Radar System Engineering

SEC.12.2] A TYPICAL RECEIVING SYSTEM 437
it experiences an amplification of some 120 db. Following the i-f amplifier
is a “second” detector and i-f filter which rectify the signals and
remove the i-f components, leaving only the video signal envelope. The
signal channel is completed by a video amplifier which delivers the
signal at proper voltage level to the input terminal of the CRT or other
indicator.
As in all superheterodynes, tuning is accomplished by controlling the
frequency of the local oscillator. Since the ratio of the bandwidth to the
carrier frequency is extremely small (of the order of one part in a few
thousand), the tuning is critical. Some form of automatic frequency
control is essential if constant manual tuning adjustments are to be
avoided. Methods of AFC in pulse radar practice are quite different
from those of radio or television; in radar, frequency control is exerted
by passing a portion of the transmitted signal through a mixer and i-f
amplifier, and maximizing its intensity by the use of a frequency discriminator
and electrical circuits, which tune the local oscillator.
When the radar is to respond to beacon signals, the receiver must be
tuned to the beacon transmitter rather than to the local one. In this
case, an alternate local oscillator (not shown in Fig. 12.1) is forced to
oscillate in resonance with a standard cavity whose frequency differs
from that of the beacon by the intermediate frequency of the receiver.
The indicating equipment consists of the cathode-ray tube together
with the auxiliary vacuum-tube circuits and other devices necessary to
the synthesis of the display.
Those elements of the cathode-ray tube essential to a general understanding
are shown in Fig. 12.1. In addition to the screen, these consist
of a hot-cathode electron source, a “control grid” whose potential determines
the beam intensity, and a mechanism for deflecting the beam. In
the example illustrated, the deflection mechanism consists of two orthogonal
pairs of parallel plates between which the beam must pass, the
deflection due to each pair being proportional to the potential across it.
In another type of tube, the deflection is produced by a magnetic field
resulting from current in a coil or combination of coils surrounding the
tube “neck.” Arrangements for focusing the beam are not illustrated.
The equipment auxiliary to the cathode-ray tube varies widely with
different situations but a few general statements can be made. Those
parts concerned with the pulse-repetition cycle are collectively called the
“timer.” The timer provides synchronization with the modulator,
sweeps and markers for the display and measurement of range, blanking
of the cathode-ray tube during unused portions of the pulse cycle, and
other related operations which may arise in special cases. The remaining
equipment, apart from the necessary power supplies, is mainly concerned
with the display and measurement of geometrical quantities other than
range.