Page 56 - Electrical Engineering Dictionary

oped by the University of Arizona and BRO, quency response curve has decreased −3dB
Inc. from the passband reference gain. Note that
f 1 and f 2 define the response passband by
APC-7 connector common term for am- marking the points at which the output power
phenol precision connector - 7mm. A “sex- has decreased to one-half the value of the in-
less” coaxial connector with butt contact be- put power. For band widths extending down
tween both the inner and outer conductors to DC, the upper −3 dB frequency is cited as
capable of low standing wave ratios to fre- the 3 dB bandwidth.
quencies up to 18 GHz.
aperture coupling a method of coupling
APD See avalanche photodiode. a transmission line to an antenna in which
fields leak through an aperture in a metallic
aperiodic convolution the convolution of ground plane separating the line from the an-
two sequences. See convolution. tenna.

aperiodic signal a signal that is not pe- aperture efficiency a figure of merit that
riodic, i.e., one for which x(t) 6= x(t + T). determines how much of the incident energy
This means that the signal x(t) has a property is captured by an aperture. It depends on the
that is changed by a time shift T . See also physical dimensions of the aperture.
periodic signal.
aperture problem given a sequence of
aperiodic waveform this phrase is used
images over time we would like to infer the
to describe a waveform that does not repeat
motion (optical flow) field. Based on local
itself in a uniform, periodic manner. Com-
image information (i.e., based on the values
pare with periodic waveform.
of those pixels falling within some aperture)
onlythecomponentofmotionalongthegray-
aperture (1) an opening to a cavity, or
level gradient can be inferred; that the com-
wave-guide, from which radiation is either
ponent of motion perpendicular to the gray-
received or transmitted. Typically used as
level gradient can only be known by resorting
antenna or a coupling element.
to global methods is known as the aperture
(2) a physical space available for beam to
problem. See optical flow, optical flux.
occupy in a device. Aperture limitations are
the physical size of the vacuum chamber; a
APL See average picture level.
magnetic field anomaly may deflect the beam
so that the full available aperture cannot be
APLC See active power line conditioner.
used.
apodization (1) a deliberate variation in
aperture antenna an antenna with a
physical opening, hole, or slit. the transmission of an optical aperture as a
function of distance from the center or edges,
in order to control optical transfer functions.
aperture correction signal compensa- (2) a deliberate variation in the strength of
tion used to correct the distortion caused by a signal with time.
the non-zero aperture of a scanning electron
beam. A standardized measure of the selec- apparent concurrency within an interval
tivity of a circuit or system. The −3dB of time more than one process executes on a
(or half-power) band width is taken to be computer, although at the instruction level,
the difference between the upper (f 2 ) and instructions from only one process run at any
lower (f 1 ) frequencies where the gain vs. fre- single point in time. See also concurrency.

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2000 by CRC Press LLC