Radar System Engineering

SEC. 17.12] 30Q-MC/SEC A-M EQUIPMENT 71!3
tude modulation make it preferable; if, on the other hand, the
interference is at low level it can be more completely suppressed
by frequency-modulation methods.
3. Frequency-modulation methods are definitely preferable for c-w
synchronization signals.
The following sections give brief descriptions of some actual equipments.
17.12. A 300-Mc/sec Amplitude-modulated Equipment.—Largely
because of its availability, the type of r-f equipment most used at Radiation
Laboratory consists of a modification of an amplitude-modulated
television transmitter-receiver combination operating in the 300-Mc/sec
region (specifically on any of 10 channels between 254 and 372 Me/see).
The transmitter provides 90 watts of peak video signal power, and 250
watts of pulse power.
A block diagram of the transmitter is shown in Fig. 17.17a. Provision
is made for grid modulation by all of the signals and for additional
plate modulation by synchronization pulses. Negative video and synchronization
signals are amplified by a three-stage broadband video
amplifier. The second stage has a gain control in the cathode to compensate
for variations in input-signal amplitude. The amplified signals drive
the grids of the 8025 r-f power amplifiers through a cathode follower.
Since the bias for the cathode follower must remain constant for all duty
ratios, a d-c restorer is used between its grid and the —105-volt supply.
The synchronization pulses are amplified by a 4-stage video amplifier operating
into a pulse transformer connected to the cathode of a diode through
which the r-f amplifiers draw their power. In the absence of pulses, the
diode is conducting and the plates of the r-f amplifiers are connected to
the high-voltage supply (800 volts). The arrival of a pulse disconnects
the diode and raises the plate potential of the amplifier by several hundred
volts, resulting in a very high instantaneous power.
The master oscillator consists of a pair of 8025’s in push-pull, the plate
and grid circuits being tuned by transmission-line elements of variable
length. The amplitude of oscillation is controlled by the capacitive
reactance of the filament line, and can also be varied by changing the
length of the filament line. .Change of channel necessitates retuning of
the r-f power amplifier by adjustment of a short-circuiting bar on the
parallel line which constitutes the plate load. The electrical length of
the coupling loop is also varied with the plate tuning. The monitor
diode rectifies a small portion of the output signal, which is displayed on a
scope for monitoring purposes.
A reflectometer, or bidirectional coupler, is coupled into the r-f line
at all times. This gives a continuous indication of the power output and