IF Amplifiers
The IF section of a radar receiver determines the
receiver’s gain, signal-to-noise ratio, and effective
bandwidth. The IF amplifier stages must have suffi-
cient gain and dynamic range to accommodate the
expected variation of echo signal power. They must
also have a low-noise figure and a bandpass wide
enough to accommodate the range of frequencies
associated with the echo pulse.
The most critical stage of a radar receiver’s IF
section is the input, or first, stage. The excellence
(figure of merit) of this stage depends on the noise
figure of the receiver and the performance of the
entire receiving system with respect to the detection
of small objects at long ranges. Not only must gain
—
and bandwidth be considered in the design of the first
IF stage, but also, and perhaps of more importance,
noise generation in this stage must be low.
Noise generated in the input IF stage will be
amplified by succeeding stages and may exceed the
echo signal in strength. The IF stages succeeding the
first stage usually achieve higher gain because the
signal level has been sufficiently increased by the
low-noise input stage to preclude problems caused by
noise generation.
A commonly used IF circuit is the single-tuned
amplifier. Each stage has only one tuning adjustment.
Inductance is varied until resonance between it and
the total shunt capacitance of the stage occurs at the
desired IF.
The IF stages require a wide bandwidth to accom-
modate the many frequencies that form the echo
pulse. Insufficient bandwidth results in transient dis-
tortion, which is the inability of the stages to amplify
transients linearly. Transient distortion may result in
ambiguities in the range of the target because of the
nonlinear rise of the leading edge of the reproduced
echo pulse.
The cascading of amplifier stages to achieve the
high gain required in microwave IF amplifiers results
in an overall bandwidth reduction. To compensate for
this effect, the bandwidth of separate stages must be
increased. This may be accomplished by several
methods, but we will only mention stagger tuning in
this chapter. For further information on these meth-
ods, consult the appropriate operating procedures for
your fire-control system.
In the stagger-tuning method, the resonant fre-
quencies of the various stages combine so that to-
gether they pass the frequency band to be amplified.
The product of each stage’s amplitude response curve
forms the overall response curve.
Gain Controls
Sensitivity time control (STC) and automatic gain
control (AGC) are commonly used to control the gain
of IF amplifiers. STC may even be used in RF ampli-
fier stages of some radar receivers. Radars detect
targets of a wide variety of sizes, ranges, and reflec-
tive area, which produce a wide range of echo signal
amplitudes that may exceed the dynamic range of a
fixed gain receiver.
SENSITIVITY TIME CONTROL.— Sensitivity
time control (STC) is used to control the gain of a
radar receiver as a function of range. Close-in target
echoes and clutter return are of greater amplitude than
when they are at greater ranges. Using STC tends to
equalize the amplitude of echoes independent of
range. There are several methods of STC, from the
simple resistance/capacitance (RC) time constant to
the more-elaborate digital schemes. The digital STC
may be controlled by a computer to provide optimum
gain as a function of range.
AUTOMATIC GAIN CONTROL.— Automatic
gain control (AGC) is common in most receiving
systems, whether radar or communications. The AGC
circuit detects the output from the IF amplifier and
produces a voltage proportional to the strength of the
detected signal and noise. For a close-in strong target
return, a larger AGC voltage is produced and the
overall receiver gain is reduced, thereby producing the
optimum signal strength out of the amplifier. This
closes the AGC loop and produces a relatively con-
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