a. The first translator mixes the 70-MHz modulated signal with the 400-MHz injection frequency to
produce the output of 470 MHz. The filter removes the lower sideband from the output signal.
b. The 470-MHz output of the first translator is mixed with the 1,800-MHz injection frequency to produce
an upper sideband of 2,270 MHz (lower sideband removed by filter). This output signal is then mixed with the
final injection frequency of 5.005 GHz to 6.105 GHz to produce the final modulated signal. This signal is in the
range of 7.25 to 8.4 GHz.
LEARNING EVENT 34: EXCITER
1. 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.
2. Variable attenuators are used to provide a means to remotely control the input power levels to the exciter
and the final-power amplifier.
LEARNING EVENT 35: POWER AMPLIFIER
1. 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.
2. 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.
LEARNING EVENT 36: FAULT DETECTION
1. 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.
2. Various types of fault detection circuits are used to protect the klystron power amplifier. Some of these
circuits are also installed to ensure the safety of the operator.
a. 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 overtemperature, coolant
underflow, or coolant overpressure.
SS0031
2-22
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