3-21.
EXCITER
a. The exciter in a microwave transmitting system can be a TWT with its associated filters, attenuators,
and couplers. Its purpose is to amplify the modulated signal to a power level that will satisfactorily drive the
power amplifier.
b. Variable attenuators are used to provide a means to remotely control the input power levels to the
exciter and the final-power amplifier.
3-22.
POWER AMPLIFIER
a. The final power amplifier employed in most microwave transmitters is a multicavity-klystron tube.
The klystron will amplify the input power by approximately 20,000.
b. Devices that operate continuously at high-power levels generally require a means of cooling to
prevent heat damage. This cooling system can be a refrigeration system, a heat-exchanger system, or a simple
blower. The study of cooling systems is known as cryogenics.
3-23.
FAULT DETECTION
a. Fault detection circuits are used throughout a communication station to protect its components from
overloads that may be caused by the failure of other components.
b. Various types of fault detection circuits are used to protect the klystron power amplifier. Some of
these circuits are also installed to insure the safety of the operator.
(1) An extensive interlock chain is provided in each station for maximum safety of personnel and
equipment. The klystron beam voltage is removed immediately whenever the high-voltage
section of the power-amplifier cabinet is opened. Beam voltage also is removed for a failure due
to improper filament current, improper focus current, body current and beam overcurrent, coolant
over-temperature, coolant underflow, or coolant overpressure.
(2) Arc protection is usually achieved by sensing a change in body current and firing a triggered
spark gap which crowbars (instantly removes) the beam supply voltage. Protection from
excessive reflected power and RF arcing is achieved by removing RF drive from the klystron
when the input-waveguide solid-state switch is reverse biased.
344 L3
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