AR 10006031, Руководство по эксплуатации и обслуживанию

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2. THEORY OF OPERATION
2.1 DESIGN OF THE AMPLIFIER
The Model 200T4G8 TWT amplifier consists of four principal subsystems. Two of these subsystems, the
microwave power assembly (A24380-030) and the TWT power supply (A22520-090) are discussed in
sections 2.2 and 2.3, respectively.
The other two subsystems are the microprocessor control system and the TWTA packaging. These both
consist of a number of subassemblies. See the build tree in section 5.1 for further information about how the
parts lists are structured.
The heart of the microprocessor control system is the control head assembly (A26946-300) and the HPA
display board (A22700-900). The microprocessor control system supervises the power supply, provides
metering display, processes operator front panel input, and enables communication with a host computer over
the IEEE-488 interface.
The control head is provided with its own power supply and, other than thru the IEEE-488 interface bus, is
electrically isolated from the amplifier. Communication with the amplifier is via fiberoptic links to the HPA
interface assembly (A25444-000).
The TWTA packaging consists of a cabinet assembly (A24381-030), a cooling system (A24382-000) utilizing
a DC powered backwards curved impeller blower and a DC power supply, and various chassis wiring
components.
2.2 DESCRIPTION OF THE RF CIRCUIT (A24380-030)
The TWTA consists of two stages of RF amplification: a solid state preamp assembly with adjustable gain
(E01198-000) and a traveling-wave tube amplifier (E30069-000).
The type N RF input connector is located on the rear panel. The RF input is fed to the input connector on the
solid state preamp. The solid state preamp's output drives the RF input of the TWT. The RF output of the
TWT is a type N female connector. The output is directed through a cable to a dual coaxial -30 dB directional
coupler. The output of the coupler is a type N female connector, which protrudes through the rear panel to
serve as the amplifier’s RF output connector.
The reverse port on the directional coupler is connected to a detector diode, whose output is used for VSWR
protection by the power supply logic board, for VSWR measurement in the leveling loop, and for reverse
power measurement on the HPA interface board.
The forward port output is split by a -3 dB splitter. One output is connected to a detector diode via a pad. The
output of the detector is used on the HPA interface board to measure forward power. The other port of the
splitter is connected to the RF sample port on the rear panel. A pad between the splitter and the port isolates
the forward power measurement from mismatches at the sample port.
Amplifier gain is determined by the solid state preamp (SSA), which has a voltage-controlled attenuator. The
control head determines the output of a digital-to-analog converter (DAC) on the HPA interface board. The
output of the DAC controls the SSA attenuator. The remote control board (A24369-000) mounted behind the
front panel is provided with a circuit that increases the attenuation so that reverse power is limited to a safe
level (on the order of 20 to 50 watts). In emergency bypass operation (see section 3.7) the attenuator signal is
Rev. F 5