TM 11-5840-281-35/1
tion as the Radar Set AN/TPN-18 pretrigger, and to a
ply. The transmitter may be operated at one-half power
delay network and trigger shaper to trigger the
for an emergency mode of operation if the magnetron is
modulator.
arcing badly and continuous operation is required, or to
burn-in a new magnetron.
NOTE
b. Trigger Generator and Shaper Circuit (Fig. 2-2).
The trigger generator discussed above is used to
The system pretrigger for Radar Set AN/TPN-18 is
develop the radar system trigger when the associated iff
generated, distributed, and shaped by the following
set is not in use. Refer to paragraph b (2) below for
circuits:
further discussion.
(1) The trigger generator circuit consists
of four-layer diode CR269, resistors R244 through R247,
(2) The pulse-forming network is charged by the
and capacitor C220. When TRIG MODE switch S201 is
high-voltage power supply through the charging choke
placed to the NORMAL position, capacitor C220 charges
and the hold-off diode during the inter- pulse period.
toward +200 volts through resistors R244, R246 and
When the thyratron modulator is triggered by the trigger
R247. When the charge on capacitor C220 equals 40
generator, the pulse- forming network discharges
volts, four layer diode CR269 conducts and becomes
through the primary winding of the pulse transformer.
almost a short circuit causing capacitor C220 to
The pulse developed across the transformer is stepped
discharge through resistor R244 and the diode. When
up across the secondary windings and applied to the
the voltage across capacitor C220 is nearly neutral- ized,
cathode of the magnetron, causing it to oscillate for the
diode CR269 ceases conduction and becomes an open
duration of the pulse period. The magnetron rf oscillator
circuit. The negative trigger developed across resistor
produces pulsed, high-frequency, rf energy at a peak
R244 is the system pretrigger and is sent to the
power level of 200 kilowatts.
sensitivity time control (stc) unit and to the control-
The frequency is adjustable between 9000 and 9600
indicator section. The pulse repetition frequency (prf) is
MHz. (In Radar Set AN/TPN-18, the frequency is set
adjusted by PRF ADJ control R247 which controls the
between 9000 and 9160 MHz for performance reasons.)
charging current and normally is adjusted for a prf of
(3) The high-voltage power supply develops an
1200 pps. When the associated iff equipment is opera-
output of approximately +4500 volts. This voltage is
ting in a normal manner, it provides the system trigger for
applied to the pulse-forming network through the
Radar Set AN/TPN-18 and this trigger generator is
charging choke and the hold-off diode.
disabled by energizing iff interlock relay K102 in Radar-
The dc resonant charge across the charging choke
Receiver Transmitter RT/TPN-18. When energized, relay
raises the voltage at the pulse-forming net- work to
K102 disables the trigger generator circuit by removing
approximately 8500 volts. This charge is held by the
hold-off diode until the thyratron is triggered. The width of
CR269 which is applied through resistor R246 and
the pulse developed across the secondary windings of
potentiometer R247. If the iff equipment is operated to
the pulse transformer is determined by the pulse-forming
network. The pulse width is either 0.2 sec (narrow
deenergize relay K102, +200 volts will be applied through
resistors R246 and R247, enabling trigger generator
pulse) or 0.8 sec (wide pulse), depending upon the
CR269 to generate the system trigger.
position of the TRANSMITTER switch (NARROW or
WIDE position). The high-voltage regulator circuit
NOTE
controls the amplitude of the charge on the pulse-
forming network by gating a silicon-controlled rectifier.
The system pretrigger applied to the control-indicators
Conduction of the silicon controlled rectifier collapses the
will be approximately 2.5 sec earlier when trigger
field of the charging choke, which terminates the
generators provide the system pretrigger than when the
charging cycle and reduces the high voltage.
pretrigger is generated by the iff equipment.
(4) The overload sensor circuit detects
(2) The system pretrigger is also coupled
excessive rectifier current, backswing current, and Zener
through resistor R245 and shorted delay line DL1 to the
current. Detection of excessive current by the overload
trigger shaper circuit. The 2.5-sec delay line reflects the
sensor will cause the main power interlock relay to
pretrigger and reverses the polar-
deenergize and open the power source to the transmitter
high-voltage power sup-
2-3
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