Page 196 - Radar Technology Encyclopedia

filter, coherent [circulating] memory filter, digital 186

.
This analog technique has been replaced in modern radar by the maximum of the relative error, d Thus for N = 16, d =
0.
digital filtering such as the fast Fourier transform. DKB 0.0136, for N = 32, d = 0062. The cutoff frequency of the
Ref.: Skolnik (1970), pp. 17.56–17.57. differentiating filter of an even order may reach half the sam-
pling frequency. IAM
A crystal filter is a filter whose resonant elements consist of
piezoelectric crystal structures. Ref.: Rabinev (1975), pp. 143, 187; Barton (1964), pp. 365–367, 422–428,
(1988), p. 426.
A dielectric filter is a microwave filter based on transmission
A diffraction filter is an analog filter consisting of a nondis-
lines with dielectric loading. The basis of the dielectric filter
persive ultrasonic delay line of wedge shape with point trans-
is usually dielectric cavities, which constitute dielectric bod-
ducers. In the process of transmitting the ultrasonic
ies of a specific shape (sphere, cylinder, torus, etc.), whose
oscillations, diffraction occurs in the acoustic medium. One
relative dielectric constant is greater than that of the sur-
distinguishes between diffraction filters with two transducers,
roundings. The frequency characteristic of the filter is formed
or with a single input-output transducer.
through reflection of waves from the boundary of the dielec-
Pulse compression diffraction filters provide high linear-
tric media with different constants.
ity of the modulation slope during the pulse, wide frequency
They are subdivided into dielectric filters using free
bands (500 MHz at a central frequency of 9,250 MHz), and
space as a medium, waveguide dielectric filters and strip
high compression ratios (1,200). The high losses (60 to 75)
(coaxial) dielectric filters. Filters of the first type usually con-
dB are the drawback of the filters. IAM
sist of a number of dielectric layers with low and low dielec-
Ref.: Skolnik (1970), p. 20.7; Shirman (1974), p. 137.
tric constants, alternating with each other. Such filters are
used for frequency and angle selection (suppression of side- A digital filter is a nonlinear filter processing digital signals.
lobes) of signals in the aperture of antenna systems. The main source of the nonlinearity is quantization of the sig-
Strip dielectric filters usually constitute a metal housing nal, but if the number of digits (bits) is sufficiently high the
with microstrip line whose dielectric substrate contains quantized signal is equivalent to a discrete signal. In this case,
dielectric cavities closely to the current conductor, usually in the basic discrete filtering equation (see linear filter) is appli-
the form of a cylinder. Their connection to the line and with cable, resulting in the following equations that relate the out-
each other is through the magnetic field. The unloaded Q-fac- put signal, y(t) to the input, x(t), both being digital codes at N
tor of such filters is 1,000 or more. sample points:
Dielectric resonator-based filters are based on the phe- M 1– N 1–
nomenon of the resonance of electromagnetic waves inside a
(
(
y = – å b × yn – j ) + å a × xn – k )
m
n
k
dielectric volume. The filter is usually a cylinder with a
m = 1 k = 0
height-to-diameter ratio of 0.3 to 0.5 on a dielectric with per-
meability e = 10 to 40 (ceramic and so forth). The filter is dis- Parameters b and a are called filter (weighting) coefficients
k
m
=
tinguished for its small size (6 mm for e 36 at a frequency and can be either constant or variable (dependent on n). For a
of 8.5 GHz), insignificant losses (inherent Q-factor 5 to 10 ´ nonrecursive filter, b = 0, and:
m
3
10 ) in the range from decimeter to centimeter and millimeter N 1–
waves. y = å a × xn – k )
(
n k
Multiresonant filters with direct links among dielectric
k = 0
resonators located on a flat plate at tuning distances from one
and a is a set of sampled filter impulse response values,
k
another are used to obtain the necessary band and shape of the
a = h . The parameter N is the filter length.
k
k
frequency response. Dielectric filters possess vitally smaller
The first expression for y provides the basis for setting
n
dimensions and mass than waveguide filters, but higher losses
the digital filter configuration, as it shows that three opera-
(1 to 3 dB). They are used in radar apparatus of moving
tions are performed by the filter: (1) storing the input samples
objects in a frequency range of 0.1 to 100 GHz with realiza-
(delay), (2) multiplication (of samples by weights), and
tion of frequency characteristics both of the Chebyshev type
(3) summation. The complexity of filter implementation is
and with a maximally flat peak. IAM
defined by the number of memory cells (registers) required to
Ref.: Kapilevich, B. Yu., Zarubezhnaya Radioelektronika, no. 5, 1980, in
store the input data and coefficients, and by the number of
Russian; Sazonov (1988), p. 125.
multiplication and summing operations.
A differentiating filter is a high-pass filter that allows evalu- Digital filtering can be realized either in the time or fre-
ation of the derivative of the input signal. The frequency quency domain. Filtering in the time domain is described by
response, H(f) of the filter has the form H(f) = j2pf. In analog the equations above, and in radar applications a nonrecursive
form, the filter is implemented with various transmission transversal filter is often used. In the frequency domain, the
lines (see differentiating circuit), either with lumped or dis- fast Fourier transform is often used in such applications as
tributed parameters. wideband pulse compression. A feature of the digital filter is
Digital optimum differentiating filters are characterized that its frequency response has a periodic structure because of
by the order N of the filter, the cutoff frequency F , which is the sampled nature of the input signals. Digital filters are the
p
the highest frequency at which the filter still operates, and by main types used in modern signal processors. Their advan-