Radar System Engineering

442 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC. 12.4
Another very important factor in radar receiver design is that of
securing a good transient response. Reception of pulses about 1 ~secin
length imposes severe requirements on the pass band of the amplifier.
At the same time, the amplifier must be capable of amplifying pulses
several hundred microseconds in length and yet be fully sensitive to a
weak signal immediately following. It must recover immediately from a
signal thousands of times stronger than the minimum discernible signal.
Frequently special requirements are placed on the receiver. For
example, it may be required that it respond to weak pulses even in the
presence of a c-w signal of considerable strength. The problem of
detecting a weak target echo in the presence of strong sea return is somewhat
similar. Special circuits which go under the general title of “anticlutter
circuits” are usually required in such cases.
The unique requirements outlined in the foregoing three paragraphs
have resulted in a receiver that is considerably different from anything
previously existing. The design of i-f and video amplifiers to meet these
requirements will be discussed in the following sections.
12.4. I-f Ampliiier Design—In an ideal i-f amplifier, all the noise
would originate in the generator connected to its input terminals-that is,
the crystal that serves as converter in
the usual radar receiver. In any practical
amplifier there are additional
sources of noise. Thermal noise from
B+
Fm. 12.4.—Pentodeinput circuit.
resistances in the input circuit and shot
noise due to the uneven flow of electrons
to the plate of the first tube are
the chief sources of excess noise, although
later stages may contribute
slightly. For this reason, care must
be taken in the design and tuning of the network coupling the crystal to
the first grid as well as in the choice of the operating conditions and type of
the first tube.
Figure 1264 shows a typical input circuit, with the primary tuned to
resonate with the crystal and mixer capacity at the intermediate frequency,
and with the secondary tuned to resonate with the input capacity
of the tube and socket. The coils are usually fixed-tuned or “ slugtuned”
1 in order to avoid any extra capacity. The only loading on the
circuit is due to the crystal and the input resistance of the tube; thus,
there are no additional resistances to contribute thermal noise. Thermal
1The inductanceof a ‘1slug-tuned” coil is loweredby the effect of eddy currents
inducedin a metallicrod (slug)insertedin one endof thecoil. The degreeof penetration,
and thereforethe inductance,can be varied by turning the slug in a threaded
support.