Electrical Engineering Dictionary - part 158

Generally, efficient results might be found

only for bounding sets parameterized by a
little number of parameters. Except for the
ellipsoids, such property is endowed only by
polyhedral sets bounding uncertain variables.
In this case, efficient results could be reached
by the use of linear programming algorithms.

settling time

(1) the time required for a

signal to change from one value to another.

(2) refers to the time that it takes stable

transients of a dynamic system to decay to a
negligible amplitude. This can be quantified
to the time it takes an exponential transient
mode to decay to a band that is

±37%, ±5%,

±2% of its initial value. See also

time

constant

setup

a video term relating to the specified

base of an active picture signal. In NTSC, the
active picture signal is placed 7.5 IRE units
above blanking (0 IRE). Setup is the separa-
tion in level between the video blanking and
reference black levels.

setup time

See

hold time

SF6

See

sulfur hexaflouride

SF6 circuit breaker

a power circuit

breaker where sulfur hexaflouride (SF

) gas

is used as an insulating and arc clearing agent.

SGNN

See

self-generating neural network

SGNT

See

self-generating neural tree

SGVQ

See

shape-gain vector quantization

SHA

See

sample-and-hold amplifier

shaded pole

a magnetic pole-face in

which part of the pole is encircled by a
shorted conductor (usually copper). The flux
through the encircled portion will be out of
phase with the flux through the other portion.
Shaded pole motors use the phase shift to pro-

duce a quasi-rotating magnetic field which
develops a weak torque, suitable primarily
for small fans. In AC relays, shaded poles are
used to prevent chatter (the attempted open-
ing and subsequent closing each time the flux
passes through zero).

shaded-pole motor

a single-phase induc-

tion type motor that uses shaded poles on the
stator to create a weak quasi-rotating mag-
netic field. Shaded-pole motors are only built
in small fractional horsepower sizes and pro-
duce a very low starting torque that is suit-
able only for fan-type loads. See also

shaded

pole

shadow casting logic gate

an optical logic

gate originally using shadow casting tech-
nique. The principle of shadow casting logic
gate can be explained as follows. First, NOT
A and NOT B are generated from inputs A
and B. Second, four products of AB, A (NOT
B), (NOT A) B, and (NOT A) (NOT B) are
produced by passing a light beam through
two transparencies that could be spatial light
modulators representing A and B, A and NOT
B, NOT A and B, and NOT A and NOT B.
Third, the four products are added optically.
The sixteen combination of four products are
the sixteen Boolean logic operations.

shadowing

(1) excess propagation loss re-

sulting from the blocking effect of obstacles
such as buildings, trees etc.

(2) the statistical variation of propagation

loss in a mobile system between locations the
same distance from a base station, usually
described by a lognormal distribution.

shaft torque

the component of the mo-

tor generated electromagnetic torque avail-
able at the shaft of the motor after overcom-
ing the operational losses of the motor dur-
ing the electromechanical energy conversion
process.

Shannon, Claude (1916–1989)

consid-

ered to be the founding father of modern elec-
tronic communications theory. His contribu-

2000 by CRC Press LLC

tions include the application of Boolean al-
gebra to analyze and optimize switching cir-
cuits and, in his classic paper “The Mathe-
matical Theory of Communication,” estab-
lished the field of information theory by de-
veloping the relationship between the infor-
mation content of a message and its repre-
sentation for transmission through electronic
media.

Shannon information

the information

content of an event

x with a probability of

occurrence of

p(x) defined as

I (x) = − log p(x).

The unit of

I (x) depends on the base of the

logarithm - “bits” for base 2, “nats” for the
natural logarithm. See also

entropy

Shannon’s law

fundamental relationship

of information theory, which states that the
lower bound on the average code-word length
for coding a discrete memoryless source
is given by the source entropy.

See also

entropy

Shannon’s sampling theorem

this math-

ematical theorem states that when an analog
signal is sampled, there is no loss of infor-
mation and the analog signal can be recon-
structed by low-pass filtering, if and only if
the largest (absolute value) frequency present
in that signal does not exceed the Nyquist
frequency, this being half the sampling fre-
quency.

Shannon’s source coding theorem

a ma-

jor result of Claude Shannon’s information
theory. For lossy source coding, it gives a
bound to the optimal source coding perfor-
mance at a particular rate (“rate” corresponds
to “resolution”). The theorem also says that
the bound can be met by using vector quan-
tization of (infinitely) high dimension. For
lossless source coding, the theorem states
that data can be represented (without loss of
information) at a rate arbitrarily close to (but
not lower than) the entropy of the data. See
also

rate-distortion theory

shape analysis

the analysis of shapes of

objects in binary images, with a view to ob-
ject or feature recognition. Typically, shape
analysis is carried out by measurement of
skeleton topology or by boundary tracking
procedures including analysis of centroidal
profiles.

shape from

the recovery of the 3-D shape

of an object based on some feature (e.g.,
shading) of its (2-D) image.

shape measure

a measure such as cir-

cularity measure (compactness measure), as-
pect ratio, or number of skeleton nodes, that
may be used to help characterize shapes as a
preliminary to, or as a quick procedure for,
object recognition.

shape-gain vector quantization (SGVQ)

a method for vector quantization where the

magnitude (the gain) and the direction (the
shape) of the source vector are coded sepa-
rately. Such an approach gives advantages
for sources where the magnitude of the input
vector varies in time.

shape-memory effect

mechanism by

which a plastically deformed object in the
low-temperature martensitic condition re-
gains its original shape when the external
stress is removed and heat is applied.

shape-memory smart materials

include

three categories, namely shape-memory al-
loys (SMA), shape-memory hybrid compos-
ites (SMHC), and shape-memory polymers
(SMP).

shaping

a traffic policing process that

controls the traffic generation process at the
source to force a required traffic profile.

shared memory

characteristic of a mul-

tiprocessor system: all processors in the sys-
tem share the access to main memory. In a
physically shared–memory system, any pro-
cessor has access to any memory location
through the interconnection network.

