Electrical Engineering Dictionary - part 51

of the response of a dielectric to an applied
electric field.

(4) an electric property of an insulator or

semi-conducting material, which describes
how differently electric fields will behave in-
side of the material as compared to air. As
an example,

= 12.9 for GaAs as com-

pared to

= 1 for air. In integrated circuits,

an effective dielectric constant (

eff

) is used,

since the electric fields supported by the sig-
nals traveling through the conductors on the
circuit flow through both air and the insulator
or semiconductor simultaneously.

dielectric discontinuity

interface be-

tween two media with different dielectric per-
mittivity properties.

dielectric medium

medium that is polar-

izable but relatively nonconducting.

dielectric resonator

an unmetallized di-

electric object of high dielectric constant and
high quality factor that can function as an en-
ergy storage device.

dielectric resonator [stabled] oscillator
(DRO)

a dielectric resonator is a cylindri-

cally shaped piece of material, or “puck,” that
has the properties of having low-loss resonant
frequencies that are determined primarily by
the size of the cylinder. Placing a dielectric
resonator near a microstrip line can form a
resonant circuit that will frequency stabilize
a voltage-controlled oscillator.

dielectric resonator antenna (DRA)

antenna where a dielectric resonator is used
as the radiation element.

dielectric slug tuner

system of two mov-

able dielectric pieces of material placed on a
transmission line for the purpose of matching
a wide range of load impedances by means
of placing the dielectrics in proper positions.

dielectric step discontinuity

the junction

between different dielectric waveguides.

dielectric waveguide

a waveguide that re-

lies on differences in permittivity among two
or more materials to guide electromagnetic
energy without the need for ground planes or
metallic strips. Such guides of rectangular,
circular, elliptical, and other cross sections
are made of dielectric materials and used for
transmitting signals. Transmission is accom-
plished by the total internal reflection mech-
anism inside the waveguide.

difference amplifier

See

differential

amplifier

difference engine

a mechanical calculator

developed by Babbage in 1823.

difference equation

the mathematical

model of a LTIL discrete time system. See
also

discrete time system

LTIL system

difference of Gaussian filter

a bandpass

filter whose point spread function is the dif-
ference of two isotropic Gaussians with dif-
ferent variances. The result is a “Mexican
hat” shape similar to the Laplacian of a Gaus-
sian (See Marr-Hildreth operator). Various
physiological sensors, including some filters
in early vision, appear to have DOG point
spread functions.

difference-frequency generation

a sec-

ond-order nonlinear optical process in which
two input beams are applied to a nonlinear
optical material and an output is produced at
the difference of the frequencies of the two
input beams.

difference-mode signal

if two arbitrary

signals

and

are applied to the inputs

of a differential amplifier, then the common-
mode signal is the arithmetic average of the
two signals. That is,

+ v

)/2

differential amplifier

an amplifier in-

tended to respond only to the difference be-

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tween its input voltages, while rejecting any
signal common to both inputs.

The differential amplifier is designed such

that the difference between the two inputs is
amplified (high differential gain), while the
signals appearing at either individual input
(referenced to ground potential) sees a very
low gain (low common-mode gain, usually
loss). The differential amplifier is usually
used as the first component at the receiving
end of a communications link using twisted
pair cable (either shielded or unshielded) as
the transmission medium. This provides a
method to reject any common-mode noise
induced onto the twisted pair transmission
line, including common-mode noise falling
within the useful bandwidth of the commu-
nications link. The figure of merit for the
differential amplifier is its common mode re-
jection ratio (CMRR), computed by dividing
the differential-mode gain by the common-
mode gain.

differential coding

a coding scheme that

codes the differences between samples. See

predictive coding

differential entropy

the entropy of a con-

tinuous random variable. For a random vari-
able

X, with probability density function

f (x) on the support set S, the differential
entropy

h(X) is defined as

h(X) = −

f (x) log f (x)

provided the integral exists.

