AR 40T26G40A Service manual

Operating and
Service Manual
40T26G40A
Model
10009284
Part Number
Serial Number
160 School House Road, Souderton, PA 18964 • 215-723-8181 • Fax [phone] • www.arworld.us

EC Declaration of Conformity
We; AR Worldwide
160 School House Road
Souderton, PA 18964
declare that our product;
the Model 40T26G40A amplifier
to which this declaration relates is in compliance with the requirements of the EEC EMC
Directive (89/336/EEC) and Low Voltage Directive (73/23/EEC) in accordance with the
relative standards listed below:
EMC:
EN 50082-1: 1992
Electromagnetic compatibility – Generic immunity standard
EN 50081-1: 1992
Electromagnetic emissions requirements for Industrial, Scientific, and
Medical (ISM) Equipment
Safety:
EN 60950 (1995)
The CE marking is affixed on the device according to the EC Directives.
Donald R. Shepherd
President

INSTRUCTIONS FOR SAFE OPERATION
Revised 0517
Observe the following safety guidelines to help ensure your
own personal safety and to help protect your equipment and
working environment from potential damage.
INTENDED USE
This equipment is intended for general laboratory use in
generating, controlling, and measuring levels of
electromagnetic Radio Frequency (RF) energy. Ensure that the
device is operated in a location which will control the radiated
energy and will not cause injury or violate regulatory levels of
electromagnetic interference.
SAFETY SYMBOLS
These symbols may appear in your user manual or on
equipment.
This symbol is marked on the equipment when it
is necessary for the user to refer to the manual for
important safety information.
The caution symbol denotes a potential hazard.
Attention must be given to the statement to
prevent damage, destruction, or harm.
Dangerous voltages are present. Use extreme
care.
Indicates a terminal intended for connection to an
external conductor for protection against
electrical shock in case of a fault, or the terminal
of a protective earth (ground) electrode.
Indicates invisible laser radiation–do not view
directly with optical instruments.
Indicates frame or chassis ground connection
terminal.
Indicates alternating current.
Indicates this product must not be disposed of
with your other household waste.
Indicates that the marked surface and adjacent
surfaces can attain temperatures that may be hot
to the touch.
EQUIPMENT SETUP PRECAUTIONS
Review the user manual and become familiar with
all safety markings and instructions. Protection
provided by the equipment may be impaired if used
in a manner not specified by AR RF/Microwave
Instrumentation (AR).
•Follow all lifting instructions specified in this document.
•Place the equipment on a hard, level surface.
•Do not use the equipment in a wet environment, for
example, near a sink, or in a wet basement.
•Position your equipment so that the power switch is easily
accessible.
•Leave 10.2 cm (4 in) minimum of clearance on all vented
sides of the equipment to permit the airflow required for
proper ventilation. Do not restrict airflow into the
equipment by blocking any vents or air intakes. Restricting
airflow can result in damage to the equipment, intermittent
shut-downs or safety hazards.
•Keep equipment away from extremely hot or cold
temperatures to ensure that it is used within the specified
operating range.
•While installing accessories such as antennas, directional
couplers and field probes, take care to avoid any exposure
to hazardous RF levels.
•Ensure that nothing rests on your equipment’s cables and
that the cables are not located where they can be stepped
on or tripped over.
•Move equipment with care; ensure that all casters and/or
cables are firmly connected to the system. Avoid sudden
stops and uneven surfaces.
BEFORE APPLYING POWER
Your AR equipment may have more than one power supply
cable. Use only approved power cable(s). If you have not been
provided with a power cable for the equipment or for any AC-
powered option intended for the equipment, purchase a power
cable that is approved for use in your country. The power cable
must be rated for the equipment and for the voltage and current
marked on the equipment’s electrical ratings label.
Incorrectly installing or using an incompatible line
voltage may increase the risk of fire or other
hazards. To help prevent electric shock, plug the
equipment and peripheral power cables into properly grounded
electrical outlets. These cables are equipped with three-prong
plugsto help ensure proper grounding. Do not use adapter plugs
or remove the grounding prong from a cable.
Do not modify power cables or plugs. Consult a licensed
electrician or AR trained service technician for equipment
modifications. Always follow your local/national wiring rules.
Do not operate the equipment if there is physical
damage, missing hardware, or missing panels.
SAFETY GROUND
This equipment is provided with a protective earth
terminal. The mains power source to the equipment
must supply an uninterrupted safety ground of
sufficient size to attach wiring terminals, power
cord, or supplied power cord set. DO NOT USE this equipment
if this protection is impaired.

