Page 301 - Radar Technology Encyclopedia

291 pi-mode oscillation oscillator, backward-wave-tube

Pi-mode oscillation appears at combination frequencies aris- amplitrons, and gyrotrons, which are also distinguished by
ing in an oscillator such as a magnetron where phases of adja- their high frequencies (up to 100 GHz).
cent resonators differ by p radians. AIL Microwave oscillators are used in transmitters and local
Ref.: Popov (1980), p. 180. oscillators of radars, in measurement apparatus, in pumping
oscillators of parametric amplifiers, exciters, and so forth.
Self-sustaining oscillation is maintained in a circuit due to
IAM
an external energy source, through automatic regulation of
the input of energy to the circuit. Losses of the energy of the Ref.: Gassanov (1988), p. 174; Rakov (1970), vol. 2, p. 5
oscillations in the circuit are compensated for automatically A amplitron oscillator is one that uses an amplitron for gen-
by energy from a source. Self-sustaining oscillations are con- eration or amplification of the frequency of the master oscil-
tinuous oscillations. AIL lator. In the first case, the oscillator is also called a stabilitron.
Ref.: Popov (1980), p. 10. Depending on the operating mode, the amplitron is used in
pre-amplification stages with a gain coefficient of 12 to 15 dB
Sinusoidal oscillations are the simplest periodical oscilla-
and low efficiency, and in terminal stages of radar transmit-
tions described as a function of time t by
ters with a gain coefficient of 6 to 8 dB and efficiency of 55 to
a(t) = A sin(wt + f) 80%. An amplitron provides high phase stability (less than 3°

to 5° per 1% change in operating voltage), bandwidth (7 to
where A is the amplitude, w is frequency, and f is initial
0
phase. In Russian literature this is termed harmonic oscilla- 20%), and output power (2 to 5 MW). To eliminate the excita-
tion inherent in a amplitron at low levels of output power, fer-
tion. AIL
rite duplexers are used at the input and output of instruments
Ref.: Popov (1980), p. 80.
in multi-stage amplitron-based transmitters.
OSCILLATOR, microwave. A microwave oscillator is a Amplitron oscillators are used in certain air-traffic con-
device that converts the energy of a power source into the trol radars. IAM
energy of microwave oscillation. Along with a self-excited Ref.: Perevezentsev (1981), pp. 167–172.
oscillator (master oscillator), the oscillator often includes a
An avalanche transit-time diode oscillator is an oscillator
frequency multiplier and power amplifiers.
that uses an avalanche transit-time diode possessing a nega-
The basic electrical parameters of an oscillator are its
tive dynamic conductivity in avalanche breakdown mode as
power level, efficiency, operating frequency and range of tun-
its active component.
ing, stability of frequency and output power, noise level close
The basic oscillation mode is the IMPATT mode, which
to the carrier, and frequency spectrum.
allows production of hundreds (tens) of milliwatts of power at
Oscillators are classified according to purpose, type of
frequencies on the order of units (tens) of gigahertz, with an
active components, value of parameters (power, frequency
efficiency of 15 to 20%. The frequency limit of operation of
range, etc.), types of stabilization of oscillation.
these oscillators is 200 GHz. With retuning of the oscillating
In terms of type of active components, oscillators are
circuit, the range of oscillation may reach an octave. Several
divided into two large classes: semiconductor and tube oscil-
times more power and efficiency up to 50 to 70% are
lators (Table O1). Low-power oscillators (units of milliwatts)
obtained in the TRAPATT mode. However, owing to the great
include klystron oscillators, tunnel-diode oscillators, and low-
amount of noise, criticality with respect to tuning, and limita-
power transistor oscillators. Other semiconductor oscillators
tion of operating frequency to 10 GHz, at which the transis-
are used in more powerful devices (hundreds of milliwatts,
tors operate effectively, oscillators using avalanche transit-
tens of watts).
time diodes are not widely used in the TRAPATT mode.
Table O1 Oscillators using avalanche transit-time diodes can have
Classification of Microwave Oscillators Based on Types a waveguide, coaxial, or microstrip design. For stabilization
of Active Components of the operating mode of the avalanche transit-time diodes,
stabilization of the bias current is required. Oscillations of the
Tube Semiconductor
current of the diodes are rich in higher harmonics, which may
klystron Gunn diode be separated by connecting an additional cavity to the diode.
magnetron avalanche transit-time diode When a diode is connected to a low-quality oscillating sys-
tem, a wideband noise oscillator is produced.
amplitron bipolar transistor
The basis use of the avalanche transit-time oscillators is
backward-wave tube field-effect transistor
in phased-array antennas and in the output stages of the trans-
traveling-wave tube mitters of radio relay stations. IAM
gyrotron Ref.: Gassanov (1988), p. 183; Voskresenskiy (1981), p. 317.
grid-control tube A backward-wave-tube oscillator is one whose basic ele-
ment is a backward-wave tube. The generation mode in a
In radar applications, high-power oscillators (up to
backward-wave tube arises when there is equality of the
megawatts) use magnetron tubes, backward wave tubes,
transfer rate of electrons and the phase rate of the reverse spa-