Page 210 - Electrical Engineering Dictionary

dewar a vacuum insulated, cryogenic ra- when the intelligence bandwidth exceeds that
diation insulated, low conduction heat loss of the modulator.
vessel for cryogenic fluids. A dewar is used
for keeping material, chiefly liquids, cold or diagonal dominance a measure of
hot. The common thermos bottle is an ex- the amount of interaction that exists be-
ample of a dewar. In the accelerator business tween variables in a multi-input–multi-
dewars are often used to store large quantities output
of cryogenic liquids. (MIMO) system. It is quantified by Gersh-
gorin circles or bands that are often plotted
DF See dissipation factor. on an inverse Nyquist array (or INA) diagram
that shows the frequency response matrix of
DFB laser See distributed feedback laser. the system in a graphical form. Its practical
significance relates to the fact that a diago-
DFD See displaced frame difference. nally dominant system can be controlled by
multiple single variable controllers, whereas
DFT See discrete Fourier transform. a nondominant process might require so-
phisticated and costly multivariable tech-
Dhrystone See Dhrystone benchmark. niques for effective control. See also inverse
Nyquist array.
Dhrystone benchmark synthetic bench-
mark program consisting of a representative diamagnetic materials with magnetiza-
instruction mix used to test the performance tion directed opposite to the magnetizing
of a computer. Does not compute anything field, so that the permeability is less than one;
in particular. Another synthetic benchmark metallic bismuth is an example.
is the Whetstone benchmark.
die an individual MMIC circuit or subsys-
DIAC atwo-terminalACdevicethat, once tem that is one of several identical chips that
gated on by sufficient forward voltage, per- are produced after dicing up an MMIC wafer.
mits the flow of current until reverse biased.
It is often used as a trigger device to drive the dielectric (1) a medium that exhibits neg-
gate of other power electronic devices. ligible or no electrical conductivity and thus
acts as a good electrical insulator.
Diac’s delta function See delta function. (2) a medium characterized by zero con-
ductivity, unity relative permeability, and a
diagnostic (1) one of a set of tests to run relative permittivity greater than one. Also
through a system that determines whether the known as an insulator.
system is functioning correctly. Dielectries are usually used to separate
(2) pertaining to the detection and isola- two conducting bodies such as to form a ca-
tion of faults or failures. For example, a di- pacitor.
agnostic message, a diagnostic manual.
dielectric constant (1) a quantity that de-
diagonal clipping distortion that occurs scribes how a material stores and dissipates
in an AM demodulator (usually associated electrical energy.
with diode detection), where the capacitor (2) ratio of the electrical capacity of a con-
discharge time constant is set too long for the denser, which has a given material as the di-
detector to accurately follow fast changes in electric, to the capacity of an identical con-
the AM signal envelope. Sometimes referred denser, but with air as the dielectric.
to as “failure to follow distortion,” diagonal (3) permittivity of a medium normalized
clipping can also occur in AM modulators to the permittivity of free space; a measure

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2000 by CRC Press LLC