Page 108 - Radar Technology Encyclopedia

98 CORRELATOR COTTON-MOUTON EFFECT

CORRELATOR. A correlator is a device that calculates the and also for coding and decoding. Sometimes this correlator
correlation function of random processes. It comprises a is called a surface-charge correlator. IAM
device to cross-multiply the input signals and an integrator in Ref.: Gassanov (1988), p. 227.
which the products from the multiplier are summed. The
A functional correlator is one in which the multiplication of
existence of a statistical relationship between the input sig-
the analyzed processes is replaced by their functional trans-
nals results in the appearance of constant components at the
form f (x,y). An example of a functional correlator is the so-
output of the multiplier, fluctuations in which are reduced by
called sign correlator, or polarity coincidence correlator, for
the integrator.
which f (x,y) = f (x) f (y) = sign(x)sign(y).
2
1
Depending upon the type of multiplier, correlators are
Correlators based on an annular balanced mixer are often
categorized as being either direct or indirect. Direct correla-
used at radio and video frequencies. The mixer is operated as
tors use physical effects that generate quantities proportional
a nonlinear transform device, in the “switch” mode, with one
to the product of the inputs: various types of modulation, the
of the voltages significantly greater than the other. When
Hall effect, and so on. Indirect correlators use operations
there is a correlation between the analyzed signals, there
other than time-domain multiplication; for example, multipli-
appears at the output a signal at the difference frequency with
cation of the signals’ Fourier transforms with the inverse
amplitude proportional to the correlation coefficient. IAM
transform (as in a functional correlator). Some types of corre-
Ref.: Vinokurov (1972), p. 51.
lators are described below. IAM
An optical correlator is one that uses optical devices to per-
Ref.: Vinokurov (1972), p. 51; Zmuda (1994), Ch. 15.
form the required multiplication and integration operations.
An acousto-optical correlator is an optoelectronic device in
This relies on the change in amplitude and phase of a light
which correlation is performed on the basis of the diffraction
wave as it passes through an inhomogeneous medium and on
of coherent optical radiation by two acoustic waves, created
the focusing action of a lens. Depending on whether the illu-
by the input signals and propagating in opposite directions.
mination is coherent, such correlators are categorized as
There are two types of acousto-optical correlator: those that
being either coherent-optical or incoherent-optical correla-
perform spatial integration of the output light distribution and
tors.
those that perform temporal integration of the output signal
Common to all optical correlators are a pair of imprinted
with a photoreceiver. These two types of correlator differ in
transparencies that modify the amplitude and phase of the sig-
the construction of the optics and the existence in the spatial
nal in accordance with the Fourier transform of the imprinted
correlator of a strip of photodetectors the output currents of
signals; one of several types of optical lens; and a photodetec-
which are integrated, in place of a single photodetector with
tor at the output (e.g., a photoelectronic multiplier).
an integrating filter. The duration of signals processed in a
Coherent optical correlators use coherent illumination
spatial correlator is on the order of microseconds or tens of
and possess a number of advantages relative to noncoherent
microseconds, while a temporal correlator may process sig-
optical correlators. They are suitable for operation with com-
nals up to tens of milliseconds in duration.
plex functions, the accuracy of the correlation calculation
The main elements of an acousto-optical correlator are a
does not depend upon diffraction effects within the optics,
pair of acousto-optical modulators, which determine the basic
and it is possible to form filters with given transfer character-
properties of the correlator: the carrier frequency (on the
istics with great flexibility and simplicity.
order of 1 GHz), and the passband (several hundred mega-
The advantages to incoherent optical correlators include
hertz). IAM
their simple construction and the fact that they may be used
Ref.: Kulikov (1989), pp. 110, 113; Zmuda (1994), p. 405.
with objects that emit their own (generally incoherent) light.
A charge-coupled device correlator is a correlator con- In accordance with the manner in which the correlation
structed with charge-coupled devices, generally containing function is calculated, optical correlators are further classified
two delay lines, from the output taps of which samples of the as being either spatially independent, in which case all terms
input signals are taken to be multiplied and summed. A distin- of the correlation function are calculated simultaneously, pro-
guishing feature of a charge-coupled device delay line is its ducing the correlation function as the output field, and spa-
invariance to the delay of one signal relative to another. A tially dependent. A spatially dependent correlator calculates
large portion of the correlator is occupied by the multiplier, one term of the correlation function for a given spatial posi-
which is usually implemented using field-effect transistor tion of the transparencies, thus greatly increasing the calcula-
technology. tion time and the required number of samples of the signals
The advantages of charge-coupled device correlators are being analyzed. IAM
realized at video frequencies. The maximum clock frequency Ref.: Baklitskiy (1986), p. 179; Nathanson (1990), p. 311, Brookner (1977),
at which the correlator may operate is inversely proportional pp. 235–237, 244; Zmuda (1994), p. 403.
to the length of the multiplier, and, for an accuracy exceeding COTTON-MOUTON EFFECT. The Cotton-Mouton effect
1%, is 5 to 8 MHz. is a phenomenon entailing the conversion of the linear polar-
Charge-coupled device correlators are used in synchro- ization of waves into elliptical polarization as a result of the
nizers and optimum filters, as functional transform devices, transverse propagation (perpendicular to the direction of the