Radar System Engineering

470 THE RECEIVIArG SYSTEM—RADAR RECEIVERS [SEC. 12.11
cavity. This phase shift is detected in the phase-comparison tube VT
by applying the amplified crystal output to the control grid and the audio
oscillator output to the suppressor grid. Both grids are biased to cutoff,
so that no current flows in the tube unless the signals are in phase. The
circuit is so arranged that the 180° shift in phase of the audio signal produces
phase agreement, thereby providing pulses of plate current on
alternate half cycles of the audio signal. These pulses, further amplified
in Vs, then trigger thyratron V4 to stop the sweep.
The change from search to beacon operation is performed remotely
by relays which switch the grid input to V~, switch from the reflector
voltage-control potentiometer PI to Pz, change over the plate power
from the search to the beacon local oscillator, and superpose the sinusoidal
modulation on the sawtooth.
Switch S1 enables a maintenance man to tune the local oscillators
manually to the proper frequencies for signal reception with P1 and Pt
(for the search or beacon local oscillators respectively), and then to
revert to AFC operation centered at the correct frequency. This switch
is spring-loaded so that the circuit cannot be left on manual tuning.
Jacks are provided at “which the radar AFC crystal current and the
beacon AFC crystal current can be measured. A pin-jack test point is
provided at the grid of Vs to aid in aligning the discriminator, and another
at the plate of Vs for checking the control signals to the thyratron.
This AFC unit requires a power source supplying 5 ma at +300 volts,
23.5 ma at +105 volts, 2 ma at – 300 volts, 1.55 amp at 6.3 volts, and 0.8
amp at 6.3 volts at —225 volts from ground. N’one of these voltages
needs to be regulated, although the plate voltages must be well filtered.
12.11. An Extremely Wideband Receiver.—The receiver shown in
Figs. 12.22 and 12.23 is a good example of a design for a receiver having a
very wide bandwidth. It was designed for a 3-cm radar system with a
O.l~psec pulse width. In order to reproduce such short pulses and still
maintain a good signal-to-noise ratio, an i-f bandwidth of at least 10
Me/see is required; the bandwidth of the receiver is 12.5 Me/see.
The i-f amplifier consists of 12 stages, 6A’K5 tubes being used in all
but the second. The intermediate frequency is 60 Me/see, the total
gain being approximately 120 db.
An intermediate frequency of 60 Me/see instead of 30 Me/see was
chosen for two reasons. First, fairly tight coupling is required between
the primary and secondary of the i-f transformers to attain wide bandwidths.
It is easier to make this coupling tight at 60 Me/see than at
30 Me/see. Second, the AFC operation is more certain at the higher
frequency. The AFC circuit can lock the local oscillator on either side
of the transmitter, provided the local oscillator is producing power at
both frequencies. However, since the correct beat frequency is produced