18WuD2x

516 parasitic choke • pass amplifier
parasitic choke A small radio-frequency choke
coil (with or without a shunting resistor) that
suppresses or eliminates parasitic oscillation in a
power amplifier.
parasitic director In a multielement directional
antenna, a parasitic element acting as a director;
usually, it is a few percent shorter than the driven
element.
parasitic element An electrical conductor that
comprises an important part of an antenna system,
but that is not directly connected to the feed
line. Such elements are used for the purpose of
obtaining directivity and power gain. They operate
via electromagnetic coupling to the driven element(s).
The principle of operation was first
discovered by the Japanese engineers Yagi and
Uda, who observed that antenna elements parallel
to a driven element but not connected to anything,
at a specific distance from the driven
element, and having a certain length, cause the
radiation pattern to show gain in one direction
and loss in the opposite direction. See DRIVEN
ELEMENT, PARASITIC ARRAY, PARASITIC DI-
RECTOR, PARASITIC REFLECTOR.
parasitic-element directive antenna See PARA-
SITIC ARRAY.
parasitic eliminator See PARASITIC SUPPRES-
SOR.
parasitic excitation Excitation of a beam-antenna
element without a direct connection to the transmitter.
Thus, a director or reflector element can
be excited by the field of the radiator element.
parasitic inductance Stray inductance (e.g., the
internal inductance of a wirewound resistor).
parasitic oscillation Extraneous, useless oscillation
present as a fault in an electronic circuit,
particularly a radio-frequency power amplifier.
parasitic reflector In a multielement beam antenna,
a parasitic element acting as a reflector;
usually, it is a few percent longer than the driven
element.
parasitic resistance Stray resistance (e.g., the inherent,
internal resistance of a multilayer coil).
parasitic suppressor A small resistor, coil, or parallel
combination of the two, connected in series
with the plate or collector of a vacuum tube or
transistor to eliminate parasitic oscillations in a
radio-frequency power amplifier.
PARD Abbreviation of PERIODIC AND RANDOM
DEVIATION.
parity 1. At par (with respect to the even-or-odd
state of the characters in a group). 2. Having the
quality that the number of bits (or the number of
similar bits) are even or odd, as intended.
parity bit 1. In computer operations, a logic 1
added to a group of bits so that the number of 1s
in the group is, according to specification, even or
odd. 2. In computer operations, a check bit that
can be a logic 1 or 0, depending on the parity (see
PARITY, 1) of the total of 1s in the bit group being
checked.
parity check A check of the integrity of data being
transferred by adding the bits in, for example, a
word, and then determining the parity bit needed
and comparing that with the transmitted parity
bit.
parity error An error disclosed by a parity check.
parity tree A digital device used to check parity.
parsec Abbreviation, pc. The distance at which the
mean radius of the earth’s orbit around the sun
subtends an angle of 1 second of arc; 1 pc =
3.0857 × 10 13 kilometers or 3.2616 light years.
part See CIRCUIT COMPONENT, 1.
part failure The usually destructive breakdown of
a circuit component.
partial One of the frequencies in a complex musical
tone. It might be a harmonic of the fundamental
frequency, although this is not always the
case.
partial carry The temporary storage of some or all
of the carry information in a digital calculation.
particle 1. A tiny, discrete bit of matter. 2. A unit
of matter smaller and lighter than an atom. See,
for example, ANTIPARTICLE, ELECTRON, ME-
SON, NEUTRETTO, NEUTRINO, NEUTRON, NU-
CLEON, POSITRON, and PROTON.
particle accelerator See ACCELERATOR, 1.
particle theory of radiation In physics, a model
that explains the nature of electromagnetic radiation
(radio waves, infrared, visible light, ultraviolet,
X rays, and gamma rays) in terms of discrete
particles. Each particle, called a PHOTON, carries
a certain amount of energy that depends on the
wavelength of the radiation.
particle velocity 1. The speed and direction of
the particles from a source of atomic radiation.
2. The speed and direction of the molecules in the
medium of an acoustic disturbance.
partitioning In computer operations, breaking
down a large block of data into smaller blocks
that can be better handled by the machine.
parton model A model for atomic nuclei, in which
protons and neutrons are made up of smaller
particles called partons. Subparticles have been
found, commonly called quarks.
Pascal A high-level computer programming language,
similar to BASIC or FORTRAN in structure.
It is used in some schools to teach computer
programming.
pascal Symbol, Pa. The SI (derived) unit of pressure;
1 Pa = 1 N/m 2 = 1.4503 × 10 –4 lb/in 2 .
Paschen-Back effect See ZEEMAN EFFECT.
Paschen’s law For a two-element, parallel-plate,
gas-discharge tube, the plate-to-plate sparking
potential is proportional to Pd, where P is the gas
pressure, and d is the distance between plates.
pass amplifier A tuned amplifier having the response
of a bandpass filter. Like the filter, the
amplifier passes one frequency (or a narrow
band of frequencies) readily while rejecting or
attenuating others. Compare REJECT AMPLI-
FIER.