Also writ-

ten

h(f ), emphasizing the sole dependence

upon the density. See also

differential gain

the amplification factor

of a circuit that is proportional to the differ-
ence of two input signals. The differential
gain may be expressed in percentage form
by multiplying the above amplification fac-
tor by 100, or in decibels by multiplying the
common logarithm of the differential gain by
20.

differential inclusion

a multivalued dif-

ferential equation,

˙x ∈ F(t, x) ,

where F

(t, x) is a nonempty set of velocity

vectors at x

∈ R

for each time

t on some

time interval. The set F

(t, x) can be viewed

as the set of all possible “velocities”

˙x(t) of a

dynamical system modeled by the multival-
ued, or multifunction, differential equation.
A solution x

(t) is an absolutely continuous

function on some time interval whose veloc-
ity vector

˙x lies in the set F(t, x) for almost

all

t. See also

Filippov method

differential kinematics

equation

v =

J (q) ˙q can be interpreted as the differential
kinematics mapping relating the

n compo-

nents of the joint velocity vector to the

r ≤ m

components of the velocity vector

v of con-

cern for the specific task. Here

n denotes

number of degrees of mobility of the struc-
ture,

m is the number of operational space

variables, and

r is the number of operational

space variables necessary to specify a given
task. See also

geometric Jacobian

differential length vector

the vector sum

of the differential length changes in each of
the three coordinate directions along a given
curve.

differential mode gain

for a differential

amplifier, the ratio of the output signal ampli-
tude to the amplitude of the difference signal
between the amplifier input terminals.

differential pair

a two-transistor BJT

(FET) amplifier in which a differential in-
put signal is applied to the base (gate) ter-
minals of the two transistors, the output is
taken differentially from the collector (drain)
terminals, and the emitter (source) terminals
are connected together to a constant current
source. Also known as an emitter-coupled
pair (BJT) or source-coupled pair (FET). This
configuration is often used as the basis of the
differential input stage in voltage feedback
op-amps.

2000 by CRC Press LLC

differential pair oscillator

a device used

instead of a transistor in any LC-oscillator.
Two distinct advantages result from employ-
ing the differential pair as the active element.
The first is that the output signal may be taken
at the collector of transistor that is external
to the oscillator feedback loop, and second is
that, if a tuned circuit is used as a load, the dis-
tortion of the output signal is much less than
it would be for a single transistor oscillator.
The second advantage follows from the fact
that the differential pair collector currents do
not include even harmonic components and,
in addition, the amplitudes of existing high-
order harmonics are smaller than they are for
a single transistor.

differential peak detector

a circuit com-

monly used for the demodulation of FM sig-
nals; it utilizes two peak detectors, a differ-
ential amplifier and a frequency selective cir-
cuit. Also known as a balanced peak detector.

differential protection

a protective relay-

ing scheme in which the currents entering and
leaving the protected line or device are com-
pared.

differential protection unit

a protective

unit based on the difference of currents flow-
ing in and out of a protected zone.

differential

pulse

code

modulation

(DPCM)

(1) a class of methods for

pulse code modulation (or scalar quantiza-
tion) where (linear) prediction is used in or-
der to utilize the temporal redundancy in the
source signal to enhance performance. Also
referred to as predictive PCM or predictive
SQ. See also

pulse-code modulation

scalar

quantization

adaptive differential pulse code

modulation

(2) in image processing, a lossy predictive

coding scheme. In this scheme m pixels in
a causal neighborhood of the current pixel is
used to estimate (predict) the current pixel’s
value. The basic components of the predic-
tive coder comprises predictor, quantizer, and
code assigner.

differential relay

a differential relay is

a protective relay that measures current go-
ing into a device from all sources by means
of a network of paralleled current transform-
ers. Ideally, the operational current is zero for
normal conditions, and rises to a high value
(proportional to fault current) when a fault
comes on inside the differential zone. Dif-
ferential relays are commonly applied in bus
protection, transformer protection, generator
protection, and large motor protection.

