Page 249 - Radar Technology Encyclopedia

239 limiter, diode limiter, transistor

Diode limiters are both for low- and high-power levels At low power levels, the diode is closed. At high power lev-
(e.g., as protectors at the input of the receiver of a monostatic els, during the positive half-period of the microwave field, the
radar). In the latter case, multidiode limiters or avalanche- diode is shunted by the low forward resistance. Through the
multiplication limiters are used. charge accumulation effect of the minority carriers, the resis-
The general shortcoming of diode limiters is their consid- tance of the diode remains low also during the negative half-
erable thermal noise level, which must be taken into consider- period. As a result of reflection and partial absorption of
ation when calculating the noise temperature of the low-noise energy, power is limited in the diode at the level of 3 to
input circuit of the receiver. IAM 10 mW. Counterparallel connection of the diodes is more
Ref.: Gassanov (1988), p. 142; Skolnik (1970), pp. 5.37–5.38. effective.
Microwave power limiters are used for protection of low-
The ferrite limiter is based on the properties of ferrite mate-
noise amplifiers of microwave receivers from loads during
rials to suppress RF power passing in different directions and
seepage of the power of its own transmitter to the input, or of
can be used in receiver protectors. In this application the fer-
another interfering signal, for equalization of the power of the
rite limiter is followed by a diode limiter, needed to reduce
local oscillators, exciters, low-power tunable oscillators in the
the spike leakage to a safe level, since the ferrite by itself does
operating frequency band, and so forth. (See also varactor
not give sufficient spike suppression at high peak power. SAL
limiter.) IAM
Ref.: Skolnik (1970), p. 8.35, (1980), p. 364.
Ref.: Gassanov (1988), p. 142.
A hard limiter is one in which the gain is sufficient to place
A multidiode limiter is a limiter of high microwave power,
the noise level in the limiting region of response, such that the
using uniform distribution of microwave power between all
output for any input condition is a rectangular wave. When a
diode structures with identical removal of heat from them
signal with high signal-to-noise ratio is passed through a hard
(see diode limiter). In the decimeter waveband, the limiter
limiter, the loss in detection performance is approximately
consists of a large number of diodes connected in parallel to a
1 dB. However, when the noise (or other interference)
coaxial line with increased size of cross-section. In the centi-
exceeds the signal a small-signal suppression effect occurs,
meter band, a serial-parallel arrangement of diodes covering
and the loss reaches or exceeds 6 dB. DKB
the waveguide section, placement of diodes in the slot of a
Ref.: Nathanson (1969), p. 113.
metal iris, and connection of diodes in a waveguide resonance
An instantaneous value limiter is a limiter of pulse ampli- grid of metal bars placed within the section of the waveguide
tudes. Instantaneous value limiters are divided into maximum are used.
limiters (or top limiters) and minimum limiters (or bottom The increase in input power is limited by the number of
limiters). diodes in the section of the transmission line, which is
Pulsed radar devices used them for formation of a flat decreased proportionally to the increase in frequency.
peak of pulses, for reduction in the duration of leading edges, Multidiode limiters are used to protect the input circuits
and for stabilization of pulse amplitudes; (e.g., in circuits of the receiver at high power levels of the input signal, as
used for maintaining a constant false alarm rate). Limiters are occurs for example during operation of the radar receiver and
also used for selection of pulses based on amplitude or polar- transmitter with a common antenna. IAM
ity. IAM Ref.: Lebedev, I. V., et al., Radiotekhnika, no. 8, 1982, pp. 3–5.
Ref.: Grigor’yants (1981), p. 46.
A transistor limiter is a limiter using a transistor as the
A microstrip limiter is used up to the shortwave part of the active element. Usually controlled transistorized limiters are
centimeter band, where resonant circuits designed for a rela- used. In the simplest limiter, the transistor operates in the
tively narrow band of operating frequencies are used to strong-signal mode and the operating point moves beyond the
improve the electrical parameters of the diode limiters. Reso- limits of the linear region of the characteristic: on one side to
nant circuits are based on nonhomogeneous microstrip lines the saturation region, and on the other to the cutoff region. In
and the reactive parameters of diodes. microwave circuits, single- and double-gated field-effect
Microstrip limiters are used as protective devices at the transistors with a Schottky barrier are used. Microwave
input of microwave receivers, in antenna switches, and in power limiters using field-effect transistors have been devel-
radio measuring apparatus. IAM oped basically in the form of wideband amplifier-limiters
Ref.: Lazunin, Yu. A., Radiotekhnika, no. 10, 1987, p. 61, in Russian. with automatic gain adjustment. Limiters based on field-
A microwave power limiter is a circuit whose output power effect tetrodes have automatic gain regulation at the second
is constant over a broad range of power levels of the input gate. Automatic gain regulation of multistage amplifier-limit-
signal. Controlled power limiters include attenuators whose ers covers several stages, which prevents their saturation and
level of limiting power is regulated by the control voltage distribution of signal shape, in contrast to diode limiters.
(see also transistor limiter). An uncontrolled limiter is a sec- The amplitude characteristic of the amplifier-limiter
tion of a transmission line in which a limiting diode with a p- based on field-effect Schottky transistors differs from the
n or p-i-n structure in the short-circuit mode with constant characteristic of diode limiters by the steeper break with the
current is connected in parallel to the line (see diode limiter). transition to saturation and the higher level of output power.