Electrical Engineering Dictionary - part 5

than the thermal energy

kT . Contrast with

45 Mbs DPCM for NTSC color video
a codec wherein a subjectively pleasing pic-
ture is required at the receiver. This does
not require transparent coding quality typical
of TV signals. The output bit-rate for video
matches the DS3 44

.736 Megabits per second

rate. The coding is done by PCM coding the
NTSC composite video signal at three times
the color subcarrier frequency using 8 bit per
pixel. Prediction of current pixel is obtained
by averaging the pixel three after current and
681 pixels before next to maintain the sub-
carrier phase. A leak factor is chosen before
computing prediction error to main the quali-

ty of the image. For example a leak factor of

31
32

the prediction decay is maintained at the

center of the dynamic range.

−

= 128 +

−

− 128

Finally, a clipper at the coder and decoder

is employed to prevent quantization errors.

90% withstand voltage

a measure of

the practical lightning or switching-surge im-
pulse withstand capability of a piece of power
equipment. This voltage withstand level is
two standard deviations above the BIL of the
equipment.

2000 by CRC Press LLC

a posteriori probability

See

posterior

statistics

a priori probability

See

prior statistics

A-mode display

returned ultrasound

echoes displayed as amplitude versus depth
into the body.

A-site

in a ferroelectric material with the

chemical formula ABO

, the crystalline lo-

cation of the A atom.

A/D

See

analog-to-digital converter

AAL

See

ATM adaptation layer

ABC

See

absorbing boundary condition

ABCD

propagation of an optical ray

through a system can be described by a sim-
ple 2

×2 matrix. In ray optics, the character-

istic of a system is given by the correspond-
ing ray matrix relating the ray’s position from
the axis and slope at the input to those at the
output.

ABCD formalism

analytic method using

two-by-two ABCD matrices for propagating
Gaussian beams and light rays in a wide va-
riety of optical systems.

ABCD law

analytic formula for trans-

forming a Gaussian beam parameter from
one reference plane to another in paraxial op-
tics, sometimes called the Kogelnik transfor-
mation. ABCD refers to the ABCD matrix.

ABCD matrix

the matrix containing

ABCD parameters. See

ABCD parameters

ABCD parameters

a convenient mathe-

matical form that can be used to characterize

two-port networks. Sometimes referred to
as chain parameters. ABCD parameters are
widely used to model cascaded connections
of two-port microwave networks, in which
case the ABCD matrix is defined for each
two-port network.

ABCD parameters can

also be used in analytic formalisms for prop-
agating Gaussian beams and light rays. Ray
matrices and beam matrices are similar but
are often regarded as distinct.

ABC parameters have a particularly use-

ful property in circuit analysis where the
composite ABCD parameters of two cas-
caded networks are the matrix products of
the ABCD parameters of the two individual
circuits. ABCD parameters are defined as

A B

C D

where

and

are the voltages on ports one

and two, and

and

are the branch currents

into ports one and two.

aberration

an imperfection of an optical

system that leads to a blurred or a distorted
image.

abnormal event

any external or program-

generated event that makes further normal
program execution impossible or undesir-
able, resulting in a system interrupt. Exam-
ples of abnormal events include system de-
tection of power failure; attempt to divide by
0; attempt to execute privileged instruction
without privileged status; memory parity er-
ror.

abort

(1) in computer systems, to termi-

nate the attempt to complete the transaction,
usually because there is a deadlock or be-
cause completing the transaction would re-
sult in a system state that is not compati-
ble with “correct” behavior, as defined by a
consistency model, such as sequential con-
sistency.

(2) in an accelerator, terminating the ac-

celeration process prematurely, either by in-
hibiting the injection mechanism or by re-
moving circulating beam to some sort of

2000 by CRC Press LLC

dump. This is generally done to prevent in-
jury to some personnel or damage to acceler-
ator components.

ABR

See

available bit rate

absolute address

an address within an

instruction that directly indicates a location in
the program’s address space. Compare with

relative addressing

absolute addressing

an addressing mode

where the address of the instruction operand
in memory is a part of the instruction so that
no calculation of an effective address by the
CPU is necessary.

For example, in the Motorola M68000 ar-

chitecture instruction ADD 5000,D1, a 16-bit
word operand, stored in memory at the word
address 5000, is added to the lower word in
register D1. The address “5000” is an exam-
ple of using the absolute addressing mode.
See also

addressing mode

absolute encoder

an optical device

mounted to the shaft of a motor consisting
of a disc with a pattern and light sources and
detectors. The combination of light detectors
receiving light depends on the position of the
rotor and the pattern employed (typically the
Gray code). Thus, absolute position infor-
mation is obtained. The higher the resolution
required, the larger the number of detectors
needed. See also

encoder

absolute moment

The

pth order absolute

moment

of a random variable X is the

expectation of the absolute value of X raised
to the

pth power:

= E[|X|]

See also

. See also

absolute pressure

units to measure gas

pressure in a vacuum chamber with zero be-
ing a perfect vacuum. Normally referred to
as psia (pounds per square inch absolute).

absolute sensitivity

denoted

(y, x), is

simply the partial derivative of

y with respect

x, i.e.,

(y, x) = ∂y/∂x, and is used to

establish the relationships between absolute
changes. See

sensitivity

sensitivity measure

relative sensitivity

semi-relative sensitivity

absolute stability

occurs when the net-

work function

H (s) has only left half-plane

poles.

absorber

generic term used to describe

material used to absorb electromagnetic en-
ergy.

