Page 213 - Electrical Engineering Dictionary

differential pair oscillator a device used differential relay a differential relay is
instead of a transistor in any LC-oscillator. a protective relay that measures current go-
Two distinct advantages result from employ- ing into a device from all sources by means
ing the differential pair as the active element. of a network of paralleled current transform-
The first is that the output signal may be taken ers. Ideally, theoperationalcurrentiszerofor
at the collector of transistor that is external normal conditions, and rises to a high value
to the oscillator feedback loop, and second is (proportional to fault current) when a fault
that, ifatunedcircuitisusedasaload, thedis- comes on inside the differential zone. Dif-
tortion of the output signal is much less than ferential relays are commonly applied in bus
it would be for a single transistor oscillator. protection, transformer protection, generator
The second advantage follows from the fact protection, and large motor protection.
that the differential pair collector currents do
not include even harmonic components and, differential volume element inagiven
in addition, the amplitudes of existing high- coordinate system, the product of the differ-
order harmonics are smaller than they are for ential length changes in each of the three co-
a single transistor. ordinate directions.

differential peak detector a circuit com- differential-mode coupling pick-up from
monly used for the demodulation of FM sig- an electromagnetic field that induces a
nals; it utilizes two peak detectors, a differ- change in potential on both signal leads of
ential amplifier and a frequency selective cir- equal magnitude but opposite phase relative
cuit. Alsoknownasabalanced peak detector. to the ground reference potential.

differential protection a protective relay- differentially compounded a compound
ingschemeinwhichthecurrentsenteringand machine in which the flux produced by the
leaving the protected line or device are com- MMF of the shunt field winding and the
pared. flux produced by the MMF of the series
field winding oppose each other. Most of-
differential protection unit a protective ten obtained by incorrectly connecting the
unit based on the difference of currents flow- machine, the differentially compounded ma-
ing in and out of a protected zone. chine may demonstrate very erratic behavior.

differential pulse code modulation differentiator a filter that performs a dif-
(DPCM) (1) a class of methods for ferentiation of the signal. Since convolution
pulse code modulation (or scalar quantiza- and differentiation are both linear operations,
tion) where (linear) prediction is used in or- they can be performed in either order.
der to utilize the temporal redundancy in the
0
0
0
source signal to enhance performance. Also (f ∗ g) (x) = f (x) ∗ g(x) = f(x) ∗ g (x).
referred to as predictive PCM or predictive
SQ. See also pulse-code modulation, scalar Thus, instead of filtering a signal and then
quantization, adaptive differential pulse code differentiating the result, differentiating the
modulation. filterandapplyingittothesignalhasthesame
(2) in image processing, a lossy predictive effect. This filter is called a differentiator.
coding scheme. In this scheme m pixels in A low-pass filter is commonly differentiated
a causal neighborhood of the current pixel is and used as a differentiator.
used to estimate (predict) the current pixel’s
value. The basic components of the predic- diffracted beam diffraction that takes
tivecodercomprisespredictor, quantizer, and place when the wavelength of an incident
code assigner. beam is short compared to the interaction

c
2000 by CRC Press LLC