Page 253 - Radar Technology Encyclopedia

243 local oscillator, reflex klystron LOSS, in radar

A reflex klystron local oscillator is a local oscillator in the with minimum accompanying phase noise. DKB
centimeter or millimeter waveband that uses a reflex klystron Ref.: IEEE (1993), p. 1,273; Skolnik (980), p. 105, (1990), p. 3.12.
with an external or internal three-dimensional cavity as its
A tracking LO is one whose frequency is automatically
active component. Klystrons with external cavities are used in
tuned to the frequency of the received (analyzed) signal. It is
the lower microwave and UHF bands (4 to 60 cm wave-
used, for example, in doppler trackers. In this case, the fre-
length) and allow frequency retuning of ±15%. Local oscilla-
quency of the low-frequency modulation of a signal of the
tors based on klystrons with an external cavity are
local oscillator is tuned to a value close to the doppler shift.
distinguished by high resistance to mechanical influences, but
The tuning is done by the control circuit, which measures the
they have a smaller retuning range, not exceeding ±10%.
frequency difference of the local oscillator and the doppler
Stabilized power sources in the circuits of the reflector,
signal and contains the low-pass filter, an amplitude and
cavity, and filament are used for stabilization of the frequency
phase detector, and an integrator. IAM
of klystron local oscillators. A high-quality three-dimensional
Ref.: Supryaga (1974), p. 109; Skolnik (1990), p. 19.13.
cavity is connected to the klystron. The local oscillator
requires reliable shock isolation. A vacuum-tube LO uses a vacuum tube with grid control, or
Klystron local oscillators are used in landing and naviga- a microwave tube. Usage of the former (special types of
tional noncoherent radars, test equipment, and other appara- microwave triodes and tetrodes) is limited to frequencies of 2
tus. IAM to 3 GHz. Because of the high phase instability and frequency
restrictions, their use is limited in the local oscillator of mod-
Ref.: Ridenour (1947), pp. 414–416; Pereverzentsev (1981), p. 242; Zherebt-
sov (1989), p. 330. ern radars. Klystron sources with cavity stabilization are more
suitable for coherent radar systems. IAM
A resonator-stabilized LO uses high-quality microwave res-
Ref.: Ridenour (1947), p. 414; Druzhinin (1967), p. 373.
onators. Depending on the frequency band and the require-
ments for frequency stability, the following may be used: LOCK-ON, radar (see TARGET lock-on).
surface-acoustic-wave resonators (up to 1 to 2 GHz); three-
dimensional resonators of material with a low temperature LOSS, in radar. A loss is a value opposite to a gain in a radar
coefficient of linear expansion, for example invar (in the deci- channel. It shows the degree by which the power P of a sig-
2
meter and centimeter bands); or dielectric resonators (in a nal in the presence of a specified degrading factor is less than
band of 1 to 20 GHz), which have become common thanks to the power P without that factor, or the amount by which the
1
their high stability of characteristics and compatibility of input signal must be increased to overcome the factor. The
design with hybrid-integrated technology. usual notation is L, and when expressed in decibels it is
The resonators are connected to the frequency-generating P 1
circuit of the local oscillator; for example, in the gate circuit L dB = 10log ------ 2
P
in an oscillator based on a field-effect transistor in a circuit
In this encyclopedia, values and equations for loss will be in
with a common source, or in a frequency autotuning circuit of
terms of the power ratio, rather than the decibel value, unless
a klystron oscillator. In the last case, the autotuning circuit
the latter is specifically noted. The loss may be defined in
contains a microwave discriminator based on a hollow reso-
terms of signal-to-noise ratio, rather than signal power only
nator, an operational amplifier-integrator that provides the
(e.g., see matching loss)
transient characteristics necessary for keeping the frequency
The total radar loss consists of many components origi-
of the klystron very close to the average frequency of the dis-
nating throughout the radar channel, that consists of the tar-
criminator. IAM
get, the propagation medium, and the radar itself. Proper
Ref.: Gassanov (1988), p. 176; Skolnik (1970), pp. 5.13–5.15. calculation of the loss budget is important for accurate predic-
A semiconductor LO uses a Gunn diode, field-effect transis- tion of radar performance. Although each component of radar
tor, or tunnel diode for generation of low power. Stabilization loss can be small, the total can degrade performance consider-
of oscillations is done with an external high-quality resonator ably. For that reason, the origin of radar losses must be under-
with a Q-factor up to 20,000. Synchronization of semicon- stood and their value minimized by proper radar design.
ductor local oscillators can be done in various modes: at input Radar losses are of four distinct types:
operating frequency, or at a harmonic or subharmonic of input (Type 1) Antenna and propagation loss factors, resulting
frequency. To increase stability of oscillations of diode local from effects other than attenuation, and from target location
oscillators, their self-synchronization is used. Toward this in the nonscanning coordinate.
end, some of the output power is sent to the oscillator through (Type 2) Transmit RF loss factors, which attenuate the
an additional feedback circuit. Semiconductor local oscilla- signal passing from the transmitter to the target, relative to
tors are used in transmitting and receiving modules of ideal equipment operating over a free-space path.
antenna arrays and in modern solid-state receivers. IAM (Type 3) Receive RF loss factors, which attenuate the
Ref.:Fink (9182), p. 13.112; Gassanov (1988), pp. 177, 180. signal passing from the target to the receiver, relative to ideal
equipment operating over a free-space path.
A stable LO (STALO) is the first local oscillator in a coher-
ent radar system, providing downconversion to the first IF