Generally made of polyurethane

foam and impregnated with carbon (and fire-
retardant salts), it is most frequently used to
line the walls, floors and ceilings of anechoic
chambers to reduce or eliminate reflections
from these surfaces.

absorbing boundary condition (ABC)

fictitious boundary introduced in differential
equation methods to truncate the computa-
tional space at a finite distance without, in
principle, creating any reflections.

absorption

(1) process that dissipates en-

ergy and causes a decrease in the amplitude
and intensity of a propagating wave between
an input and output reference plane.

(2) reduction in the number of photons of a

specific wavelength or energy incident upon
a material. Energy transferred to the material
may result in a change in the electronic struc-
ture, or in the relative movement of atoms in
the material (vibration or rotation).

(3) process by which atoms or molecules

stick to a surface. If a bond is formed, it is
termed chemisorption, while the normal case
is physisorption. The absorption process pro-
ceeds due to, and is supported by, the fact that
this is a lower energy state.

absorption coefficient

(1) in a passive de-

vice, the negative ratio of the power absorbed
(

absorbed

= p

−p

out

) ratioed to the power in

(

= p

incident

− p

reflected

) per unit length (l),

usually expressed in units of 1/wavelength or
1/meter.

2000 by CRC Press LLC

(2) factor describing the fractional atten-

uation of light with distance traversed in a
medium, generally expressed as an exponen-
tial factor, such as

k in the function e

−kx

with units of (length)-1. Also called attenu-
ation coefficient.

absorption cross section

energy ab-

sorbed by the scattering medium, normal-
ized to the wavenumber. It has dimensions
of area.

absorption edge

the optical wavelength

or photon energy corresponding to the sep-
aration of valence and conduction bands in
solids; at shorter wavelengths, or higher pho-
ton energies than the absorption edge, the ab-
sorption increases strongly.

absorption grating

(1) a diffraction

grating where alternate grating periods are
opaque.

(2) an optical grating characterized by

spatially periodic variation in the absorption
of light. Absorption gratings are generally
less efficient than phase gratings.

absorption optical fiber

the amount of

optical power in an optical fiber captured
by defect and impurity centers in the energy
bandgap of the fiber material and lost in the
form of longwave infrared radiation.

See

alternating current

AC bridge

one of a wide group of

bridge circuits used for measurements of re-
sistances, inductances, and capacitances, and
to provide AC signal in the bridge transducers
including resistors, inductors, and capacitors.

The Wheatstone bridge can be used with

a sinusoidal power supply, and with an AC
detector (headphones, oscilloscope), one can
use essentially the same procedure for mea-
surement of resistors as in DC applications.
Only a small number of other AC bridges are
used in modern electric and electronic equip-
ment. A strong selection factor was the fact
that in a standard capacitor the electrical pa-

rameter are closest to the parameters of an
ideal capacitor. Hence, not only a capaci-
tance is measured in terms of capacitance (in
resistive ratio arms bridges), but the induc-
tance as well is measured in terms of capac-
itance (Hay and Owen bridges).

The AC bridges with ratio arms that are

tightly coupled inductances allow measure-
ment of a very small difference between cur-
rents in these inductances, and this fact is
used in very sensitive capacitance transduc-
ers.

AC circuit

electrical network in which the

voltage polarity and directions of current flow
change continuously, and often periodically.
Thus, such networks contain alternating cur-
rents as opposed to direct currents, thereby
giving rise to the term.

AC coupling

a method of connecting two

circuits that allows displacement current to
flow while preventing conductive currents.
Reactive impedance devices (e.g., capacitors
and inductive transformers) are used to pro-
vide continuity of alternating current flow
between two circuits while simultaneously
blocking the flow of direct current.

AC motor

an electromechanical sys-

tem that converts alternating current electri-
cal power into mechanical power.

AC plasma display

a display that em-

ploys an internal capacitive dielectric layer
to limit the gas discharge current.

AC steady-state power

the average

power delivered by a sinusoidal source to a
network, expressed as

P =| V | · | I | cos(θ)

where

√

· | V | and

√

· | I | are the peak

values, respectively, of the AC steady-state
voltage and current at the terminals.

θ rep-

resents the phase angle by which the voltage
leads the current.

2000 by CRC Press LLC

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