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Electrical Engineering Dictionary - part 137

of exposure and reduce the effects of stand-
ing waves. For a chemically amplified resist,
this bake also causes a catalyzed chemical
reaction.

post-metal anneal (PMA)

a process used

in semiconductor processing after metalliza-
tion to improve device properties and circuit
performance. Typically it is performed at a
modest temperature (300–450 C

◦

) in a form-

ing gas (e.g., 5% hydrogen with 95% nitro-
gen) ambient.

postbake

See

hard bake

posterior statistics

the empirical statistics

of a random quantity (scalar, vector, process,
etc.), based on the a priori statistics supple-
mented with experimental or measured ob-
servations. See also

prior statistics

Bayes’

rule

postfix

See

postfix notation

postfix notation

a notational or program-

ming scheme in which both operands of a
two-operand operation are written before the
operator is specified. Example:

ab+ is the

postfix representation of a sum; this could be
implemented in a programming model based
upon an evaluation stack by the operation se-
quence PUSH a, PUSH b, ADD. Postfix no-
tation is used in programming zero-address
computers.

postincrementation

an assembly lan-

guage addressing mode in which the address
is incremented after accessing the memory
value. Used to access elements of arrays in
memory.

pot head

a fork-shaped transition between

a three-phase buried cable and an overhead
three-phase electric power line.

potassium dihydrogen phosphate (KDP)

a strong linear electro-optic material, be-

longing to the same crystal class as am-

monium dihydrogen phosphate (ADP). Its
chemical form is

P O

potential

an auxiliary scalar or vector

field that mathematically simplifies the solu-
tion process associated with vector boundary
value problems. See also

Hertzian potential

potential coil

a long, finely wound,

straight coil, similar in operation to a Chat-
tuck coil, that is used with a fluxmeter to mea-
sure magnetic potential difference between a
point in a magnetic field and a flux-free point
in space.

potential source rectifier exciter

a source

of energy for the field winding of a syn-
chronous machine obtained from a rectified
stationary AC potential source. The AC po-
tential can be obtained from the machine
phase voltages, or from an auxiliary source.
The components of the exciter are the po-
tential source transformer and the rectifiers
(including possible gate-circuitry).

potential transformer (PT)

a device

which measures the instantaneous voltage of
an electric power line and transmits a signal
proportional to this voltage to the system’s
instruments. See

voltage transformer

potentially unstable

active circuit or de-

vice, where particular impedance termina-
tion placed on its input and/or output ports
causes it to become unstable and oscillates.

potentiometric sensor

a chemical sen-

sor that measures the concentration of a sub-
stance by determining the electrical poten-
tial between a specially prepared surface and
a solution containing substance being mea-
sured.

Potier reactance

the leakage reactance

obtained in a particular manner from a test
on a synchronous machine at full load with
a power factor of zero lagging. The test re-
quires little power but supplies the excitation
for short circuit and for normal rated volt-

2000 by CRC Press LLC

age both at full-load current at zero power
factor. The Potier reactance is determined
by a graphical manner from the open circuit
characteristic and the short circuit point for
full-load current.

power

(1) a measurable quantity that is the

time rate of increase or decrease in energy.
Units are in watts.

(2) ratio of energy transferred from one

form to another (i.e., heat, radio waves, light,
etc.) to the time required for the transfer,
expressed in watts.

power added efficiency

dimensionless

ratio of RF power delivered from a device to
the load minus the input incident RF power
versus the total DC power dissipated in the
device.

power amplifier

amplifier operating at

either audio or radio frequency range deliv-
ering power to a termination such as speaker,
antenna, or resistive load.

power angle

the angular displacement of

the rotor from the stator rotating magnetic
field while the machine is on load. The power
angle is also the angle between the terminal
voltage

of a synchronous machine and the

generated voltage

, respectively, for

a generator or motor. This angle denoted by
δ is also referenced as power angle or torque
angle or the load angle in a synchronous ma-
chine. It signifies the limits of the machine
to remain in synchronism. See also

torque

angle

power angle curve

a curve shown the re-

lationship between the active power output
of a generator and its power angle.

power broadening

increase in linewidth

of a homogeneously broadened transition due
to stimulated emission.

power conditioner

a device designed to

suppress some or all electrical disturbances
including overvoltages, undervoltages, volt-

age spikes, harmonics, and electromagnetic
interference (EMI). Example power condi-
tioners are active filters for the reduction
of harmonics, metal-oxide varistors (MOVs)
and isolation transformers for the protection
against voltage spikes, and EMI filters.

power control

(1) the use of a mechanism

to adjust a transmitter’s power, usually in or-
der to improve the overall performance of a
communications network.

(2) in CDMA, a technique to increase the

radio capacity. This is due to the fact that
a CDMA system is interference-limited and
that all users in a cell operate at the same
frequency. The power control scheme used
at the forward link of each cell can reduce the
interference to the other adjacent cells. The
less the interference generated in a cell, the
higher the capacity.

(3) in a TDMA system, used to reduce co-

channel interference or interference in adja-
cent cells. By having all cells (which oper-
ate at different frequencies with a cell reuse
factor of 7 for a TDMA system) at approx-
imately the same power, interference is re-
duced.

power density

generally refers to the av-

erage power density, which is a measure of
the power per unit area of a propagating EM
wave. Mathematically, it is defined as the
time average of the Poynting vector.

power disturbance

a variation of the

nominal value of the voltage or current.

power divider

passive electronic circuit

consisting of one input and two or more out-
puts. A signal applied to the input is divided
into equiphase output signals, generally of
equal amplitude.

power factor

in an AC system, the ratio

of the (active component) real power

P to the

apparent power

S; it is given by the cosine of

the angle subtended by

S on the real, P axis.

