18WuD2x

154 critical frequency • cross modulation
critical frequency For a particular layer of the
ionosphere, the high frequency at which a vertically
propagated wave is no longer reflected back
to the earth.
critical inductance In a choke-input powersupply
filter, the minimum inductance that will
maintain a steady value of average load current.
critical potential The potential difference required
for an electron to excite or ionize an atom with
which it collides.
critical voltage The voltage at which a gas ionizes.
critical wavelength The wavelength that corresponds
to CRITICAL FREQUENCY.
CRO Abbreviation of cathode-ray oscilloscope.
Crookes dark space In a glow-discharge tube, the
narrow dark space next to the cathode. Also see
CROOKES TUBE.
Crookes tube A glow-discharge tube containing an
anode, cathode, and a small amount of gas under
low pressure.
cross antenna An antenna in which two (usually
equal-length) horizontal radiators cross each
other at right angles and are connected together
to a feeder at their point of intersection. It takes
its name from its horizontal-cross shape.
cross assembler A program used with one computer
to translate instructions for another computer.
crossband operation 1. Communications in which
two frequency bands are used. Station X, for example,
might transmit on frequency f A in band A
and receive on frequency f B in band B; station Y
would then transmit on f B and receive on f A . 2. In
satellite communications, the use of two frequency
bands to facilitate full-duplex operation
and to allow the satellite transponder to effectively
function. The transponder receives signals
from the earth within a specific frequency band,
and converts this entire band of signals to a set of
signals that occupies an equal amount of spectrum
space on another frequency band. The converted
signals are then retransmitted back to
earth.
crossbar switch A three-dimensional array of
switch contacts in which a magnetic selector
chooses individual contacts, according to their
coordinates in the matrix.
cross bearings A method of radionavigation, in
which directional readings are taken from a receiving
station (such as a ship or aircraft) for two
fixed transmitting stations whose locations are
known. Lines are drawn on a map from the transmitting
stations, in directions 180 degrees opposite
the bearings obtained from the receiving
station. The intersection point of these lines is the
location of the receiving station.
cross beat A spurious frequency arising from
CROSS MODULATION.
cross-check To compare the result of a calculation
or computer routine with the result obtained by a
different method.
cross color In the chrominance channel of a color
television receiver, crosstalk interference caused
by monochrome signals.
cross-connected neutralization Neutralization of
a push-pull amplifier by feedback through two
capacitors—each connected from the output circuit
of one transistor to the input circuit of the
other.
cross-coupled multivibrator A multivibrator circuit
in which feedback is provided by a coupling
capacitor between the output of the second stage
and the input of the first stage; the stages are
forward-coupled by a capacitor of the same value.
cross coupling 1. The state of being cross-coupled
(see, for example, CROSS-COUPLED MULTIVI-
BRATOR). 2. Undesired coupling between two circuits.
cross current A current that flows in the opposite
direction from some other current.
crossed-pointer indicator 1. Also called crossedneedle
meter. A combination of two analog metering
instruments in one case. Each needle has its
own independently calibrated scale. A third scale
corresponds to the intersection point of the needles.
Commonly used in directional wattmeters
that simultaneously show forward power, reflected
power, and standing-wave ratio (SWR). 2.
A two-pointer meter used in aircraft to show the
position of the aircraft, relative to the glide path.
crossed-wire thermoelement Two wires or strips
of dissimilar metals joined or twisted at a point
that constitutes a thermoelectric junction. In
usual operation, a high-frequency current is
passed through one wire, and a proportional
direct-current (dc) voltage, generated by thermoelectric
action, appears at the other wire.
cross flux The magnetic flux component that is
perpendicular to the flux produced by field magnets.
cross-hair pattern A television test pattern consisting
of a single vertical line and a single horizontal
line, which form a simple cross. The
pattern resembles the cross hairs of an optical instrument.
crosshatch generator A modulated radiofrequency
(RF) signal generator that produces a
crosshatch pattern on a picture-tube screen.
crosshatch pattern A grid of horizontal and vertical
lines produced on a picture-tube screen by a
cross-hatch generator. It is used in checking horizontal
and vertical linearity.
cross modulation 1. A type of radio-frequency interference
(RFI) between two strong stations that
are close in frequency. The desired carrier is modulated
by the interfering signal. 2. The production
of signals by rectifier junctions in pipes and
wiring near a radio receiver. These objects pick
up waves and deliver energy at a different frequency,
which finds its way into the receiver. Also
called external cross modulation. 3. The interaction
between signals of different frequency when