AR 10010475, Operating And Service Manual

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2. THEORY OF OPERATION
2.1 DESIGN OF THE AMPLIFIER
The Model 40T18G26A TWT amplifier consists of four principal subsystems. Two of these subsystems, the
microwave power assembly (A28051-042) and the TWT power supply (A22826-093), 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 (A27752-300), which consists of
the CPU board (A25450-000), the HPA display board (A25425-000), and the data link board (A22488-003).
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 through the IEEE-488 interface bus,
is electrically isolated from the amplifier. Communication with the amplifier is via fiber-optic links to the
HPA interface assembly (A25444-000).
The TWTA packaging consists of cabinet assembly (A27742-000). The cooling system utilizes a 400 Hz fan
with a dedicated 400 Hz power supply (A23692-000).
2.2 DESCRIPTION OF THE RF CIRCUIT (A28051-042)
The TWTA consists of two stages of RF amplification: a solid state preamp assembly (E01357-000) and a
traveling-wave tube amplifier (E00969-000).
The type K RF input connector is located on the rear panel. The RF input is fed to the solid state preamp via
an input isolator (E20374-000). The solid state preamp's output passes through a voltage-controlled attenuator
(E20370-000) to drive the RF input of the TWT. Each unit is provided with an RF equalizer (E01461-000)
adjusted to achieve the flatness specification. The RF output of the TWT is a type WR-42 waveguide flange.
The output is directed through a waveguide assembly provided with a harmonic filter and a -30/-30 dB dual
directional coupler. The output WR-42 waveguide flange of the coupler protrudes through the rear panel of
the TWTA to function as the RF output connector.
The reflected port on the directional coupler is connected to a detector diode, whose output is used for
Voltage Standing-Wave Ratio (VSWR) protection by the power supply logic board, for VSWR measurement
in the leveling loop, and for reflected 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 -10 dB coupler. The direct output is connected to a detector diode via a
pad. The output of the detector diode 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. Its nominal coupling factor is -
42 dB, due to approximately 2 dB of additional loss in the cabling.
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