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Electrical Engineering Dictionary - part 56

of black and white dots is associated with
each gray-level, and error-diffusion dither-
ing, where each gray-level pixel in turn is
assigned an available color, and the error is
spread to its unprocessed neighbors (as in the
Floyd-Steinberg) method. See

halftone

divergence

the angle that the trajectory of

each particle makes with the beam axis. Ac-
celerator systems always try to reduce beam
divergence.

divergence theorem

let us consider a vol-

ume

V bounded by a surface S and an outer

normal

n. Let also F be a vector function

which, together with its partial derivatives,
is continuous at all points of

V and S. The

divergence theorem states that

divFdx =

· ndS .

The above equation may also be used to de-
fine the divergence.

diversity combining

a communication

technique that combines the signals re-
ceived through different, possibly indepen-
dent channels. The most common methods
of combining are maximum-ratio combining,
equal gain combining, selection combining,
and switched combining.

diversity frequency

a method for in-

creasing the reliability of digital communi-
cations in which multiple copies of the sig-
nal, or other types of redundant informa-
tion, are transmitted.

Frequency diversity

implies that the received signal occupies a
much wider bandwidth than the minimum
bandwidth needed to carry the information.

diversity path

a form of diversity in which

multiple copies of the signal are created via
different paths from the transmitter to re-
ceiver.

diversity selection

a form of diversity

reception in which the receiver selects the

strongest signal among the copies received.
The weaker signals are simply ignored.

divide by zero

error occurring when a

division per zero is operated. This case is
mathematically undefined. In many cases,
this problem is detected directly at level of
microprocessor that activates an exception
and leaving true a status flag. The excep-
tion can be managed for recovering the error
and avoiding the interruption of the program
execution. Also known as divide per zero.

divide per zero

See

divide by zero

divider

functional unit consisting of

circuits that implement either integer or
floating-point division.

DLL

See

delay-locked loop

DLT

directory look-aside table. See also

translation lookaside buffer

DMA

See

direct memory access

DMS

See

distribution management system

DOA

direction of arrival. See also

angle

of arrival

DOF

See

depth of focus

domain

module or area of execution that

is to be kept isolated from other domains; a
domain may have special properties that de-
fine the nature of the isolation and the limits
on communication to and from the domain.
In the context of secure system design, a do-
main may be an execution of a process that
has specific security attributes.

dominant mode

the mode of a waveguide

having the lowest cutoff frequency or of a
cavity having the lowest resonant frequency.

don’t care

a function that can be taken

either as a minterm or a maxterm at the con-
venience of the user.

2000 by CRC Press LLC

donor

an impurity in a semiconductor

that donates a free electron to the conduction
band.

door

See

lid

dopant

an impurity substance (such as

phosphorus or boron) added in very small
controlled quantities to a semiconductor base
material (such as silicon or gallium arsenide),
thereby changing the material conduction
characteristics by modifying electron and/or
hole concentration. A donor dopant is one
that gives rise to electrons and an acceptor
dopant is one that gives rise to holes.

doping

the process of introducing impu-

rity atoms into pure silicon to change its elec-
trical properties. The impurities may be ei-
ther n-type (introducing an additional con-
ducting electron) or p-type (introducing the
absence of a conducting electron, also called
a “hole”).

Doppler broadened lineshape function
spectral function that results from Doppler
shifts caused by the velocity distribution of
atoms or molecules in a gas; a Gaussian func-
tion for a Maxwellian velocity distribution.

Doppler broadening

broadening of a

spectral line due to Doppler shifts caused by
the random motion of atoms or molecules in
a vapor.

Doppler effect

See

Doppler shift

Doppler filter

a filter used to resolve tar-

gets from each other and from extraneous re-
turns from other objects (called clutter) by
filtering in the velocity or Doppler domain.
So-called because the Doppler effect causes
frequency shifts proportional to velocity vari-
ations in tracked objects.

Doppler frequency

a shift in frequency of

the returned power from a target as a result of
the target’s motion relative to the illuminating
source.

Doppler linewidth

characteristic width

of a Doppler-broadened spectral line; usually
the full width at half maximum when the line
is Gaussian.

Doppler power spectrum

a function

characterizing the spread of average received
power as a function of Doppler shift. Can be
obtained from the scattering function by in-
tegrating over the delay variable. See also

scattering function

multipath propagation

Doppler radar

radar-based technique

used in measuring the velocity of a moving
target or wind by measuring the Doppler shift
(Doppler effect).

Doppler shift

a frequency shift in a re-

ceived signal caused by time-variant trans-
mission delay, or equivalently time-variant
propagation path length.

This in turn is

caused by movement of terminals with re-
spect to each other, or by movement of re-
flecting objects. In optics, frequency shifts
imposed on laser beams such as when used
in laser radar or when diffracted by an acous-
tic wave.

The Doppler shift depends on the fre-

quency of the signal and the angle of arrival of
the signal relative to the direction of move-
ment of the receiver. For a signal consist-
ing of a range of frequencies, each frequency
component will experience a different shift.
Hence the received signal will have a dif-
ferent bandwidth than the transmitted signal
(Doppler dispersion).

For a tone (continuous-wave) signal of fre-

quency

f , the Doppler shift f

observed on

a single propagation path of changing length
is given by

f v

where

v is the rate (in m/s) of path length

change, and

c is the speed of light.

