Radar System Engineering

540 THE RECEIVING SYSTEM—INDICATORS [SEC. 13.17
Automutic Triggering of the Transmitter. —Waveforn balace by triggering
the transmitter as the secondary current reaches zero is illustrated
in Fig. 13.48. Since thevoltages inthesecondary circuits arelow, the
signal from the cathode resistor in the sweep amplifier is used to provide
the required information. This signal is passed through a network whose
voltage response is equivalent to the current response of the deflectioncoil
circuit, and from there to the first stage of a high-gain amplifier. The
grid of the first amplifier tube is so biased that in the absence of signal it
T T2
Rectangular 4+
wavefofm
u
generator
Sawtmth
~: sweep
waveform
generator
A
I
+
I
Synchro J’
+T
I
:
,,
Transmitter (C’) ?’1‘z i
trigger
A A
Intensifying
swnal out
.- -..
Alternative -4 LA--
waveform for — — —
A and B
FIG. 13.4S.—Reaolved-currentPPI usingautomatic transmitter triggering.
is exactly at cutoff.
Thus when a sweep is being applied this grid will
pass upward through its cutoff ppint at exactly the same time that the
deflecting current is passing through zero. The remainder of the sawtooth
is therefore amplified and, by suitable overdriving and clipping, can
be turned into a rectangular wave. The transmitter trigger is then
derived from the front edge of this rectangular wave, and the wave itself
serves to intensify the cathode-ray tube.
If a simple sawtooth is used, the maximum useful sweep duration has
an upper limit of half the pulse period; somewhat less can be realized in
inadequate high-frequency responsein the synchro and amplifiers, and from crosacoupling
in the former, were troublesome only on fast sweeps. Furthermore the
“corners” were at the 45° rather than the 90° positions.