Radar System Engineering

SEC.12.11] AN EXTREMELY WIDEBAND RECEIVER 473
in only one of these cases, it is necessary for the local oscillator to ceaae
oscillating when it is on the “wrong” side of the transmitter frequency.
This can be counted on only when the intermediate frequency is high.
Double-tuned circuits were chosen in preference to stagger-tuned,
because it was felt that they would give a smaller change in the shape of
the response curve for a given change in tube capacity. The circuits were
loaded on both sides to reduce still further the sensitivity to tube-capacity
changes. Equal values of primary and secondary Q would have been
best in this respect; actually a compromise Q-ratio of 2.2 was adopted
to give a little more gain than equal Q’s would provide. The transformer
coils are wound adjacent to each other on a powdered-iron core to give
tight coupling. A thin spacer between the two coils is of such thickness
as to make the coupling just transitional. The gain per stage is approximately
11 db, and the single-stage bandwidth 25 Me/see.
The 6AK5 tube is chosen in preference to the 6AC7 because it is
smaller, takes less power, and has a smaller variation in input and output
capacity. It also gives a better receiver noise figure when used in, the
first stage. A 6J6 is used in the second stage because of its low plate-tocathode
capacity, an important factor in grounded-grid triode operation.
The first two stages are operated as triodes; in spite of the high intermediate
frequency and the broad bandwidth, an i-f noise figure of 3.5 db
can usually be attained.
Both signal and AFC mixers are of the balanced type. This effectively
cancels out local-oscillator noise and pi-ovides good isolation
between the two channels. There are two crystals in each mixer, provision
being made for monitoring the current in each of the four crystals.
The local oscillator is a 2K25 klystron, which can be tuned mechanically
over a large range and electrically over a restricted range of some
30 Me/see. The electrical tuning can be done manually, by means of a
remote potentiometer, or automatically.
The AFC circuit operates as follows. The beat frequency obtained
by mixing a small part of the transmitter power with local-oscillator
power is amplified by two i-f stages and then applied to the discriminator.
The video pulses from the disitiminator undergo one stage of amplification
before reaching the control tube, Vb (Fig. 12.23). Tube VCgenerates
a sawtooth sweep which moves the reflector voltage of the local oscillator
through a range determined by the setting of Rte; the sweep is stopped at
the correct voltage to receive signals by the firing of the control tube.
The video amplifier consists of a limiter-amplifier stage which drives a
cathode follower operating into a line terminated with 75 ohms. Limited
signals of 1.5 volts amplitude appear across the line. The video amplifier
is very wide, the bandwidth of each stage being about 22 Me/see.
“L’hk bandwidth is obtained by means of a shunt-series peaking network