Electrical Engineering Dictionary - part 182

Unibus

bus standard used by Digital

Equipment Corporation for its PDP and VAX
computers.

unidirectional bus

a group of signals that

carries information in one direction. Exam-
ple: The address bus of the microprocessor is
unidirectional; it carries address information
in one direction — from the microprocessor
to memory or peripheral.

unidirectional laser

ring laser in which

either the clockwise or counter-clockwise
circulating wave is negligible.

unidirectional resonator

ring resonator

in which the electromagnetic waves circulate
in either the clockwise or counter-clockwise
direction but not both.

unidirectional transducer (UDT)

transducer capable of launching energy from
primarily one acoustic port over a desired
bandwidth of interest.

unified cache

a cache that can hold both

instructions and data. See also

cache

unified power flow controller

a device

for both series and shunt reactive compensa-
tion, brought about by thyristor-based con-
trol.

unified transaction

a transaction, which

can be a hardware instruction or a program
segment, that performs a read-modify-write
operation, that is allowed to complete without
interruption by other processes.

uniform array

an array where the antenna

elements that make up the array are uniformly
spaced. Typical examples of this are the lin-
ear array and circular array.

uniform boundedness

a property of dy-

namical systems possessing solutions of state
equations uniformly bounded.

The solutions of

˙x = f(t, x) are uniformly

bounded if for any given

d > 0, there exists

b = b(d) < ∞ so that for each solution

(t) starting from the initial condition x(t

)

such that

kx(t

)k ≤ d,

kx(t)k ≤ b for all t ≥ 0

where

k · k is any H¨older norm on R

uniform chromaticity scale

a chart,

which allows for the quick calculation of the
X, Y, and Z tristimulus values of the CIE col-
orimetry system.

uniform circular array (UCA)

in array

processing, an array with evenly spaced sen-
sors placed on the perimeter of a circle.

uniform controllability

a dynamical

where it is controllable in any time interval
[t

, t

uniform distribution

a probability distri-

bution in which all events are equiprobable,
i.e.,

p(x) = k subject to

∞

−∞

p(s)ds = 1.

uniform length coding

a coding scheme

that assigns the same number of bits to dif-
ferent messages no matter what probabilities
the messages assume.

uniform linear array (ULA)

in array pro-

cessing, an array with evenly spaced sensors
placed on a straight line.

uniform memory access

refers to a class

of shared memory multiprocessor systems
in which accesses to all parts of the shared
memory take the same time independently
of which processor makes the access.

uniform plane wave

a special class of

electromagnetic problems where the E and
H field are locally contained in a plane, and
in each local plane, the E and H fields have a
constant value over all that plane.

uniform sampling

the sampling of a con-

tinuous signal at a constant sampling fre-
quency.

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uniform scalar quantization

a structured

scalar quantizer where the distance between
reproduction levels is a given fixed number.
Also known as uniform SQ.

uniform SQ

See

uniform scalar quantization

uniform stability

the constant equilib-

rium solution x

∈ R

˙x = f(t, x) or

(k + 1) = f(k, x) is uniformly stable, in the

sense of Lyapunov, if for any

ε > 0, t

≥ 0,

there corresponds

δ = δ(ε) independent of

such that

(t) − x

< ε

for

t ≥ t

whenever

) − x

< δ ,

where

k · k is any H¨older norm on R

uniform ultimate boundedness

the so-

lutions of

˙x = f(t, x), x ∈ R

, are uni-

formly ultimately bounded, or practically sta-
ble, with respect to the ball

= {x : kxk ≤ r} ,

if for every

d > 0, there exists T (d) > 0 such

that for each solution x

(t) starting from the

initial condition x

) such that kx(t

)k ≤ d,

(t) ∈ B

for

t ≥ T (d) ,

where

k · k is any H¨older norm on R

. See

also

uniform boundedness

uniform variate

pseudo-random variate

generated by computer that is equally likely
to be any place within a fixed interval, usually
[0, 1].

uniformly asymptotically stable state
the equilibrium state of a dynamic system
described by a first-order vector differential
equation is said to be uniformly asymptoti-
cally stable if it is both uniformly convergent
and uniformly stable. See also

uniformly

stable state

and

uniformly convergent state

uniformly convergent state

the equilib-

rium state of a dynamic system described by a
first-order vector differential equation where
there exists a

δ, independent of the t

, such

that

k x (t

) − x

k< δ ⇒ lim

t→∞

x(t) = x

See also

convergent state

uniformly excited equally spaced linear ar-
ray (UEESLA)

an antenna array in which

all the centers of the antennas are collinear
and equally spaced and each antenna has a
constant harmonic value but each antenna can
have a unique phasing.

uniformly stable equilibrium

an equilib-

rium point of a nonautonomous system where
the solutions that start “sufficiently close,”
stay “close” in time irrespective of the choice
of initial time.

uniformly stable state

the equilibrium

state of a dynamic system described by a
first-order vector differential equation where
if given

 > 0 there exists a δ = δ(), inde-

pendent of the initial

, such that

k x (t

) − x

k< δ ⇒k x(t) − x

∀t ≥ t

See also

stable state

unilateral gain

special case of the trans-

ducer power gain of a 2-port network. The
transducer power gain is the ratio of the
power delivered to the load to the power
available from the source. The unilateral gain
is the nonreciprocal case of transducer power
gain (

= 0).

unilateral transducer power gain

a spe-

cial case of transducer power gain,

T u

where

= 0.

unimplemented instruction

(1) a nu-

meric pattern in an instruction stream that
does not correspond to any defined machine
instruction.

2000 by CRC Press LLC

A typical UPS.

