlinear circuit. Numer-
ous figures of merit have been adopted to de-
scribe various aspects of signal distortion.

distortion cross modulation

nonlinear

distortion of a system or carrier signal char-

2000 by CRC Press LLC

acterized by the appearance in the output
of frequencies equal to the sum and differ-
ence of the desired carrier’s frequency and
the unwanted cross modulating carrier’s fre-
quency. Although harmonic components are
also caused to be present, they are not usually
a part of measurements of this effect.

distortion factor

See

total harmonic dis-

ortion

distortion-rate theory

See

rate-distortion

theory

distributed amplifier

(1) composed of

two main artificial transmission-line sections
consisting of series inductors and shunt ca-
pacitors, which are usually supplied by the
FET transistor.

Excellent bandwidth per-

formers are obtainable and the amplifier can
be designed as wideband low-noise ampli-
fiers and are relatively easy to simulate and
fabricate.

(2) the input and output capacitance of

the active devices can be absorbed into dis-
tributed circuits (i.e., transmission lines) to
obtain a very broad bandwidth.

distributed antenna

typically consists of

a set of discrete radiators fed by a common
cable from a single signal source.

distributed arbitration

a scheme used

for bus arbitration where multiple bus mas-
ters can access the bus. Arbitration is not
done centrally (by a bus arbiter), but instead
done in a distributed fashion. A mechanism
to detect when more than one master tries to
transmit on the bus is included. When this
happens, one (or all) stops transmitting and
will reattempt the transmission after a short
(e.g., random) time delay.

Compare with

centralized arbitration

distributed computing

an environment

in which multiple computers are networked
together and the resources from more than
one computer are available to a user. See
also

distributed computing environment

distributed computing environment (DCE)

an industry-standard, comprehensive, and

integrated set of services that supports the
development, use, and maintenance of dis-
tributed computing technologies. DCE is in-
dependent of the operating systems and net-
work types. It provides interoperability and
portability across heterogeneous platforms,
and provides security services to protect and
control access to data. DCE also provides
services that make it easy to find distributed
resources; for instance, directory service, a
DCE component, is a central repository for
information about resources in a distributed
system. Distributed time service (DTS), an-
other DCE component, provides a way to
synchronize the times on different hosts in
a distributed system. DCE gives a model for
organizing widely scattered users, services,
and data. It runs on all major computing plat-
forms and is designed to support distributed
applications in heterogeneous hardware and
software environments. Particularly impor-
tant for the World Wide Web and security of
distributed objects.

distributed computing system

a system

whose different parts can run on different
processors.

distributed control

a control technique

whereby portions of a single control process
are located in two or more places.

distributed element

a circuit element in

which dimension is not negligible relative to
the wavelength. The characteristics of a dis-
tributed element depend upon a dimension
such as a length.

distributed feedback laser (DFB)

a laser

source where the optical feedback is dis-
tributed throughout the length of the gain
medium.

Feedback then occurs through

Bragg diffraction, and the laser operates in
only one optical mode.

2000 by CRC Press LLC

distributed generation

small power

plants at or near loads and scattered through-
out the service area.

distributed memory

denotes a multipro-

cessor system where main memory is dis-
tributed across the processors, as opposed
to being equally accessible to all.

Each

processor has its own local main memory
(positioned physically “close”), and access
to other processors’ memory takes place
through passing of messages over a bus. The
term “loosely coupled” could also be used to
describe this type of multiprocessor architec-
ture to contrast it from shared-memory archi-
tectures, which could be denoted as “strongly
coupled.”

distributed memory architecture

a mul-

tiprocessor architecture in which physical
memory is distributed among the processing
nodes, as opposed to being in a central loca-
tion, equidistant from all processors.

distributed refresh

in a DRAM, carrying

out refresh operations one at a time, at regular
intervals. Requires that all rows be refreshed
in a time less that the time before which any
given row needs to be refreshed. See also

burst refresh

distributed sample scrambling

a modi-

fication of the reset scrambling technique in
which information regarding the state of the
scrambling sequence generator is embedded
into the encoded sequence in a distributed
fashion for purposes of synchronizing the de-
scrambling sequence generator.

distribution

(1) the possibility to execute

different parts of a system on different pro-
cessors.

(2) that class of electric power system

work which is concerned with the distribu-
tion of electric power within a load area such
as a residential or commercial area, or within
an industrial installation. The distribution
circuit extends from the local substation and
terminates at the customer’s meter.

distribution function

See

cumulative

distribution function

distribution management system (DMS)

a system that helps manage the status of the

distribution network, crews and their work
flow, system safety, and abnormal conditions.

distribution switchboard

a switchboard

used in the distribution system, typically
within a building.

distribution transformer

a transformer

designed for use on a power distribution sys-
tem (typically 2.4 kV to 34.5 kV) to supply
electrical power to a load at the proper uti-
lization voltage.

disturbance

a sudden change or a se-

quence of changes in the components or the
formation of a power system. Also called
fault.

disturbance decoupling of generalized 2-D
linear systems

given the second general-

ized 2-D Fornasini–Marchesini model with
disturbances

i+1,j+1

= A

i+1,j

+ A

i,j+1

+ B

i+1,j

+ B

i,j+1

+ H

i+1,j

+ H

i,j+1

= Cx

i, j ∈ Z

(the set of nonnegative integers),

find a state-feedback

= Kx

such that

the output

is not affected by the distur-

bances for

i, j ∈ Z

where

∈ R

, u

∈

, y

∈ R

, z

∈ R

are semistate vec-

tor, input vector, output vector, and distur-
bance vector, respectively, and

E, A

(k = 1, 2, ) are real matrices with E pos-

sibly singular or rectangular.

dithering

computer technique allowing

the display and printing of gray-level images
on devices having a small number of avail-
able colors (generally two, as in black-and-
white CRTs and printers). The two main ap-
proaches are matrix dithering, where a matrix

2000 by CRC Press LLC

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