Radar System Engineering

468 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC.1210
impedance not over 2000 ohms), which is applied to the control grids of
the second and third i-f stages to control the gain.
The video amplifiers for the cathode-ray tubes (Fig. 12.20) have two
stages and use 6AK5 tubes. The input signal is – 1 volt peak; the
amplifier output supplies —30 volts peak to the cathode of the indicator
tube. The half-power bandwidth is 4 Me/see.
The chassis also contains a 6AL5 dual-diode tube. One half of this
tube is used in a circuit to lengthen the duration of beacon reply pulses
and so give a brighter spot on the CRT screen. On the rise of the pulse
the diode conducts, and the input capacity of the second tube is charged
rapidly. When the amplitude of the pulse starts to decrease, the diode
no longer conducts, and the charge on the grid of the second tube leaks
away S1OW1 y through a 2.2-megohm resist or.
The other half of the 6AL5 is used as a d-c restorer for the cathode-ray
tube.
This amplifier is mounted on a chassis having the form of an annular
ring which fits around the neck of the cathode-ray tube. The power
supply requirements are 20 ma at + 140 volts for plates and screens, and
0.5 amp at 6.3 volts for heaters.
Automatic- frequency-control Circuit.-The AFC subunit provides
automatic control of the radar and beacon local-oscillator frequencies.
The schematic view of the eight-tube circuit is shown in Fig. 12.21.
Both search and beacon AFC systems are of the search-and-lock type
(Sec. 12.7). A sawtooth generated by the recovery of the grid circuit
of a blocking oscillator, V5, sweeps the reflector voltage and thus the
frequency of a klystron. In radar operation, i-f signals from a separate
AFC crystal are amplified by V, and applied to the discriminator formed
by Va and associated circuits, which is centered at 30 .Mc/sec. The output
pulses from the discriminator are amplified in V~ and applied to the
grid of the thyratron Vq. When the pulses change sign and become
positive at the crossover, they trigger the thyratron and stop the sweep
voltage.
In beacon operation, power is transferred from the search local
oscillator to the beacon local oscillator, which (as described in Sec. 12.7)
is coupled to the receiver crystal and to a reference cavity tuned to a
frequency 30 Me/see below the beacon frequency. A small 1000-cycle
sinusoidal modulation, supplied by oscillator VE, is superimposed on the
sawtooth applied to the reflector. The output of the beacon crystal,
which is coupled to the local oscillator through the reference cavity, is
amplified in V6 and then applied to VT.
As explained in Sec. 12.7, the phase of the a-f amplitude modulation
of the signal from the beacon crystal will change by 180° when the frequency
of the local oscillator crosses the center frequency of the reference