Radar System Engineering

SEC. 128] PROTECTION AGAINST EXTRANEOUS RADIATIONS 459
With the switch in the position shown, the time constant of the
coupling circuit is 10,000 psec so that moderately long blocks of signal
will be passed. With the switch reversed, the time constant is 1 ~sec
so that individual pulses will be passed with some differentiation; long
pulses and modulation frequencies below 20 kc/see will be greatly attenuated.
Even for modulation frequencies as high as 200 kc/see, the
circuit has considerable effect. It should, therefore, be switched in only
when interference makes it necessary.
To plate of last i.f stage +
1 If amplifier
v
FIG. 12.15.—Instantaneous automatic gain control.
Differentiation circuits, of wtilchthe FTC circuit described above isa
good example, are of real value so long as the i-f amplifier is not overloaded.
When overloading occurs, the gain of the receiver must be ‘
reduced. As long as the jamming signal is constant in amplitude, manual
gain control is a very satisfactory means of accomplishing the gain reduction.
However, if the radar antenna is scanning or the c-w signal is
modulated, manual control is much too slow to be effective. For this
reason, various schemes for reducing the gain automatically and rapidly
have been devised. These are variously called “instantaneous automatic
gain control (IAGC),” “amplified back-bias,” “back-bias,” etc. The
circuit shown in Fig. 12.15 shows the essential elements of an IAGC
circuit.
Signals appearing at the output terminals of the last i-f stage are
rectified by the detector and applied as a negative voltage to the grid of
the cathode follower, which in turn controls the grid bias of one of the
i-f amplifiers in such a way that the presence of a signal tends to lower
the i-f gain. The time required for this reduction in gain to take place is
determined by the constants of the circuits, as well as by the strength of
the signal. In the particular example shown, this time is a few microseconds,
so that the gain is not appreciably reduced by a single pulse but
is ctit down by long blocks of signals or by a c-w signal modulated at a
low frequency.
The detector shown in Fig. 12.15 may be either the normal signal.
detector or a separate detector. The output of the IAGC circuit may
be fed back to the grid of the last stage or to that of the preceding stage.
(Instability results from feeding back across too many stages.) When