Page 293 - Radar Technology Encyclopedia

283 MTI, PRF-diversity MULTIVIBRATOR

R =c/2f is the unambiguous range. The disadvantage of called reactive. A reactive 2N multiport is characterized by
r
u
batch processing is that only a single output is obtained from N(N + 1)/2 real parameters.
each batch, decreasing the number of integrated pulses by the Examples of nondissipative multiports include ideal
factor m + p for an m-pulse canceler with p fill pulses. DKB directional couplers and ideal circulators.
Ref.: Barton (1988), p. 239. A reciprocal multiport is a microwave multiport that has
a symmetrical, nonnormalized conductance matrix. For recip-
Staggered-PRF MTI uses a train of pulses with pulse-to-
rocal multiports, the theorem, familiar from electrodynamics,
pulse stagger (change of PRI) to reduce target loss in blind
regarding the reciprocity of any two inputs applies (see also
regions of the velocity response. The clutter cancellation
RECIPROCITY). Normalized matrices of conductivity,
notch remains at zero velocity and is usually repeated at some
resistance, and scattering are also symmetrical.
multiple of the average blind speed (see Fig. M30). Cancela-
A reciprocal 2N-pole is described by N(N + 1) complex
tion of multiple-time-around clutter is impossible when using
parameters.
pulse-to-pulse stagger. DKB
The property of reciprocity is ensured by the absence of
Two-frequency MTI uses two carrier frequencies that are anisotropic electromagnetic media inside the multiport (e.g.
beat together to generate a difference frequency as the IF. magnetized ferrites or plasma).
When this is processed in an MTI canceler, the resulting blind A symmetrical multiport is one for which renumeration
speed is that of a carrier equal to the difference frequency. of inputs is possible, without leading to a change in the multi-
The spectral spread of the clutter is increased by the beating port matrices. We distinguish between geometrical and elec-
of the two signals, resulting in decreased improvement factor. trical symmetry. The latter is achieved through special
It has been found that the two-frequency MTI gives no net selection of the nominal characteristics of the elements of the
advantage over a properly designed single-frequency system, multiport. Geometrical symmetry is manifested in the fact
in most applications, and it has not found wide use. SAL that the multiport remains like itself during symmetrical con-
Ref.: Skolnik (1980), p. 147. versions (rotations of the multiport about its axis, mirror
images relative to planes of symmetry). Geometric symmetry
MULTIPATH (see ERROR, radar; PROPAGATION).
is easily established a priori and entails electrical symmetry.
MULTIPLEXING is the combining of two or more signals Mathematically, symmetry of a multiple is described by the
into a single wave (the multiplex wave) from which the sig- introduction of a special matrix of symmetry G, which is
nals can be individually recovered. commutative with the matrices of the multiple parameters Z:
GZ º ZG
MULTIPORT, microwave. A microwave multiport is any
These equations for the resistance matrix, conductance
combination of conductors, dielectrics, and other microwave
matrix, and scattering matrix are used to reduce the number of
components that has several inputs in the form of transverse
independent parameters of a multiport. A double waveguide
sections of transmission lines with type types of waves in
T-bridge is an example of a symmetrical multiport. IAM
each line.
A certain fictive pair of poles in the supply transmission Ref.: Sazonov (1988), pp. 86–92.
line is assigned to each input of a microwave multiport. For A MULTIVIBRATOR is a two-stage oscillator with positive
this reason the term “2N-pole” or “2N-port” means N supply feedback between the stages, which generates video pulses
transmission lines with N types of waves. with the shape close to a rectangular one. They can operate in
Multiports are subdivided into passive (without amplifi- two modes: a self-oscillations mode and a waiting mode. The
cation or microwave power generation inside the multiport) first class device is a two-stage amplifier with resistors, the
and active (with the opposite properties). The parameters of a output of each amplifier being connected with input of
multiport are written in the form of multiport matrices, which another one via a separating capacitor (Fig. M33). The pro-
mathematically describe the properties of reciprocal, nondis- cess of generation consists of opening one transistor and clos-
sipative, symmetrical, and other types of multiports. ing another one in succession. As a result, the pulses of
In a nondissipative multiport there are no internal losses different polarity and duration appear at transistors output.
of electromagnetic energy (losses are vanishingly small Pulse duration t, pulse repetition period T , and pulse
p
p
against the background of the total power applied to the amplitude A are defined as
p
inputs). A condition of nondissipation is this equation for the 2 + ( I R ¤ E )
c0 b1
c
resistance matrix Z: t p1 = R C ln ----------------------------------------
b1 2
1 +
E )
(
I R
¤
c0 b1 c
T*
Z + Z = 0
2 + ( I R ¤ E )
c0 b2
c
where T is the transposition sign, and * is the complex conju- t p2 = R C ln ----------------------------------------
b2 1
E )
I R
¤
1 +
(
c0 b2
c
gation sign. There is an analogous condition for the conduc-
tance matrix Y. T = t + t ; A = A = E c
p2
p1
p
p2
p1
A nondissipative and reciprocal multiport often has where I = initial current at collector.
c0
purely imaginary matrices Z and Y and for this reason are A waiting multivibrator possesses a single stable state
and generates a single pulse when external voltage is applied.