known. The most frequently used are worst-
case measure and quadratic measure of sensi-
tivity. See also
sensitivity
,
sensitivity index
.
sensitivity reduction
reducing sensitivity
is very desirable in active filter realization.
Some general suggestions, at the stage of ap-
proximation, ensuring that filter realizations
will have low sensitivities to component vari-
ations include the following:
1. Increasing the order of approximation
and introducing a redundancy.
2. Using predistorted (tapered) specifica-
tions in the vicinity of the passband edge.
3. Using transfer functions with a limited
value of the maximum
Q of the transfer func-
tion poles. The realization itself gives the cir-
cuits lower sensitivities if the filter is realized
as the doubly terminated lossless structure or
its active equivalent.
sensor
a transducer or other device whose
input is a physical phenomenon and whose
output is a quantitative measurement of that
physical phenomenon. Physical phenomena
that are typically measured by a sensor in-
clude temperature or pressure to an internal,
measurable value such as voltage or current.
sensor alignment
alignment of sensors so
as to correct the time delay differences arising
from spatial differences.
sensorless control
a control method in
which mechanical sensors are replaced by an
indirect estimation of the required variable.
separability
the separable property for the
signal or system such that the signal or system
representation can be expressed by the prod-
uct of component terms, each depending on
fewer independent variables.
separable data
a 2-D signal that can be
written as a product of two 1-D signals.
separable filter
a filter that can be ap-
plied in two or more operations without any
change in overall function, thereby gaining
some computational advantage. In particu-
lar, a 2-D mean filter can be re-implemented
identically as two orthogonal 1-D mean fil-
ters, and is therefore separable. However,
a 2-D median filter is non-separable, as its
action is not in general identical to that of
two orthogonal 1-D median filters. See also
separable transform
.
separable image transform
a 2-D sepa-
rable transform used to transform images.
separable kernel
for a 2-D transform a
kernel that can be written as the product of
two 1-D kernels. For higher dimension trans-
forms a separable kernel can be written as the
product of several 1-D kernels. See
separable
transform
.
separable transform
a 2-D transform that
can be performed as a series of two 1-D trans-
forms. In this case the transform has a separa-
ble kernel. The 2-D continuous and discrete
Fourier transforms are separable transforms.
In higher dimensions a separable transform
is one that can be performed as a series of
1-D transforms. See also
separable filter
.
separately excited DC machine
a DC
machine where the field winding is supplied
by a separate DC source. Separately excited
generators are often used in feedback con-
trol systems when control of armature volt-
age over a wide range is required.
sequency
in a transform, the number of
zero-crossings of a particular basis function.
By extension used to refer to the transform
coefficient that corresponds to a particular
basis function. For example, in the discrete
cosine transform the zero sequency coeffi-
cient is the one for which the basis function
is flat (and therefore has no zero crossings),
often called the DC coefficient. Sequency is
roughly analogous to frequency; higher se-
quency basis functions correspond to higher
frequency components of signal energy.
c
2000 by CRC Press LLC