AR 10037990, Operating And Service Manual, Page: 25 / 54

Rev A 5
2. THEORY OF OPERATION
2.1 DESIGN OF THE AMPLIFIER
The Model 250T6G18 TWT amplifier consists of four principal subsystems. Two of these subsystems, the
microwave power assembly (A24380-948) and the TWT power supply (A22520-062) 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.
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 fiber optic links to the HPA
interface assembly (A25444-001).
The TWTA packaging consists of a cabinet assembly (A24381-400), 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
The TWTA consists of two stages of RF amplification: a solid state preamp assembly with adjustable gain
(E01247-000) and a traveling-wave tube amplifier (E08194-000).
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 WRD-650 for Model 250T6G18. The output is directed through a straight length of waveguide and a 180º E-
plane bend into a dual -30 dB directional coupler. The output of the coupler is a type WRD-650 flange, 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 provided locally on the
remote control board. The foldback circuit remains on line in emergency bypass operation.