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Electrical Engineering Dictionary - part 172

ing the information is applied to the other.
When used in this manner, the LO is of-
ten referred to as a beat frequency oscillator
(BFO). Low-pass filtering the output results
in retrieval of the intelligence signal, super-
imposed upon a DC voltage (or current, de-
pendent upon the actual device). The DC
value may either be discarded via high-pass
filtering, or used as a received signal strength
indicator for use in automatic gain control
circuits.

synchronous digital hierarchy (SDH)

international interface specification for high-
speed optical fiber transmission networks
that allows different manufacturers’ equip-
ment to be interconnected with full mainte-
nance and signal transparency. Specifies the
optical parameters and the basic rates and for-
mats of the signal. Emphasizes protection
from faults and fast restoration of service af-
ter service interrupts.

synchronous drive

a magnetic drive char-

acterized by synchronous transmission of
torque, typically using a salient pole struc-
ture. There is no slip between the driver and
the follower.

synchronous dynamic RAM (SDRAM)
a type of dynamic random access memory
with an added synchronizing clock signal that
allows for burst mode access of a series of
successive bits. See also

dynamic random

access memory

synchronous machine

an AC electrical

machine that is capable of delivering torque
only at one specific speed (

), which is deter-

mined by the frequency of the AC system (

f )

and the number of poles (

P ) in the machine.

The relationship between synchronous speed
and the other variables is

= 120f/P

synchronous motor

an AC motor in

which the average speed of normal operation
is exactly proportional to the frequency to

which it is connected. A synchronous mo-
tor generally has rotating field poles that are
excited by DC.

synchronous operation

an operation that

is synchronized to a clocking signal.

synchronous optical network (SONET)
a U.S. interface specification for high-speed
optical fiber transmission networks that al-
lows different manufacturers’ equipment to
be interconnected with full maintenance and
signal transparency.

SONET emphasizes

protection from faults and fast restoration of
service after service interrupts.

synchronous reactance

the inductive re-

actance of the armature windings in syn-
chronous machines under steady-state con-
ditions. Designated by the symbol

, ex-

pressed in ohms per phase, the synchronous
reactance is a function of the stator induc-
tance and the frequency of the stator currents.

synchronous reference frame

a two-

dimensional space that rotates at an angu-
lar velocity corresponding to the fundamen-
tal frequency of the physical stator vari-
ables (voltage, current, flux) of a system.
In electric machines/power system analysis,
an orthogonal coordinate axis is established
in this space upon which fictitious wind-
ings are placed. A linear transformation is
derived in which the physical variables of
the system (voltage, current, flux) are re-
ferred to variables of the fictitious wind-
ings.

See also

arbitrary reference frame

rotor reference

frame

stationary reference frame

synchronous reluctance machine

a type

of synchronous machine that has no rotor
winding. The rotor consists of salient poles,
which causes the reluctance to vary as a func-
tion of position around the airgap. When op-
erated as a motor, a rotating magnetic field is
established by the stator windings that causes
a reluctance torque on the rotor as the path

2000 by CRC Press LLC

of lowest permeability stays aligned with the
peak of the stator flux wave.

synchronous reluctance motor

a syn-

chronous motor that depends on a reluctance
variation on the rotor for the mechanism of
torque production. The rotor shape is de-
signed to provide a high difference in the re-
luctances between the

d and q axes.

synchronous speed

speed of the rotating

magnetic flux produced by three-phase cur-
rents in stationary coils in three-phase AC
machines. The synchronous speed is calcu-
lated by a knowledge of the number of poles
of the machine and the frequency of the stator
currents as

= 120f

synchronous transfer mode

a method

of multiplexing messages onto a channel in
which each period of time (also called a
frame) on the channel is divided into a num-
ber of slots; one slot is allocated to each
source for the transmission of messages, and
a slot’s worth of data from each source is sent
every period.

synchronous updating

all units in a neu-

ral network have the values of their outputs
updated simultaneously.

synchronously pumped-modelocked (SPM)
laser

a laser in which periodic pump pul-

sations arrive at the amplifying medium of a
laser oscillator in synchrony with the circu-
lating mode-locked pulses, a standard tech-
nique for obtaining sub-picosecond pulsa-
tions.

synchroscope

a device used to determine

the phasor angle between two 3-phase sys-
tems. It is normally used to indicate when
two systems are in phase so that they can be
connected in parallel.

syndrome

bit pattern used for error-

detection and correction that is formed by

multiplying the received vector by the parity-
check matrix. Any two

n-tuples that have the

same syndrome can differ at most by a code
word.

