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swinging choke • symmetrical conductivity 667
swinging choke A filter choke that exhibits relatively
high inductance when low current flows
through it, and lower inductance when high current
flows through it. This inductance, which
swings under conditions of varying load current,
permits the use of a high-resistance bleeder resistor.
Compare SMOOTHING CHOKE.
Swiss-cheese packaging A method of packaging
an electronic circuit, in which components are inserted
into the assembly through holes drilled or
punched in parallel, stacked printed-circuit
boards.
switch 1. A circuit or device (electronic, electromechanical,
or mechanical) for opening and closing
a circuit or for connecting a line to one of several
different lines (e.g., rotary selector switch). 2. To
change the logic state of a circuit or device. 3. In
a computer program, a branch instruction directing
the program to a line number dependent on
the value of a variable or result (e.g., BASIC’s
GOTO). 4. To cause an electrical circuit to change
state, as from low to high or vice versa.
switch current 1. The current flowing through a
switch. 2. The current flowing through a switching
diode or transistor. 3. The minimum current
necessary to produce switching of a transistor,
specified in milliamperes or microamperes.
switchgear Collectively, devices and systems for
making and breaking circuits—either automatically
or manually.
switchhook In a telephone set, the spring-andswitch
device that engages the line when the receiver
is lifted.
switching characteristics Technical data describing
the performance and capabilities of switching
devices and circuits.
switching circuit An on-off type of circuit containing
electronic or mechanical switches.
switching diode See COMPUTER DIODE.
switching frequency The frequency at which a
repetitive switch operates. Also see SWITCHING
RATE.
switching mode Operation in which a device, such
as a transistor or diode functions as a binary digital
device, rather than as an analog device. The
current is generally either zero (cutoff or pinchoff)
or some value that depends on the bias and on
the applied voltage.
switching rate The rate (e.g., closures per second)
at which a repetitive switch operates. Also see
SWITCHING FREQUENCY.
switching speed The time required for a switch to
open or close or for a switching device to change
states (as from cutoff to saturation). Also see
SWITCHING TIME.
switching time The time required, after the application
of a pulse, for an electronic switch to
change state. Also see SWITCHING SPEED.
switching transistor A transistor designed especially
for on-off operation. Such units exhibit
short recovery time and low capacitance.
switching voltage The largest voltage that a
switching device can handle without malfunctioning.
switch leakage current 1. The current flowing
through a switching device when it is supposed to
be nonconducting. 2. In a switching transistor,
for a given voltage, the leakage current between
the emitter and collector when the device is supposed
to be nonconducting.
SWL Abbreviation of SHORTWAVE LISTENER.
SWR Abbreviation of STANDING-WAVE RATI0.
SWR bridge A four-arm resistance bridge for measuring
voltage standing-wave ratio. This radiofrequency
bridge has noninductive resistors in
three of its arms and the device under test in the
fourth arm. The bridge is balanced first with an
equivalent noninductive resistor that replaces
the device, and the output voltage is noted. Then
the device is substituted for the test resistor, and
the change in voltage is noted. The standing-wave
ratio is determined from the voltage ratio.
SWR meter See SWR BRIDGE.
S Y band A section of the S BAND, extending from
2600 to 2700 MHz.
syllable compandor A device that compresses or
expands the amplitude of an audio signal. The
time constant is fast enough to allow response to
individual syllables. Compression is generally
used at the transmitting station, and expansion
at the receiving station.
sym 1. Abbreviation of symmetrical. 2. Abbreviation
of SYMBOL.
symbol 1. A letter or graphic device representing a
quantity or term [e.g., I (current), f (frequency),
etc.]. 2. A conventional device denoting a mathematical
operation (e.g., +, /). 3. In a circuit
diagram, a pictorial device representing a
component.
symbolic address An address in a sourcelanguage
computer program (i.e., the arbitrary
label used by the programmer).
symbolic language See SOURCE LANGUAGE.
symbolic logic A system for representing logical
relationships, such as those acted upon by computer
and switching circuits, by means of symbols
that are usually nonnumerical. Also see
BOOLEAN ALGEBRA.
symmetrical circuit A circuit having identical
configurations on each side of a dividing line,
such as the ground bus. A push-pull circuit is an
example.
symmetrical communications 1. Two-way communications
in which the volume of transmitted
data is the same, or nearly the same, in both directions.
2. Two-way communications in which
the speed of transmitted data is the same, or
nearly the same, in both directions. Compare
ASYMMETRICAL COMMUNICATIONS.
symmetrical conductivity Identical conductivity
for both positive and negative electricity. Compare
ASYMMETRICAL CONDUCTIVITY.