Page 27 - The Illustrated Dictionary of Electronics

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12 active communications satellite • active repair time

active communications satellite A satellite con- active file A computer file in use (i.e., one that is
taining receivers (which pick up beamed electro- being updated or referred to).
magnetic signals from a ground point and amplify active filter A bandpass, bandstop, highpass or
them) and transmitters (which send signals back lowpass filter, consisting of resistors, capacitors,
to the surface of the earth). Also called active and operational amplifiers, arranged to pass a de-
comsat. Compare PASSIVE COMMUNICATIONS sired frequency response. Commonly used at au-
SATELLITE. dio frequencies.
active component 1. A device capable of some dy- active infrared detection Detection of infrared
namic function (such as amplification, oscilla- rays reflected from a target to which they were
tion, or signal control) that usually requires a beamed.
power supply for its operation. Examples include active jamming Transmission or retransmission
bipolar transistors, field-effect transistors, and of signals for the purpose of disrupting communi-
integrated circuits. Compare PASSIVE COMPO- cations.
NENT. 2. In an ac circuit, a quantity that con- active junction A pn junction in a semiconductor
tains no reactance so that the current is in phase device that has been created by a diffusion pro-
with the voltage. cess.
active component of current See ACTIVE CUR- active leg An element within a transducer that
RENT. changes one or more of its electrical characteris-
active computer A computer in an installation or tics in response to the input signal of the trans-
network that is processing data. ducer. Also called active arm.
active comsat See ACTIVE COMMUNICATIONS active lines In a U.S. television picture, the lines
SATELLITE. (approximately 488) that make up the picture.
active control system A device or circuit that The remaining 37 of the 525 available lines are
compensates for irregularities in the operating blanked and are called INACTIVE LINES.
environment. active material 1. In a storage cell, the chemical
active current In an ac circuit, the current compo- material in the plates that provides the electrical
nent that is in phase with the voltage. This is in action of the cell, as distinguished from the sup-
contrast to reactive current, which is not in phase porting material of the plates themselves. 2. A ra-
with the voltage, and is “inactive,” with respect to dioactive substance. 3. The phosphor coating of a
power in the circuit. The active current is equal to cathode-ray tube screen. 4. The material used to
the average power divided by the effective voltage. coat an electron-tube cathode.
active decoder An automatic ground-station de- active mixer A signal mixer using one or more ac-
vice that gives the number or letter designation of tive components, such as transistors or in-
a received radio beacon reply code. tegrated circuits. An active circuit provides
active device 1. An electronic component, such as a amplification, input-output isolation, and high
transistor that needs a power supply, and/or that input impedance, in addition to the mixing ac-
is capable of amplifying. 2. Broadly, any device (in- tion. Compare PASSIVE MIXER.
cluding electromechanical relays) that can switch active modulator A modulator using one or more
(or amplify) by application of low-level signals. active components, such as transistors or inte-
active electric network A network containing one grated circuits. An active circuit provides gain,
or more active devices or components, usually input-output isolation, and high input impe-
amplifiers or generators, in addition to passive dance, in addition to modulation. Compare PAS-
devices or components. SIVE MODULATOR.
active element The driven or RF-excited element active network See ACTIVE ELECTRIC NET-
in a multielement antenna or antenna array. WORK.
active pressure The electromotive pressure that
produces a current in an ac circuit.
active pull-up An arrangement using a transistor
as a pull-up resistor replacement in an integrated
circuit, providing low output impedance and low
power consumption.
active RC network 1. A resistance-capacitance
(RC) circuit that contains active components
(transistors or integrated circuits), as well as pas-
sive components (capacitors and resistors). 2. An
RC network in which some or all of the resistors
and capacitors are simulated by the action of ac-
tive components.
active repair time The time during which mainte-
nance is done on a system and the system is out
of operation.