Amplifier Research 200T4G18, Operating And Service Manual

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2. THEORY OF OPERATION
2.1 DESIGN OF THE AMPLIFIER
The model 20T4G18 TWT amplifier consists of four principal subsystems. Two of these subsystems, the
microwave power assembly (A28826-000) and the TWT power supply (A23720-099) 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 (A25403-302), which consists of
the CPU board (A25450-000), the HPA display board (A25425-001), and the data link board (A22488). The
microprocessor control system supervises the power supply, provides metering display, processes operator
front panel inputs, 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 cabinet assembly (A28827-000). The cooling system utilizes a DC blower
with a dedicated DC power supply.
2.2 DESCRIPTION OF THE RF CIRCUIT
The TWTA consists of two stages of RF amplification: a solid state preamp assembly with adjustable gain
(E01122-000) and a traveling-wave tube amplifier (E08137-000).
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 TNC coaxial connector. The output is directed through a short cable to a -35/-35 dB dual
directional coupler. The output TNC connector of the coupler protrudes through the rear panel of the TWTA
to function as the RF output connector.
The reverse port on the directional coupler is connected to a crystal detector, 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 -10 dB coupler. The direct output is connected to a crystal detector via a
pad. The output of the crystal detector is used on the HPA interface board to measure forward power. The
side port of the coupler is connected to the RF sample port on the rear panel. 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.
Rev B 5
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