AR 10012514, Operating And Service Manual

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2. THEORY OF OPERATION
2.1 DESIGN OF THE AMPLIFIER
The Model 300T2G8 TWT amplifier consists of four principal subsystems. Two of these subsystems, the
microwave power assembly (A24380-066) and the TWT power supply (A22520-052) 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 (A24381-060), 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 (A24380-066)
The TWTA consists of two stages of RF amplification: a solid state preamp assembly with adjustable gain
(E01167-000) and a traveling-wave tube amplifier (E01538-000).
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 S-C 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. With an optional reverse power sample port on the rear panel, an
additional –10 dB coupler is connected to the reverse port of the system’s dual directional coupler. The output
port of the –10 dB coupler is connected to a detector diode whose output is used on the HPA interface board to
measure reflected power and for VSWR foldback and protection. The coupled port of the –10 dB coupler is
connected to the reverse power sample port on the rear panel.
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.
Rev A 7