Also known as Doppler effect.

Doppler spread

the increase of band-

width of a signal due to doppler shifting of

2000 by CRC Press LLC

multipath energy arriving at a mobile receiver
from differing directions.

Doppler-free spectroscopy

a spectro-

scopic technique in which two or more light
fields are used to produce a spectral line that
is not Doppler shifted or Doppler broadened.
For example, in the case of lambda systems
or vee systems, this requires that the elec-
tromagnetic fields co-propagate, whereas for
cascade systems, the fields must counter-
propagate.

DOS

See

disk operating system

dose to clear

the amount of exposure

energy required to just clear the resist in a
large clear area for a given process.

dosimeter

an instrument used for mea-

suring or evaluating the absorbed dose of ra-
diation. It may depend on the measurement
of ionization for its operation or may simply
involve the darkening of a piece of photo-
graphic film ("film badge").

dot pitch

the center-to-center distance be-

tween adjacent green phosphor dots of the
red, green, blue triad in a color display.

dot-matrix printer

a printer that pro-

duces readable characters by imprinting a
large number of very small dots.

dots per inch (DPI)

a measure of the den-

sity of line-printer plots in dots per inch.

double bridge

a Wheatstone bridge mod-

ification designed to increase the precision
of measurements for low-value resistors. To
avoid the error due to resistance of the con-
nection (called yoke) between the unknown
resistor and the standard resistor, the ends of
the unknown resistor and the standard resis-
tor are connected to the balance detector (usu-
ally a galvanometer for this type of bridge)
via two small resistors, the ratio of which is
the same as the ratio of resistors in the ratio
arms. Then the yoke resistance is eliminated

from the balance condition and the unknown
resistor can be found using the same formula
as in an ordinary Wheatstone bridge.

double buffering

(1) generally, any

buffering scheme where two identical mem-
ory images are used. One memory image
is used to hold received data, while data is
simultaneously read from the other memory
image.

(2) in terminal-to-computer communica-

tion scheme where a number of remote ter-
minals are connected to a single computer
through a “multiplexer.” This unit connects
n low-speed bit-serial transmission lines onto
a single high-speed bit-serial line (running

times faster) using STDM (synchronous time
division multiplex) of the connection. In the
multiplexer, each low-speed line is connected
to a “one-character buffer,” converting the re-
ceived low-speed bitstream to a high-speed
bitstream using two shift-registers (buffers).
The first one receives the low-speed line char-
acter bits (8 bits), clocked by the low-speed
“receive clock.” When a complete charac-
ter has been received, it is moved to the
second shift-register, where it is stored un-
til it can be shifted out on the high-speed
line on the appropriate time slot.

Mean-

while, the next character is assembled in the
first shift-register. For full-duplex operation
(simultaneous two-way communication), a
similar structure is needed for the opposite
(computer-to-terminal) connection.

double conversion

the process where an

incoming RF signal is mixed with a local os-
cillator (LO) signal to produce the first inter-
mediate frequency (IF). This IF is then mixed
with a second fixed LO to produce a second
IF signal.

double conversion receiver

a receiver

that uses two heterodyne operations be-
fore detection generating two intermedi-
ate frequencies, first intermediate frequency
(IF) and second IF, respectively. See also

intermediate frequency

2000 by CRC Press LLC

double heterostructure dye laser

laser in

which the amplification takes place in a dye;
a broadly tunable laser.

double lambda system

a quantum me-

chanical system composed of two interact-
ing lambda systems that share common lower
states. A double lambda system is also a dou-
ble vee system. Double lambda systems can
be used to construct resonant closed loop in-
teractions in which some linear combination
of the phases of the four fields is constant in
time.

double line to ground fault

See

double

phase ground fault

double phase ground fault

a fault with

two transmission lines being connected to the
ground.

double revolving-field theory

See

counter-

rotating field theory

double sideband modulation (DSB)

modulation scheme resulting in a spectrum
consisting of the carrier frequency, one signal
that is the sum of the carrier and the modu-
lating signal, and one signal that is the differ-
ence between the carrier and the modulating
signal.

double tuned

a circuit, amplifier, or other

device having a response that is the same as
two single-tuned circuits.

double word

data block that contains

twice the number of bits as the machine word
size in a microprocessor.

double-cage rotor

See

dual-cage rotor

double-frequency recording

See

magnetic

recording code

double-line contingency

a malfunction

of a power system which involves the simul-
taneous failure of two transmission lines.

double-sided assembly

a packaging and

interconnecting structure with components
mounted on both the primary and secondary
sides.

double-sided disk

a disk in which both

sides of a platter are covered with magnetic
material and used for storing information.

doublet

a system of two quadrupole mag-

nets in close proximity and with opposite
polarity used to simultaneously constrain
the beam size in two dimensions at some
point downstream.

Doublet (Quadrupole)

is a beam optical system consisting of two
quadrupoles of opposite sign, which provides
net particle focusing in all planes.

doubly-fed induction motor

an induc-

tion motor with a wound-rotor. The rotor
and stator windings are connected to sepa-
rate sources of electric energy. The machine
can be used as a generator to provide fre-
quency conversion or precise speed control
in the motor-mode.

down-sampling

See

decimation

downconductor

the cable which connects

an air terminal or lightning rod to ground. See

air terminal

download

to bring data from a remote

source to local storage.

downstream

in power distribution work,