Radar System Engineering

464 THE RECEIVING SYSTEM—RA DAR RECEIVERS [SEC. 12.10
control. The sweep voltage is generated by the 884 thyratron operating
as a relaxation oscillator. When the discriminator crossover point is
reached, positive pulses from the video amplifier VI 1fire the gas tube VIZ,
thus stopping the sweep.
To obtain the control pulses a very small part of the transmitter r-f
power is mixed with a portion of the local-oscillator output signal in the
.4FC crystal. The beat frequency generated by the crystal is then
amplified by Vg and applied to the discriminator detector. The discriminator
output signal, in the form of a video pulse, is amplified by VII
before being applied to the control tube.
12.10. Lightweight Airborne Receiver. ~This receiver is part of a
lightweight 3-cm airborne radar (AN/APS-10) intended primarily for
navigation. It was designed to require a minimum of field test equipment
and a minimum number of highly trained service personnel.
Accordingly, as far as possible the system was divided into small units
that could be replaced, rather than repaired, in the field.
The receiver is distributed among several subunits, which, except for
the video amplifiers, are enclosed (along with the r-f components, transmitter,
modulator, and most of the power supplies) in a pressurized container
forming one of the major units of the system. These subunits of
the receiver are the follo\ving:
1. A unit containing a double crystal, mounts for 723A/B radar and
beacon local oscillators, and a 1ow-Q reference cavity for the beacon
~AFC.
2. An AFC unit for both radar and beacon local oscillators.
3. An i-f strip containing the entire i-f amplifier, the second detector,
and a video cathode follower.
4. Power supply.
The receiver supplies low-level video signals to either one or two cathoderay
tubes, each of m-hich is equipped with a video amplifier.
The i-f amplifier, a schematic view of which is shown in Fig. 12.18,
contains eight 6AK5 pentode tubes. Its construction is shown in Fig.
12,19. The input circu;t is a T-network consisting of three self-inductances.
A crystal-current jack and decoupling filter are provided. The
first six tubes are i-f amplifiers whose interstage coupling networks are
single-tuned circuits arranged in two stagger-tuned triples. The center
frequencies of these circuits are, in order, 30.0, 33.7, 26.7, 33.7, 26.7, and
30.0 Me/see, giving an average over-all i-f bandwidth of 5.5 Me/see
centered at 30 ~ 1.5 Me/see. Fixed tuning is employed in this replaceable
subunit. The i-f bandwidth is made considerably wider than that
I By L. Y, Beers and R. L. Sinsheimer.