differential volume element

in a given

coordinate system, the product of the differ-
ential length changes in each of the three co-
ordinate directions.

differential-mode coupling

pick-up from

an electromagnetic field that induces a
change in potential on both signal leads of
equal magnitude but opposite phase relative
to the ground reference potential.

differentially compounded

a compound

machine in which the flux produced by the
MMF of the shunt field winding and the
flux produced by the MMF of the series
field winding oppose each other. Most of-
ten obtained by incorrectly connecting the
machine, the differentially compounded ma-
chine may demonstrate very erratic behavior.

differentiator

a filter that performs a dif-

ferentiation of the signal. Since convolution
and differentiation are both linear operations,
they can be performed in either order.

(f ∗ g)

(x) = f

(x) ∗ g(x) = f (x) ∗ g

(x).

Thus, instead of filtering a signal and then
differentiating the result, differentiating the
filter and applying it to the signal has the same
effect. This filter is called a differentiator.
A low-pass filter is commonly differentiated
and used as a differentiator.

diffracted beam

diffraction that takes

place when the wavelength of an incident
beam is short compared to the interaction

2000 by CRC Press LLC

distance. Particles exhibit wave-like charac-
teristics in their passage through matter. In
striking a target the incident beam scatters off
nucleons. The scattered waves then combine
according to the superposition principle, and
the peak of this scattered wave is called the
diffracted beam.

diffraction

(1) distortion of an electro-

magnetic wave due to the proximity of a
boundary or aperture.

(2) a bending or scattering of electromag-

netic waves. Basically a redistribution within
a wavefront when it passes near the edge of
an opaque object.

(3) the propagation of light in the presence

of boundaries. It is the property of light that
causes the wavefront to bend as it passes an
edge.

diffraction angle

angle corresponding ap-

proximately to the rate of spreading of an
electromagnetic wave that has been trans-
mitted through an aperture; with Gaussian
beams the far field half angle for a radius
equal to the spot size.

diffraction coefficient

in the Geometric

Theory of Diffraction, the coefficient that is
proportional to the contribution to the scat-
tered field due to the fringe currents near an
edge or corner of a scattering target.

diffraction efficiency of Bragg cell

ra-

tio of the intensity of the principal diffracted
beam to the intensity of the undiffracted
beam.

diffraction grating

an array of reflecting

or transmitting lines that mutually enhance
the effects of diffraction.

diffraction loss

loss from an electromag-

netic beam due to finite aperture effects.

diffraction tomography

generalization

of computerized tomography incorporating
scattering effects.

diffuse density

signal that has uniform

energy density, meaning that the energy flux
is equal in all parts of a given region.

diffuse intensity

the energy scattered in

all directions out of the forward or specular
directions. Sometimes also called incoherent
component of the intensity.

diffuse multipath

the result of multipath

propagation observed as overlapping signal
components, due to delay differences of mul-
tipath components being less than the delay
resolution of the signal. Observable in the de-
lay power spectrum as a continuous distribu-
tion of power over delay. See also

multipath

propagation

delay power spectrum

diffuse scattering

the component of the

scattering from a rough surface that is not in
the specular direction. It is caused by reflec-
tions from local surfaces oriented in planes
different from that of the mean surface. See
also

specular scattering

diffuse transmittance

a transmitted sig-

nal that has uniform energy density.

diffusion

a region of a semiconductor into

which a very high concentration of impurity
has been diffused in order to substantially in-
crease the majority carrier concentration in
that region.

diffusion pump

second stage of the vac-

uum system. Hot oil showers the particles in
a vacuum and creates a better vacuum. Af-
ter a mechanical (roughing) pump is used to
remove about 99.99% of the air in the beam
tube, the remaining air can then be removed
by a diffusion pump, down to about 1

−9

torr.

diffusion under field (DUF)

a local thin

layer of semiconductor with a very high car-
rier concentration located under and in con-
tact with the collector of a vertical bipolar
transistor to provide a low-resistivity connec-
tion to it.

2000 by CRC Press LLC

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