Radar System Engineering

462 THE RECEIVIXG SYSTEJ-RADAR RECh’11’ERS [%c. 129
12.9. A General-purpose Receiver. —This receiver was designed
as part of a 3-cm radar set installed on small ocean-going vessels for seasearch
and navigational purposes. The receivers built intoa steel box
which fits into the r-f head. The general construction and layout can be
seen from Fig. 12.16. Separate AFC and signal mixers are incorporated
in the receiver; the choke joints for connection to the r-f system can be
seen in the side view. A shutter is sho\vn across one of the waveguides in
the position it occupies when the transmitter is off. This shutter provides
crystal protection against signals picked up on the antenna when
the TR switch is not firing. At ~he other end of the receiver are a videooutput
jack and a terminal strip for bringing in power and certain control
voltages.
The local oscillator is a 723A/B klystron. It supplies r-f power to
both the AFC mixer and the signal mixer. Its frequency can be varied
over a wide range by mechanical tuning and over a smaller range by
varying the reflector voltage.
Except for the common local oscillator, the receiver is divided into
two portions, the AFC channel and the signal channel (see Fig. 12.17).
The latter consists of six i-f amplifier stages, a detector, and a video
stage. Each stage of i-f amplification uses a 6AC7 pentode with singletuned
coupling circuits. All stages but the last have a 1200-ohm load
resistance, 1 giving a single-stage bandwidth of a little over 6 lIc/sec and
an over-all i-f band~\-idth of nearly 2 Me/see. The nominal gain of the
i-f amplifier is 120 db. A 6H6 diode second detector is capacitively
coupled into a single video stage.
The video stage serves as a combined line driver and limiter. It
~perates as a cathode follower, the cathode resistance of 100 ohms being
located at the end of the video line. The screen voltage is set at such a
value that the tube draws approximately 10 ma of current with no signal;
this puts 1 volt across the 100-ohm line. N“egative signals from the
detector may drive the grid to cutoff, thereby reducing the cathode voltage
to zero. The video signals at the cathode are then negative and
limited to 1 volt in amplitude.
The gain of the i-f amplifier is adjusted by varying the voltage on the
grid of the second and third stages by means of an external control.
The receiver can be tuned either manually by means of an external
potentiometer, or automatically by means of a 5-tube .\FC circuit.
The AFC circuit is of the hunt-lock type briefly described in Sec. 12.7.
The reflector of the 723A/B local oscillator is swept over a range of about
30 volts, the center of the range being set by the manual frequency
1R-f chokes, self-resonant at 30 31c/see,are wound on the load resistorsto reduce
the power dissipation in the resistor and to keep the plate voltage up to the screen
voltage.