AR 10027190, Operating And Service Manual

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Rev – 5
2. THEORY OF OPERATION
2.1 DESIGN OF THE AMPLIFIER
The TWT amplifier consists of four principal subsystems. Two of these subsystems, the microwave power
assembly and the TWT power supplies 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, which consists of the CPU
board, the HPA display board, and the data link board. 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 fiber optic links to the HPA
interface assembly.
The TWTA packaging consists of two cabinet assemblies secured as one. The cooling system utilizes two 60
Hz fans to cool the HV power supplies and a pair of hi speed 400Hz fans to cool the TWTs.
2.2 DESCRIPTION OF THE RF ASSEMBLY
The TWTA consists of two stages of RF amplification. The first stage is a solid-state preamp assembly with
adjustable gain followed by the last stage using a pair of power combined traveling-wave tubes.
The Type K RF input connector is located on the rear panel, or optionally on the front panel depending on
configuration. The RF input is fed through an input isolator to the input connector on the solid-state preamp.
The solid-state preamp's output drives into a magic tee/splitter assembly that incorporates phase shifters to
permit adjustment for optimized phase matching. The two outputs drive the RF input of each TWT. The RF
output of each TWT is through a waveguide flange. Each output passes through a reverse directional coupler
used to monitor for high reflected power due to excessive load VSWR and/or internal RF component failure.
High reflected power will cause a fault and take the unit out of Operate mode.
The output power of both two couplers is combined at a magic tee assembly before being directed through a
dual directional coupler. The output coupler provides forward and reverse samples for both monitoring and
protection. The reflected port on this directional coupler is routed to a detector diode. The detector output is
used for reverse power measurement on the HPA interface board. A sample of the reverse power is optionally
located on the front or rear panels.
The forward port of the directional coupler is split with a 10 dB coupler. One 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 other output is connected to the RF sample port on the rear panel or, optionally, on the front
panel depending on configuration. The coupler waveguide terminates in a WR-42 waveguide flange in the
18G26 and WR-28 waveguide flange in the 26G40 that protrudes through the rear panel of the TWTA.