Widrow-Hoff learning rule
a gradi-
ent descent learning rule for calculating the
weight vector
Ew for a linear discriminant
which minimizes the squared error objective
function. The vector
Ew is modified as
Ew(n + 1) = Ew(n) + α(n)(d − Ew
T
(n)Ex)Ex
where
Ex is an input vector, d is the desired
output, and
Ew(n) is the weight vector at iter-
ation
n.
Wien bridge
a bridge circuit where one
arm is a series connection of a resistor and ca-
pacitor, another arm is a parallel connection
of a resistor and capacitor (these two arms are
called reactance arms), and two other arms
(called ratio arms) are resistors. The balance
detector is connected between the common
point of ratio arms and the common point of
reactance arms, a sinusoidal voltage source
is connected to another bridge diagonal. The
Wien bridge was initially designed as a fre-
quency measuring circuit; now the main part
of its application is the Wien bridge oscilla-
tor.
Wien bridge oscillator
(1) an oscillator
where the Wien bridge is used in the am-
plifier feedback. The frequently used cir-
cuit includes equal resistors and equal ca-
pacitors in reactance branches of the bridge;
this arrangement provides easy continuous
tuning of the oscillation frequency.
Tun-
ing of the bridge providing high indirect fre-
quency stability of oscillations is easily com-
bined with application of an operational am-
plifier as oscillator active element. The cir-
cuit of amplitude control is also easily at-
tached. All these advantages provide wide
spread of Wien bridge oscillators in high and
low radio frequency ranges of applications.
(2) a form of feedback oscillator that uses
a noninverting amplifier along with a feed-
back path that produces a phase shift of zero
degrees at the operating frequency. The feed-
back network contains only two reactive ele-
ments of the same type.
Wiener, Norbert (1894-1964)
mathe-
matician whose contributions include Brow-
nian motion, stochastic processes, general-
ized functions, harmonic analysis, control
theory, and optimal filtering. Established the
field of cybernetics, author of “Cybernetics:
or Control and Communication in the Animal
and the Machine.”
Wiener filter
filter that attempts to re-
duce signal noise by separating and sup-
pressing the power spectrum of the noise
from the power spectrum of the signal. The
uncertainty in the estimation of the noise
power spectrum will cause the signal to be
smoothed. Also known as the least-mean
square filter.
Wilkinson coupler
a coupler that splits
a signal into a number of equiphase and
equiamplitude parts. It provides isolation be-
tween output terminals by connecting resis-
tors between each output terminal and a com-
mon junction. A coaxial type coupler was
first proposed by Dr. Wilkinson. In recent
years, not only coaxial type but also MIC
(microwave integrated circuit) type Wilkin-
son couplers are practically used for various
kinds of microwave circuits.
Williams tube memory
a memory device
based on electric charges being stored on the
screen of a cathode ray tube. Now obsolete.
Wilson central terminal
reference point
for forming most of the standard ECG leads.
It is the average of the right arm, the left arm,
and the left potentials. It is a time-varying
reference.
Winchester disk
a type of magnetic disk
for data storage. Its characteristic property
is that the disk and the read-write head are
placed in a hermetically sealed box. This
allows higher recording density as the read-
write head can be moved closer to the disk
surface. See also
disk head
.
c
2000 by CRC Press LLC