2-41
Figure 2-29.—Stc voltage waveform.
The combination of stc and iagc circuits results in better overall performance than with either type of
gain control alone. Stc decreases the amplitude of nearby target return signals, while iagc decreases the
amplitude of larger-than-average return signals. Thus, normal changes of signal amplitudes are adequately
compensated for by the combination of iagc and stc.
Antijamming Circuits
Among the many circuits used to overcome the effects of jamming, two important ones are GATED
AGC CIRCUITS and FAST-TIME-CONSTANT CIRCUITS. A gated agc circuit permits signals that
occur only in a very short time interval to develop the agc. If large-amplitude pulses from a jamming
transmitter arrive at the radar receiver at any time other than during the gating period, the agc does not
respond to these jamming pulses.
Without gated agc, a large jamming signal would cause the automatic gain control to follow the
interfering signal. This would decrease the target return signal amplitude to an unusable value. Gated agc
produces an output signal for only short time periods; therefore, the agc output voltage must be averaged
over several cycles to keep the automatic gain control from becoming unstable.
Gated agc does not respond to signals that arrive at times other than during the time of a target return
signal. However, it cannot prevent interference that occurs during the gating period. Neither can gating
the agc prevent the receiver from overloading because of jamming signal amplitudes far in excess of the
target return signal. This is because the desired target is gated to set the receiver gain for a signal of that
particular amplitude. As an aid in preventing radar receiver circuits from overloading during the reception
of jamming signals, fast-time-constant coupling circuits are used. These circuits connect the video
detector output to the video amplifier input circuit.
A fast-time-constant (ftc) circuit is a differentiator circuit located at the input of the first video
amplifier. When a large block of video is applied to the ftc circuit, only the leading edge will pass. This is
because of the short time constant of the differentiator. A small target will produce the same length of
signal on the indicator as a large target because only the leading edge is displayed. The ftc circuit has no
effect on receiver gain; and, although it does not eliminate jamming signals, ftc greatly reduces the effect
of jamming.
Q41. Which of the two types of automatic gain control, agc or iagc, is most effective in radar use for
the Navy?
Q42. Immediately after the transmitter fires, stc reduces the receiver gain to what level?
