RECEIVER SPECIAL CIRCUITS
The performance efficiency of radar receivers is often greatly decreased by interference from one or
more of several possible sources. Weather and sea return are the most common of these interference
sources, especially for radar systems that operate above 3,000 megahertz. Unfavorable weather conditions
can completely mask all radar returns and render the system useless. Electromagnetic interference from
external sources, such as the deliberate interference by an enemy, called jamming or electronic counter
measures (ECM), can also render a radar system useless. Many special circuits have been designed to
help the radar receiver counteract the effects of external interference. These circuits are called VIDEO
ENHANCEMENT FEATURES, ANTIJAMMING CIRCUITS, or ELECTRONIC COUNTER-
COUNTERMEASURES (ECCM) CIRCUITS. This section will discuss, in general terms, some of the
more common video enhancement features associated with radar receivers.
Automatic Gain Control (AGC)
Most radar receivers use some means to control the overall gain. This usually involves the gain of
one or more IF amplifier stages. Manual gain control by the operator is the simplest method. Usually,
some more complex form of automatic gain control (agc) or instantaneous automatic gain control (iagc) is
used during normal operation. Gain control is necessary to adjust the receiver sensitivity for the best
reception of signals of widely varying amplitudes. Agc and iagc circuits are designed with, a shut-off
feature so that receiver gain may be adjusted manually. In this way, manual gain control can be used to
adjust for best reception of a particular signal.
The simplest type of agc adjusts the IF amplifier bias (and gain) according to the average level of the
received signal. Agc is not used as frequently as other types of gain control because of the widely varying
amplitudes of radar return signals.
With agc, gain is controlled by the largest received signals. When several radar signals are being
received simultaneously, the weakest signal may be of greatest interest. Iagc is used more frequently
because it adjusts receiver gain for each signal.
The iagc circuit is essentially a wide-band, dc amplifier. It instantaneously controls the gain of the IF
amplifier as the radar return signal changes in amplitude. The effect of iagc is to allow full amplification
of weak signals and to decrease the amplification of strong signals. The range of iagc is limited, however,
by the number of IF stages in which gain is controlled. When only one IF stage is controlled, the range of
iagc is limited to approximately 20 dB. When more than one IF stage is controlled, iagc range can be
increased to approximately 40 dB.
Sensitivity Time Control (STC)
In radar receivers, the wide variation in return signal amplitudes make adjustment of the gain
difficult. The adjustment of receiver gain for best visibility of nearby target return signals is not the best
adjustment for distant target return signals. Circuits used to adjust amplifier gain with time, during a
single pulse-repetition period, are called stc circuits.
Sensitivity time-control circuits apply a bias voltage that varies with time to the IF amplifiers to
control receiver gain. Figure 2-29 shows a typical stc waveform in relation to the transmitted pulse. When
the transmitter fires, the stc circuit decreases the receiver gain to zero to prevent the amplification of any
leakage energy from the transmitted pulse. At the end of the transmitted pulse, the stc voltage begins to
rise, gradually increasing the receiver gain to maximum. The stc voltage effect on receiver gain is usually
limited to approximately 50 miles. This is because close-in targets are most likely to saturate the receiver;
beyond 50 miles, stc has no affect and the receiver operates normally.