2000 by CRC Press LLC

shared memory architecture

a computer

system having more than one processor in
which each processor can access a common
main memory.

sharpening

the enhancement of detail in

an image.

Processes that sharpen an im-

age also tend to strengthen the noise in it.
See also

SHDTV

See

super high definition

television

shed

a circular roof-like feature of an

electrical insulator which presents a long
electrical leakage path while permitting the
drainage of rainwater.

sheet resistance

lateral resistance of an

area of thin film in the shape of a square.

shell-type transformer

a power trans-

former in which the magnetic circuit sur-
rounds and normally encloses a greater por-
tion of the electrical winding.

shield wire

(1) a ground wire placed above

an electric transmission line to shield the con-
ductors from lightning strokes.

(2) a ground wire buried directly above

a buried communications cable for lightning
protection.

shielding effectiveness

a measure of the

reduction or attenuation in the electromag-
netic field strength at a point in space caused
by the insertion of a shield between the source
and that point. Typically expressed in deci-
bels: SE

= 20 log

) dB, where E

field strength measured without shield and
E

= field strength measured after shielding

is applied.

shift instruction

a program instruction

in which data in a register or memory loca-
tion is shifted one or more bits to the left or
right. Data shifted off the end of the regis-

ter or memory location is either shifted into
a flag register, used to set a condition flag,
or dropped, depending on implementation of
the instruction. See also

rotate

instruction.

shift invariance

a characteristic of a prop-

erty that in some domain it is invariant to dis-
placements within that domain. Particularly
important sorts of shift invariance are space
invariance and time invariance. The impulse
response of a system is independent of the
spatial (or temporal) location of the impulse.

shift register

a register whose contents

can be shifted to the left or right.

sifting property

the ability of the impulse

function to select a particular value of another
function. In discrete time

x(n) =

∞

i=−∞

x(n)δ(n − i).

In continuous time

x(t) −

∞

−∞

x(s)δ(t − s)ds.

Shockley, William

(1910–1989) Born:

London, England

Shockley is best known as one of the de-

velopers of the transistor. In 1956 Shockley,
John Bardeen, and Walter Brattain received
the Nobel Prize for their work. Schockley
led the group at Bell Labs responsible for the
semiconductor research that led to the de-
velopment of the “point-contact transistor.”
In later life, Shockley became known for his
public pronouncements on various political
and genetic issues.

Shoeffler sensitivity

when the multipa-

rameter sensitivity row vector is known, then
the measure of this vector determined as

(F ) =

i=1

F (jω,x)

is called Shoeffler multivariable sensitivity.

2000 by CRC Press LLC

short circuit

a condition on the power

system where energized conductors come in
contact (or generate an arc by coming in close
proximity) with each other or with ground, al-
lowing (typically large) fault currents to flow.

short circuit admittance

the admittance

into an N-port device when the remaining
ports are terminated in short circuits. For port
1 of a 2-port device, it is the input admittance
into port 1 when port 2 is shorted.

short circuit gain–bandwidth product

measure of the frequency response capabil-
ity of an electronic circuit. When applied to
bipolar circuits, it is nominally the signal fre-
quency at which the magnitude of the current
gain degrades to one.

short code

in a spread-spectrum sys-

tem, periodic spreading code (spreading se-
quence) with a period equal to a bit duration.
See also

long code

short duration

See

voltage variation

short-circuit protection

the beneficial ef-

fect provided by an overcurrent device when
it acts to interrupt short-circuit current.

short-circuit test

a transformer test con-

ducted by placing a few percent of rated volt-
age on the voltage side while the low voltage
winding is shorted. By measuring the volt-
age, current, and input power, it is possible to
calculate the equivalent winding impedance
for the transformer equivalent circuit.

shortened code

a code constructed from

another code by deleting one or more mes-
sage symbols in each message.

Thus an

(n, k) original code becomes an (n−1, k−1)
code after the deleting of one message sym-
bol.

shot noise

noise voltage developed at in-

ternal device boundaries, such as solid-state
junctions, where charges cross from one type

of material into another.

Also known as

Schottky noise.

shotgun

a specialized hot stick that is used

to install a hot tap.

Shubnikov–de Haas oscillation

quantum

oscillations in resistance as a function of ap-
plied magnetic field.

shunt

(1) a device having appreciable

impedance connected in parallel across other
devices or apparatus and diverting some of
the current from it. Appreciable voltage ex-
ists across the shunted device or apparatus,
and an appreciable current may exist in it.

(2) an inductive element connected across

a power line or bus. Those connected to buses
are known as bus-connected reactors, while
those connected across a power line are called
line-connected reactors.

shunt capacitor

a capacitor or group of

capacitors which are placed across an electric
power line to provide a voltage increase or to
improve the power factor of the circuit. A
switchable shunt may be disconnected from
the circuit when conditions warrant, while a
fixed shunt is permanently connected to the
power line.

shunt DC machine

a DC machine with

the field winding connected in shunt with
the armature. In shunt generators, residual
magnetism must be present in the machine
iron in order to initiate the generation pro-
cess. These machines are also known as self-
excited, since they supply their own excita-
tion.

shunt field

a field winding of a DC ma-

chine consisting of many turns of fine wire,
connected in parallel with the armature cir-
cuit. It may be connected to the same source
as the armature or a separate source.

shunt peaking

use of a peaking coil in a

parallel tuned circuit branch connecting the
output load of one amplifier stage to the in-

2000 by CRC Press LLC

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