Radar System Engineering

460 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC. 12.8
it is necessary to prevent earlier stages from becoming overloaded, they
can be shunted by another similar loop. The gain of the feedback 100p
can be increased either by using a plate detector instead of a diode, or by
converting the cathode follower into a d-c amplifier. With a single
feedback loop over two i-f stages, the loss in signal detectability resulting
from a 60-db c-w jamming signal is only 12 db.
Although considerable thought and effort have gone into the problem
of designing a receiver that can discriminate against more complex types
of interference (pulses and noise), this is a much more difficult problem
which will not be treated here. 1
Anticlutter Circuits. —Land, rough sea, and storm clouds reflect a
considerable amount of r-f energy and can, therefore, interfere ~vith the
detection of objects in the area they occupy. If the desired signal is
\veaker than the “clutter” from the land, sea, or cloud, it is difficult to
modify the receiver to discriminate against the clutter; for the frequencies
contained in the clutter echoes are in the same range as those of the
desired signals. If, on the other hand, the signal return is larger than
the clutter, it should be possible to see the signal. However, the signal
can be missed if the clutter is so strong as to saturate the receiver completely.
This can be avoided by reducing the rccciver gain to the point
where the clutter is below saturation. Such a procedure would be satisfactory
if the cl~ltter ]vere everywhere uniform, which it never is. Therefore,
if the gain is reduced to the point required by strong clutter, it will
be too low for regions of less or no clutter, and wcafi signals ~rill still be
lost. The IAGC circuit previously described is ~’ery ~aluable under such
conditions and has been included in many receivers primarily for this
reason.
In the case of sea return, the clutter is fairly constant at all azimuths.
Also, the amount of sea return is a steadily decreasing function of range.
It is possible to devise a circuit that lo\vew the receiver gain immediately
following the transmitter pulse and then increases it ~teadily, arriving at
maximum gain at the time the sea return has disappeared. This is
kno~vn as a “ sensitivity y-time-control (STC) circuit”; it has the disadvantage
that controls must be provi{ded to adjust it for varying sea conditions.
N-evertheless, STC has proven of considerable value at sea, and
has also been wed on some ground-based radar sets \vhere control of aircraft
close to the set is desired.
TYPICAL
RECEIVERS
BY ~f”. H. ,JoRD.\S
The practical applicat irm of thc foregoing considerations }~ill be illustrated
by clrscrillin~ a fe\vtypical recei~-ers chosen to cm-m :Lwide variety
of purposes.
1See hIicrowave Receivers, I’ol 23, Chap. 10