synonym

in a virtual addressed cache,

when a real address has more than one vir-
tual address, the name given to the virtual
addresses.

syntax

the part of a formal definition of

a language that specifies legal combinations
of symbols that make up statements in the
language.

synthesis filter

a bank of filters that

recombines the components decomposed
by analysis filters from different frequency
bands.

synthesizer

a software program that cre-

ates GDS2 data from a hardware description
language specification such as VHDL or Ver-
ilog.

synthetic aperture radar (SAR)

a tech-

nique for overcoming the need for large
antennas on side-looking airborne radar
(SLAR) systems. The effect of a large an-
tenna is synthesized by using Doppler shifts
to classify the return signals, generating a
very small effective beamwidth. The process
is quite similar to that of holography, since
the amplitude and frequency of the signals is
recorded over time.

synthetic diamond

diamond grown arti-

ficially, usually as a film, for industrial pur-
poses such as hardness, thermal conductivity,
or optical properties.

system

a physical process or device that

transforms an input signal to an output signal.
For example, the following figure describes
a system consisting of a RC circuit with an
input of voltage, and an output of measured
voltage across the capacitor:

Often the behavior of systems is governed

by a physical law, which when applied yields

2000 by CRC Press LLC

System example.

a mathematical description of the behavior.
For example, application of Kirchoff’s volt-
age law and Ohm’s Law leads to the follow-
ing differential equation description of the
above electric circuit input/output behavior:

y(t) +

y(t) =

f (t)

system bus

in digital systems, the main

bus over which information flows.

system identification

a field of control

engineering dealing with the derivation of
mathematical models for the dynamics of
processes, often by a detailed study of its in-
put and output signals. It includes the design
of experiments for enhancing the accuracy of
the models.

system implementation

a phase of soft-

ware development life cycle during which a
software product is integrated into its opera-
tional environment.

system interaction

a stream of energy,

material, or information exchanged between
the sub-systems of a large-scale system. Rel-
evant attributes of those streams are, respec-
tively, interaction inputs or interaction out-
puts). Interactions are described by the in-
teraction equations, which relate interaction
inputs to a given subsystem to interaction out-
puts from other subsystems.

system noise factor

a value, in decibels,

representing the ratio of the signal-to-noise
ratio (S/N) appearing at the input of a system
to that appearing at the output.

system NF

= 10 log

S/N

input

S/N

output

where S/N is the ratio (not in decibels) of
the signal power to noise power at a given
temperature. This value indicates the amount
of signal-to-noise degradation from input to
output of a system of components. If the S/N
and power gain of each individual component
in the system is known, then Friis’ formula
can be used to predict the overall system noise
factor:

system NF

= 10 log

+ (NR

− 1) /P

+ (NR

− 1) / (P

× P

)

+ (NR

− 1) / [P

× P

]

+ · · ·

+ (NR

− 1) /

× P

. . . P

G(n−1)

where all values for S/N and power gain are
in ratio (non-decibel) format, and the noise
ratio, NR, of each stage is defined as NR
= (S/N

input

)/(S/N

output

). Also referred to as

system noise figure.

System Performance and Evaluation Co-
operative (SPEC)

a cooperative formed

by four companies, Apollo, Hewlett-Packard,
MIPS, and Sun Microsystems, to evaluate
smaller computers.

system transfer function

the result of

sending a known test signal (often an impulse
function or sine wave) through a system and
defines what a system will do when presented
with an input signal. Test signals often must
be varied in frequency since system trans-
fer functions are often frequency dependent
(e.g., a stereo amplifier or speakers).

system with memory

a system whose out-

put at time t depends on the input at other
times (and possibly including) that instant t.
If the output of the system at time t depends
only on the input to the system at time t the
system is said to be memoryless.

2000 by CRC Press LLC

systematic code

a code for which the

information sequence itself is a part of the
coded sequence. For block codes, it is com-
mon to assume that the information sequence
is the first (or last) part of the codeword.

systems engineering

an approach to the

overall life cycle evolution of a product or
system. Generally, the systems engineering
process comprises a number of phases. There
are three essential phases in any systems en-
gineering life cycle: formulation of require-
ments and specifications, design and devel-
opment of the system or product, and deploy-
ment of the system. Each of these three basic

phases may be further expanded into a larger
number. For example, deployment gener-
ally comprises operational test and evalua-
tion, maintenance over an extended opera-
tional life of the system, and modification
and retrofit (or replacement) to meet new and
evolving user needs.

systolic

flow of data in a rhythmic fash-

ion from a memory through many processors,
returning to the memory just as blood flows
from and to the heart.

2000 by CRC Press LLC

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