Radar System Engineering

458 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC.12%
or like the noise produced in the receiver, the more difficult it is to discriminate
against.
One type of interference that must be guarded against is “hash,”
from rotating machinery and steep pulse wavefronts, which may leak
into the i-f amplifier. Strict attention must be paid to shielding, and
all leads into the receiver must be properly filtered and bypassed,
Perhaps the most fundamental AJ provisions are those that will
permit the receiver to operate in the presence of considerable amounts of
c-w power. If the second detector is directly coupled to the video
amplifier, then a very small amount
RFC 51 ##f of c-w signal will develop enough
voltage across the detector to bias the
first video stage out of its operating
range. This can be avoided by developing
an equal amount of counterbias,
or by coupling the detector to
2nd
detector the video amplifier through a block-
Fm. 12.14.—Fast-time-oonstantcircuit. ing condenser. The latter course is
much simpler and usually is the one
adopted. The receiver WN now continue to function in the presence of a
c-w simal until the i-f amplifier is overloaded. The i-f overload limit
can be increased by opera~ing the last stage or two with higher platesupply
voltage. Frequently a power tube, such as the 6AG7, is used in
the last stage to increase the output capabilities of the i-f amplifier,
although the 6AK5 has proven fairly satisfactory in this application. A
rather wide video bandwidth (at least as wide as the total i-f bandwidth)
is necessary when the frequent y of the jamming signal is not the same as
that of the radar transmitter. Use of a linear detector will also give the
i-f amplifier a greater dynamic range. In some receivers it is necessary
to apply a “gate,” which sensitizes the receiver during a certain interval
of time. It is desirable that such gating be tione in the video amplifier
rather than in the i-f amplifier, to avoid generating a pulse when a c-w
signal is present. Finally, the operation of the gain control early in the
i-f amplifier is of value when strong c-w signals are present. The gain
control should be of a type that does not reduce the output capabilities
of the cent rolled stages; grid gain cent rol is satisfactory in this respect.
The precautions mentioned thus far constitute good design practice
and can well be included in any radar receiver. To protect against either
frequency- or amplitude-modulated c-w signals, such precautions are still
necessary but not quite sufficient. In addition, there is needed between
the detector and video amplifier a filter that will pass individual pulses
but not the c-w modulation frequencies. Several types of filter have
been tried; the simplest is the fast time constant (FTC) circuit shown in
Fig. 12.14.