Page 58 - The Illustrated Dictionary of Electronics

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astronomical unit • atmospheric absorption noise 43

astronomical unit Abbreviation, AU. A unit of dis-
8
tance equal to 1.496 × 10 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 com-
munications 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 com-
mon 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- asynchronous input In digital circuitry, any flip-
TOR in which the source and drain cannot be in- flop input at which a pulse can affect the output
terchanged without degrading performance. independently of the clock.
asymmetrical multivibrator An unbalanced mul- asychronous motor An ac motor whose speed is
tivibrator (i.e., one in which the circuit halves are not proportional to the supply frequency.
not identical). If the time constants of the halves asynchronous transmission Data transmission
are different, the output pulses will be short and in which each character or symbol begins with a
widely separated. start signal and ends with a stop signal. This
asymmetrical sideband See VESTIGIAL SIDE- eliminates the need for the data to be sent at a
BAND. uniform speed.
asymmetrical sideband transmission See VESTI- asynchronous vibrator In a vibrator-type portable
GIAL SIDEBAND TRANSMISSION. power supply, a vibrator that only makes and
asymmetrical wave A wave whose upper (positive breaks the primary circuit of the step-up trans-
half-cycle) and lower (negative half-cycle) por- former. This is in contrast to the synchronous vi-
tions have different amplitudes or shapes. Also brator, which also makes and breaks the
called asymmetric wave. secondary circuit in synchronism with the pri-
asymmetry control An adjustment in a device in- mary. Also called NONSYNCHRONOUS VIBRA-
tended for measuring the pH (acidity/alkalinity). TOR.
This corrects the inaccuracies that results from AT A quartz crystal cut wherein the angle between
the differences between the electrodes. the x-axis and the crystal face is 35 degrees.
asymptote In analytical geometry, a fixed straight At Symbol for astatine.
line or ray L with a special relationship to a AT-cut crystal A piezoelectric crystal cut at a 35-
curve or part of a curve K that recedes to infin- degree angle, with respect to the optical axis of
ity. As the distance from the origin (0,0) in- the quartz. The frequency of such a crystal does
creases without limit, the separation between K not appreciably change with variations in tem-
and L approaches zero, but K and L never actu- perature.
ally meet. atmosphere 1. The gas surrounding a planet, par-
asymptotic breakdown voltage A voltage that will ticularly the air sheathing the earth. 2. Abbrevia-
cause dielectric breakdown if applied continu- tion, atm. A unit of pressure equal to 1.013 × 10 6
ously for a sufficiently long time. dynes per square centimeter (about 14.7 pounds
asymptotic expression An expression having a per square inch).
very small error in terms of percentage. atmospheric absorption 1. The conversion of
asynchronous 1. Not synchronous, i.e., nonrecur- electromagnetic energy into heat, with resulting
rent (as in out-of-phase waves). 2. A mode of loss, as the energy passes through the earth’s at-
computer operation in which the completion of mosphere. The extent of this effect depends on
one operation starts another. the wavelength. 2. See ABSORPTION LOSS, 2.
asynchronous device A device not regulated by atmospheric absorption noise Noise, principally
the system in which it is used, as far as its oper- above 1 GHz, resulting from atmospheric absorp-
ating frequency or rate is concerned. tion (see ABSORPTION LOSS, 2).