van der Pol oscillator
an oscillator or os-
cillating system described by the equation
d
2
x
dt
2
− µ
1
− x
2
dx
dt
+ x = 0
This equation is mentioned in almost any
book on oscillators or on nonlinear mechan-
ics. The reason is that this equation is rela-
tively simple, yet it successfully lumps to-
gether two rather complex processes asso-
ciated with oscillators, namely, the process
of generation of periodic waveform and the
process of automatically stabilizing the am-
plitude of this wave. The attempts to improve
the solutions of the van der Pol equation and
to apply to design of low-distortion oscilla-
tors can be traced to recent publications.
Vander Lugt filter
encoded optical mask
for representing, in the Fourier-transform
domain, the reference or library functions
needed in an image correlator; encoding is
performed holographically.
vanishing point
the point in the perspec-
tive projection plane in which a system of
3-D parallel lines converge.
Vapnik–Chervonenk (VC) dimension
a
measure of the expressive power of a learn-
ing system with binary or bipolar outputs.
For neural networks it is closely related to
the number of weights in the network. For
single-layer networks it is simply equal to
the number of weights (including biases) but
for multilayer and other networks, analytic
expressions for VC dimension are not avail-
able.
vapor cooling
a cooling technique for
power vacuum tubes utilizing the conversion
of hot water to steam as a means of safely
conducting heat from the device and to a heat
sink.
VAR
See
volt-ampere-reactive
.
varactor
a reverse biased PN or Schottky
diode that uses the voltage variable depletion
region as a tuning element or as a nonlinear
frequency multiplier.
varactor diode
a diode designed to have
a repeatable and high capacitance vs. reverse
voltage characteristic. A two terminal semi-
conductor device in which the electrical char-
acteristic of primary interest is the voltage
dependent capacitance.
varactor tuner
a tuning circuit at the in-
put of a television receiver that uses a varac-
tor diode. The tuning capability comes from
the characteristic of a varactor, or varicap, to
function as a voltage-sensitive capacitance.
variable bit rate (VBR)
describes a traf-
fic pattern in which the rate at which bits are
transmitted varies over time; such patterns
are also referred to as bursty. VBR sources
often result from compressing CBR sources,
for example, a 64 kbps voice source in its raw
form has a constant bit rate; after compres-
sion by removing the silence intervals, the
source becomes VBR.
variable frequency drive
electric drive
system in which the speed of the motor can be
varied by varying the frequency of the input
power.
variable length code
to exploit redun-
dancy in statistical data, and to reduce aver-
age number of bits per word luminance lev-
els having high probability are assigned short
code words and those having low probabil-
ity are assigned longer code words. This is
called variable length coding or entropy cod-
ing. See also
entropy coding
.
variable loss
machine loss that changes
with a change in the mode of machine oper-
ation such as loading, temperature and cur-
rent. For example, in a transformer, the wind-
ing losses are a function of the load current,
while the core losses are almost independent
of the load current.
c
2000 by CRC Press LLC