2) a type of trap operation executed by a

processor when an unimplemented instruc-
tion is encountered.

uninterrupted power supply (UPS)

(1)

a power supply designed to charge an en-
ergy storage medium, while providing condi-
tioned output power, during the presence of
input power and to continue providing out-
put power for a limited time when the input
to the supply is removed. These power sup-
plies are typically used in critical applications
to prevent shut-down of these systems during
power failures, power surges, or brownouts.

(2) a device that provides protection for

critical loads against power outages, over-
voltages, undervoltages, transients, and har-
monic disturbances. A typical UPS is a recti-
fier supplied battery bank for energy storage,
and a PWM inverter-filter system to convert
a DC voltage to a sinusoidal AC output. UPS
systems can be on-line, as shown in the fig-
ure, where the UPS inverter powers the load
continuously, or off-line where the load is
connected directly to the utility under normal
operation and emergency power is provided
by the UPS.

union

See

union operator

union of fuzzy sets

the fuzzy analogy to

set the set-theoretic union.

Let

A and B be two fuzzy sets in the uni-

verse of discourse

X with membership func-

tions

(x) and µ

(x), x ∈ X. The mem-

bership function of the union

A ∪ B , for all

x ∈ X, is

A∪B

(x) = max {µ

(x), µ

(x)}

See also

fuzzy set

membership function

union operator

a logical OR operator.

In a crisp (nonfuzzy) system, the union

of two sets contains elements which belongs
to either one or both of the sets. In fuzzy
logic, the union of two fuzzy sets is the fuzzy
set with a membership function which is the
larger of the two. See also

union of fuzzy

sets

unipolar neuron

neuron with output sig-

nal between 0 and

+1.

uniprocessor

See

single-instruction stream,

single data stream

uniquely decodeable

a channel code

where the correct message sequence can al-
ways be recovered uniquely from the coded
sequence as observed through the channel.
Of particular interest for multiple access
channels. See also

multiple access channel

zero-error capacity

uniqueness of solution to generalized 2-D
model

the generalized 2-D model with

variable coefficients

i+1,j+1

= A

+ A

i+1,j

+ A

i,j+1

+ B

i+1,j

+ B

i,j+1

i, j ∈ Z

(the set of nonnegative inte-

gers) has the unique solution in the rectan-
gle [0

, N

]

× [0, N

] for any sequence

for 0

≤ i ≤ N

, 0 ≤ j ≤ N

and any

boundary conditions

for 0

≤ i ≤ N

and

for 0

≤ j ≤ N

if and only if the

matrix

is nonsingular, where

∈ R

is the local semistate vector,

∈ R

the input vector

, A

(k = 0, 1, 2)

are real matrices with

possibly singular

(rectangular),

is defined by the equation

= Gu + H x

which follows from the

model for

i = 0, 1, . . . , N

− 1;

j = 0, 1, . . . , N

− 1,

, x

, . . . , x

, x

, . . . , x

2000 by CRC Press LLC

u =

, u

, . . . , u

, u

, . . . , u

, x

, . . . , x

, x

. . . , x

uniqueness theorem of electromagnetics

a theorem stating that the solutions to

Maxwell’s equations are unique, given cer-
tain boundary conditions (for the differen-
tial form of the equations) or certain initial
conditions (for the integral form of the equa-
tions). As an example, if the electric field
and magnetic field satisfy Maxwell’s equa-
tions in some volume

V of lossy material and

if the tangential fields satisfy a prescribed set
of boundary conditions on the surface

S that

defines

V , then the solution for the electric

and magnetic fields is the only one (i.e., the
unique one). Note: The uniqueness theorem
only requires one of the following three con-
ditions:

1. knowledge of the tangential electric

field on all of

2. knowledge of the tangential magnetic

field on all of

S, or

3. knowledge of the tangential electric

field on part of

S and the tangential magnetic

field on the remaining part of

unit commitment problem

the task of

minimizing the cost of production by de-
ciding which of several thermal generating
plants in a power system should be kept gen-
erating, on hot reserve or on cold reserve.

unit delay

in discrete time systems the de-

lay of a signal by a single sample interval, i.e.,
x(n − 1). Under the z-transform, z

−1

X(z).

unit impulse function

a very short pulse

such that its value is zero for

t 6= 0 and the

“force” of the pulse is 1.

In discrete time:

δ(n) = 1 for n = 0,

δ(n) = 0 for n 6= 0. See also

Kronecker

delta function

. In continuous time:

δ(t) = 0

for

t 6= 0 and

δ(s)ds = 1 for  > 0. See

also

Dirac’s delta function

unit step

See

unit step function

unit step function

a mathematical func-

tion whose amplitude is zero for all values of
time prior to a certain instant and unity for
all values of time afterwards. The unit step
signal is the integral of the unit impulse func-
tion. That is, the function

u(t) which is 1 for

all

t ≥ t

and 0 for all

t < t

unitarity in scattering

law expressing the

conservation of energy from the incident light
to that scattered and absorbed by an inhomo-
geneous medium.

unitary transform

a transform whose in-

verse is equal to the complex conjugate of its
transpose.

unity feedback

the automatic control loop

configuration shown in the figure.

Unity feedback configuration.

universal asynchronous receiver/transmit-
ter

(UART)

a standard interface often

used in small computer systems, to buffer
and translate between the parallel word for-
mat used by the CPU and the aynchronous
serial format used by slow I/O devices.

universal coding

coding procedure that

does not require knowledge of the source
statistics and yet is asymptotically optimal.
A typical example is Ziv–Lempel coding.

universal function approximation prop-
erty

uniform approximation of any real

continuous nonlinear function to arbitrary
level of accuracy in a compact set. It has been
demonstrated that some relevant model of

2000 by CRC Press LLC

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