18WuD2x

astronomical unit • atmospheric absorption noise 43
astronomical unit Abbreviation, AU. A unit of distance
equal to 1.496 × 10 8 kilometers (9.296 × 10 7
miles). Approximately equal to the mean distance
between the earth and the sun.
A supply See A POWER SUPPLY.
asymmetrical cell A photocell exhibiting ASYM-
METRICAL CONDUCTIVITY.
asymmetrical communications 1. Two-way communications
in which the volume of transmitted
data is much greater in one direction than in the
other. 2. Two-way communications in which the
speed of transmitted data is much greater in one
direction than in the other. Compare SYMMET-
RICAL COMMUNICATIONS.
asymmetrical conductivity A condition in which
a device conducts well in one direction, but poorly
in the other direction. A rectifier diode is a common
example of a component that exhibits this
effect.
asymmetrical distortion In a binary system,
lengthening or shortening of one of the states, by
comparison to the theoretical or ideal duration.
asymmetrical FET A FIELD-EFFECT TRANSIS-
TOR in which the source and drain cannot be interchanged
without degrading performance.
asymmetrical multivibrator An unbalanced multivibrator
(i.e., one in which the circuit halves are
not identical). If the time constants of the halves
are different, the output pulses will be short and
widely separated.
asymmetrical sideband See VESTIGIAL SIDE-
BAND.
asymmetrical sideband transmission See VESTI-
GIAL SIDEBAND TRANSMISSION.
asymmetrical wave A wave whose upper (positive
half-cycle) and lower (negative half-cycle) portions
have different amplitudes or shapes. Also
called asymmetric wave.
asymmetry control An adjustment in a device intended
for measuring the pH (acidity/alkalinity).
This corrects the inaccuracies that results from
the differences between the electrodes.
asymptote In analytical geometry, a fixed straight
line or ray L with a special relationship to a
curve or part of a curve K that recedes to infinity.
As the distance from the origin (0,0) increases
without limit, the separation between K
and L approaches zero, but K and L never actually
meet.
asymptotic breakdown voltage A voltage that will
cause dielectric breakdown if applied continuously
for a sufficiently long time.
asymptotic expression An expression having a
very small error in terms of percentage.
asynchronous 1. Not synchronous, i.e., nonrecurrent
(as in out-of-phase waves). 2. A mode of
computer operation in which the completion of
one operation starts another.
asynchronous device A device not regulated by
the system in which it is used, as far as its operating
frequency or rate is concerned.
asynchronous input In digital circuitry, any flipflop
input at which a pulse can affect the output
independently of the clock.
asychronous motor An ac motor whose speed is
not proportional to the supply frequency.
asynchronous transmission Data transmission
in which each character or symbol begins with a
start signal and ends with a stop signal. This
eliminates the need for the data to be sent at a
uniform speed.
asynchronous vibrator In a vibrator-type portable
power supply, a vibrator that only makes and
breaks the primary circuit of the step-up transformer.
This is in contrast to the synchronous vibrator,
which also makes and breaks the
secondary circuit in synchronism with the primary.
Also called NONSYNCHRONOUS VIBRA-
TOR.
AT A quartz crystal cut wherein the angle between
the x-axis and the crystal face is 35 degrees.
At Symbol for astatine.
AT-cut crystal A piezoelectric crystal cut at a 35-
degree angle, with respect to the optical axis of
the quartz. The frequency of such a crystal does
not appreciably change with variations in temperature.
atmosphere 1. The gas surrounding a planet, particularly
the air sheathing the earth. 2. Abbreviation,
atm. A unit of pressure equal to 1.013 × 10 6
dynes per square centimeter (about 14.7 pounds
per square inch).
atmospheric absorption 1. The conversion of
electromagnetic energy into heat, with resulting
loss, as the energy passes through the earth’s atmosphere.
The extent of this effect depends on
the wavelength. 2. See ABSORPTION LOSS, 2.
atmospheric absorption noise Noise, principally
above 1 GHz, resulting from atmospheric absorption
(see ABSORPTION LOSS, 2).