See also

2000 by CRC Press LLC

power factor correction

the addition of

reactive load to bring the combined power
factor nearer unity.

Since most industrial

loads are inductive, capacitors are often em-
ployed as passive devices for power factor
correction.

power fault arc

an arc through soil

extending from a power lines’s lightning
ground to a buried, grounded structure.
These may form when lightning strikes an
energized overhead electric power line.

power flow studies

solutions of transmis-

sion line active and reactive power flow and
bus voltages giving system load.

power flow study

the circuit solution of

an electric power system which yields the
voltage of each bus and thus the power flows
throughout the system.

power follow

a fault condition, especially

through a lightning arrester, in which power
line current flows along a path through air
or other insulation broken down by a high
voltage impulse such as a lightning stroke to
a conductor. See

power fault arc

power follow transformer

a rugged,

high-current power transformer used in tests
of lightning arresters to test the arrester’s
power follow arc suppression capability.

power flux density

a vector that gives

both the magnitude and direction of an elec-
tromagnetic field’s power flow. The units are
watts per square meter.

power fuse

a protective device that con-

sists of a fusible element and an arc quench-
ing medium. An overload or fault current
in the fuse melts the fusible element, which
creates an arc. The quenching medium then
interrupts the current at a current zero, and
prevents the arc from restriking.

power gain

dimensionless ratio of RF

power delivered to a load versus the input in-

cident RF power, normally expressed in deci-
bels. Thus a power gain of 100 is expressed
as 20 dB.

power line conditioner (PLC)

equipment

installed on the customer’s side of the me-
ter to eliminate momentary over- and under-
voltages to critical loads.

power monitoring unit (PMU)

a device

that is installed in at a consumer’s site that
will indicate to the utility whether or not an
outage condition exists. The units commer-
cially available at the present time typically
have the facility to call power off and power
on status to a central facility via a telephone
link.

power noise factor

for a photodetector,

the ratio of signal-to-noise power ratio at the
input (SNPR

) to the signal-to-noise power

ratio at its output (SNPR

out

). If the power

noise factor is denoted by

k, then

k =

SNPR

out

power on self-test (POST)

a series of di-

agnostic tests performed by a machine (such
as the personal computer) when it powers on.

power optical

power carried by an optical

frequency beam.

power output

useful output from a laser

oscillator.

power quality

(1) the concept of main-

taining appropriate voltage and current wave-
forms and frequency in transmission, dis-
tribution, and generation systems, and usu-
ally taken to mean undistorted and balanced
waveforms.

(2) a measure of an electric supply to meet

the needs of a given electrical equipment ap-
plication. As delivered by the utility, power
quality is the faithfulness of the line voltage
to maintain a sinusoidal waveform at rated
voltage and frequency.

2000 by CRC Press LLC

power signal

suppose that

f (t) is a con-

tinuous time signal and let

−T

f (t)

represent the power dissipated by

f (t) dur-

ing the interval

[−T , T ]. The signal f (t) is

a power signal if lim

T →∞

exists and sat-

isfies

< P lim

T →∞

< ∞ .

Furthermore, lim

T →∞

is the average

power of the signal. Notice that an energy
signal is not a power signal since necessarily
lim

T →∞

= 0.

Suppose that

f [k] is a discrete time signal

and let

k=−T

f (tk)

The signal

f [k] is a power signal if

lim

T →∞

exists and satisfies

< P lim

T →∞

< ∞ .

Furthermore, lim

T →∞

is the average

power of the signal. All discrete time pe-
riodic signals are power signals. See also

energy signal

periodic signal

power spectral analysis

computation of

the energy in the frequency distribution of an
electrical signal.

power spectral density

(1) a function as-

sociated with a random process that speci-
fies the distribution of the signal power over
the different frequency components of the
spectrum of the random process. The in-
tegral of the power spectral density func-
tion is the average power of the signal. The
Wiener–Khinchine theorem states that the
power spectral density

(ω) of a random

process

X(t) is equal to the Fourier transform

of the autocorrelation function

(τ) of the

process; i.e., for the process

X(t) S

(ω) =

∞

−∞

(τ)e

−jωτ

dτ, where j

= −1.

(2) the power spectral density function of

a signal is its power spectrum.

power spectrum

(1) a representation of

power contributed by each harmonic of the
spectrum.

(2) for a wide sense stationary random

process is the Fourier transform of the auto-
correlation function. The power spectrum is
a measure of the average power of a random
process as a function of frequency. The ex-
pected value of the periodogram tends to the
power spectrum as the periodogram window
length tends to infinity.

power splitter

passive device that accepts

an input signal and delivers multiple outputs
with equal power.

power supply

an electronic module that

converts power from some power source to
a form which is needed by the equipment to
which power is being supplied.

power supply unit (PSU)

See

power

supply

power system damping

a torque action

which works to reduce power system oscil-
lations as time progresses.

power system oscillation

variations in the

machine rotor angle caused by small distur-
bances in the system. The oscillations are
usually at low frequency.

power system security

an operational in-

dex that determines the capability of a system
to withstand equipment failures and external
disturbances. It could be measured by the
reserve capacity available or the number of
operating constraint violations under a given
prevailing condition or under a contingent
probability of disturbances.

power system stabilizer

a control device

that provides an additional input signal to the
AVR to damp power system oscillations.

2000 by CRC Press LLC

Content .. 135 136 137 138 ..