AR 10013130, Operating And Service Manual

Page: 17 / 68

Rev - 7
2. THEORY OF OPERATION
2.1 DESIGN OF THE AMPLIFIER
The Model 2000TP2G8B TWT amplifier consists of three main parts, the microwave power assembly, the
power supply, and the control. These will be described in greater detail below.
The heart of the microprocessor control system is the control head assembly (A27509-301), which consists of
the CPU board (A25450-000) and the data link board (A22488-013). 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. Communication from the front panel is
through the HPA display board (A25425-001).
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 fiberoptic links to the
HPA interface board (A25444-001).
2.2 DESCRIPTION OF THE RF CIRCUIT (A27502-318)
The TWTA consists of two stages of RF amplification: a solid state preamp assembly with adjustable gain
(E01136-000) and the traveling-wave tube assembly (E01037-000).
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 pre-amp. The solid state pre-amp's output drives the RF input of the TWT. The RF output of the
TWT is a type SC connector. The output is directed through a coax cable assembly to a dual (-30 dB/-40 dB)
directional coupler. The RF Output connector on the rear panel is Type-N female.
An optional external harmonic filter / switch assembly may be used to further reduce harmonic power in a
sub-band. The TWTA output connects to the input of a two-position switch via a coax cable. Switching is
controlled locally, through the TWTA front panel, or remotely over the TWTA’s GPIB interface. The switch
routes the RF to the appropriate band depending upon the sub-band selected:
•
LOW band selection: routes to a low pass filter for operation from 2.5 – 4.6 GHz.
•
HIGH band selection: routes to a low pass filter for operation from 4.6 – 7.5 GHz.
The reflected signal from 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
reflected power measurement on the HPA interface board.
The VSWR detection and reflected power Foldback circuit is provided to protect the tube from high reflected
peak power that may result from progressive failure or mismatch of the output load. The Foldback circuit
reduces the maximum pulse width of the output signal when the reflected power exceeds the factory set
value. FOLDBACK ACTIVE appears on Menu 1 of the front panel display when the Foldback activates.
When the reflected power is below the Foldback setting, the Foldback circuit de-activates.
The forward port output is split by a -10 dB coupler. The direct output is connected to a detector diode. 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 forward type N RF sample port on the rear panel.