Page 304 - Radar Technology Encyclopedia

oscillator, resonant oscillator, traveling-wave-tube (TWT) 294

grid-control tube oscillator), klystron oscillator and mag- significantly exceeds the corresponding parameters of the
netron oscillator. IAM crystal cavity in crystal oscillators. The frequency stability
Ref.: Popov (1980), p. 382. with the simplest thermostats (±4°C) is no worse than 10 - 6
per degree, which is comparable with a crystal oscillator. The
A semiconductor oscillator uses semiconductors as its
relative change in frequency due to aging is an average of
active components. Tunnel diodes and low-power transistors - 6
±10 per year.
are used in semiconductor low-power microwave oscillators
These oscillators can be produced in integrated form with
(units of milliwatts). The more powerful devices (tens and
accommodation on one substrate of both a stabilizing surface
hundreds of milliwatts or more) use avalanche transit-time
acoustic-wave component, and the electronic components of
diodes, Gunn diodes, and powerful transistors. (See transis-
the oscillator (amplifiers, transistors, and diode switches).
tor oscillator; tunnel-diode oscillator; DIODE, avalanche-
Thanks to this, and the high values of the characteristics,
transit-time; and DIODE, Gunn.) Production of large output
SAW oscillators are used widely as tunable-frequency oscilla-
powers is restricted due to the difficulty of removing the heat
tors. The tuning range in a programmable frequency synthe-
from the active zone of semiconductor instruments. To
sizer based on periodic connection of various channels of a
increase the output power, heat sinks and forced cooling are
delay line is 30%. Because of the capacity of operation in the
used, and semiconductor devices are being developed that
mode for generation of pulses with a high degree of coher-
combine several diode transistor cells in one housing. IAM
ence, SAW oscillators are used in communications systems
Ref.: Gassanov (1988), p. 174.
for formation and processing of a coherent pseudonoise sig-
A shock-excited oscillator is based on electronic tubes (or nal with frequency hopping. IAM
transistors) in which generation of oscillations occurs as a Ref.: Gratze, S. C., and Barton, R. K., Electronic Engineering, March 1975,
result of application of a pulsed signal to its input, exciting pp. 49–51; Hartemann, P., Electronics Letters 11, 1975, no. 5, pp. 119–
the circuit with the current increment. The most common is a 120.
circuit with positive feedback, in which the input pulse closes A transistor oscillator uses a bipolar or field-effect transistor
the input electronic device (tube), shunting the circuit, and as the active component and constitutes an amplifier with
causing sinusoidal oscillations to arise, supported through the positive feedback in the self-excitation mode. Oscillators
feedback circuit with a cathode follower. This circuit is used based on bipolar transistors are capable of operating at fre-
as a mark generator for producing range-scale marks. quencies up to 10 GHz, while those based on field-effect tran-
A shock-excited oscillator is used for producing short, sistors can operate up to 100 GHz). The latter are inferior to
peaked pulses. In this case, a critical mode is used for which oscillators based on bipolar transistors in terms of noise prop-
the loop in the anode circuit of the tube is shunted by the erties, but they have a lower supply voltage and lower value
¤
resistor R = ( L C ) 2 ¤ . The driving input is a negative pulse of internal feedback and therefore are becoming more and
with an amplitude exceeding the voltage of the source. more common, especially in semiconductor integrated cir-
Shock-excited oscillators are also used in circuits for pro- cuits
ducing rectangular pulses of various duration, for expansion Transistor oscillators supply output power up to 1W. To
of pulses, and in other circuits. IAM obtain higher values, parallel transistor amplifiers are used.
Ref.: Druzhinin (1967), p. 218. Thanks to the use of a frequency stabilization with a cavity in
A solid-state oscillator uses solid-state passive components: the form of a sphere of iron-yttrium garnet, it is possible to
oscillating devices, matching transformer circuits, adders, etc. obtain a high nonlinearity of tuning (to ±0.05%) over a signif-
Circuits based on discrete L and C components are used as icant band. IAM
the solid-state components of oscillating systems: cavities of A triode oscillator is one in the meter or decimeter band
traveling or standing waves in sections of transmission lines, based on a microwave triode. The tube is usually connected in
including radial, spiral, printed, and dielectric; spheres of a circuit with coaxial anode-grid and grid-cathode tuned cavi-
yttrium-iron garnet [YIG]. ties. The feedback between them is provided through the
To reduce the power consumption, mass and cost of the interelectrode capacitance of the tube, and if necessary
devices, solid-state oscillators are made in the form of hybrid through additional inductance and capacitive components.
integrated circuits. Thanks to their high reliability and small The efficiency of triode oscillators is up to 50% for
size, solid-state oscillators are widely used in multifunction waves longer than 30 cm, and lower at shorter waves. Pulse
radar modules and other radar apparatus. IAM power in the meter band reaches several megawatts, and fre-
Ref.: Gassanov (1988), p. 24. quency stability is high. IAM
A surface-acoustic-wave (SAW) oscillator is an amplifier to Ref.: Druzhinin (1967), p. 279.
which a positive feedback circuit is applied, and which A traveling-wave-tube (TWT) oscillator is a generator of
includes a stabilizing (frequency-selector) component. A microwave oscillations in which a traveling-wave tube is
delay line or SAW cavity is used as the latter. used as the active component. When an O-type traveling-
The range of the basic surface acoustic wave frequencies wave tube is used, the generation occurs due to the positive
reaches 3 GHz, with frequency deviation up to 1%, which feedback between its input and output. This connection can