INSTRUCTIONS FOR SAFE OPERATION
Revised 0517
HAZARDOUS RF VOLTAGES
The RF voltages on the center pin of an RF output
connector can be hazardous. The RF output
connector should be connected to a load before AC
power is applied to the equipment. Do not come into
contact with the center pin of the RF output connector or
accessories connected to it. Place the equipment in a non-
operating condition before disconnecting or connecting the load
to the RF output connector.
ACOUSTIC LIMITATIONS
If equipment noise exceeds 80dB, ear protection is required.
MAINTENANCE CAUTION
Adjustment, maintenance, or repair of the equipment must be
performed only by qualified personnel. Hazardous energy may
be present while protective covers are removed from the
equipment even if disconnected from the power source. Contact
may result in personal injury. Replacement fuses are required
to be of specific type and current rating.
ENVIRONMENTAL CONDITIONS
Unless otherwise stated on the product specification sheet, this
equipment is designed to be safe under the following
environmental conditions:
•Indoor use
•Altitude up to 2000m
•Temperature of 5°C to 40°C
•Maximum relative humidity 80% for temperatures up to
31°C. Decreasing linearly to 50% at 40°C.
•Main supply voltage fluctuations not to exceed ± 10% of
the nominal voltage or minimum and maximum
autoranging values.
•Pollution degree 2: Normally non-conductive with
occasional condensation. While the equipment will not
cause hazardous condition over this environmental range,
its performance may vary.
EQUIPMENT CONTAINING LASERS
AR Field Probes (FL/PL Series) and Field Analyzers
(FA Series) are Class 1 laser products containing
embedded Class 4 lasers. Under normal use, the
laser radiation is completely contained within the
fiber optic cables and poses no threat of exposure. Safety
interlocks ensure that the laser is not activated unless the cables
are properly connected. Always exercise caution when using
or maintaining laser products. Do not view directly with optical
instruments.
RF ANTENNAS
•This equipment (antenna or antenna assembly) may be
heavy, requiring two persons to lift. Use caution when
installing or removing unit. Follow all equipment setup and
lifting instructions specified in this document.
•Ensure connectors are appropriate for intended operation.
Connectors are specified in the user manual and product
specification sheet.
•Do not exceed the maximum RF input level stated in the
specifications. Refer to the user manual and product
specification sheet to determine the applicable RF levels.
•Excessive RF input could damage the equipment or
connectors, causing safety hazards.
•When in operation, the RF voltages on the antenna
elements can be hazardous. Do not come into contact with
the antenna or elements when the RF input connector is
connected to a live RF source.
•To avoid injury to personnel and accidental damage to
power amplifier or antenna, disable the RF output of power
amplifier before connecting or disconnecting the input
connection to the antenna.
•Perform periodic inspections of antenna and field probe
systems to verify calibration due date, proper operation,
and overall condition of equipment.
RACK MOUNTED TWT MODELS
Some TWT models are supplied without the removable
enclosure offered for benchtop use. These rack-mountable
models may be supplied with either carry handles or slides and
front handles installed. Follow all lifting instructions specified
in this document and installation instructions supplied in the
TWT user manual.
LIFTING INSTRUCTIONS FOR AR EQUIPMENT
Because most products must be
handled during distribution, assembly
and use, the risk of serious injury due
to unsafe product handling should be
a fundamental consideration of every
user. An authoritative guideline for
eliminating unwarranted risk of injury caused by lifting is
provided by the NIOSH Work Practices (Publication #94-110)
available at:
https://www.cdc.gov/niosh/docs/94-110/pdfs/94-110.pdf.
In general, observe the following guidelines for lifting a weight
of 50 lb or more:
•Use lifting eye (for floor standing) or side handles (table
top) to lift unit only.
•Use equipment of adequate capacity to lift and support unit.
•If using forklift to move unit, be sure forks are long enough
to extend beyond the side of the unit.
•For additional information, follow the link specified above.

ADDITIONAL WARNINGS & NOTES
WARNING:
This equipment operates at potentially lethal voltages. Only trained,
qualified personnel should operate, maintain, or service it.
Hazardous energy may be present while protective covers are removed
from the equipment even if disconnected from the power source. Contact
may result in personal injury.
CAUTION:
Adjustment, maintenance, or repair of the equipment must be performed
only by qualified personnel.
CAUTION:
Replacement fuses are required to be of specific type and current rating.
CAUTION:
The information in this document was obtained from reliable sources and
was believed to be accurate at the time of publication. Since subsequent
modifications to the machine may have been made, use this information
only as a guide. Carefully compare the unit's actual configuration and
operation to the descriptions in this manual before you undertake to
operate, service, or modify this machine. Any variance or modification
should be noted, dated, and initialed in the discrepant part of all manuals
on hand for future reference. If you have technical or editorial comments
you wish to make to the manufacturer, please write them on photocopies
of the relevant sheets.
NOTE: The contents of this document are the property of the manufacturer and this document is delivered on
the express condition that it not be disclosed, reproduced in whole or in part, or used for manufacture for
anyone other than the manufacturer without its written consent, and that no right is granted to disclose or so
use any information in this document.

Suggested Periodic Maintenance for TWT Amplifiers
1. Keep monthly log of the voltages, currents and temperatures as shown on Menus. Also record
Date, “Console” and “Operate” hours. Take readings in Operate mode with the gain at zero
(0%) percent. Leave unit in Operate mode for 20 minutes (Max Duty if Pulsed Unit), and then
record data. Review the log to identify trends and contact factory if required.
2. Keep monthly log of performance with active RF. At mid-band frequency, with Gain set to
100%, apply RF drive that will provide rated power. This will help indicate if the gain or power
is changing and if the traveling wave tube or pre-amp needs service. Record the following:
a. RF Drive Level
b. Forward Power
c. Reverse Power
d. Body Current (Iw)
e. Sample Port Power reading (if possible).
3. Remove air intake filter and clean using compressed air and/or vacuum cleaner.
4. Ensure air inlet and outlet are unobstructed.
5. Check that AC Input connections are secure. Make sure the AC cable is not damaged or
deteriorated.
6. Check that both input and output RF connections (waveguide or coax) are connected tightly and
un-damaged.
Rev A

i
TABLE OF CONTENTS
TABLE OF CONTENTS .................................................................................................. i
1. DESCRIPTION AND SPECIFICATIONS ....................................................... 1
1.1 TWTA Description ................................................................................................ 1
1.2 Suggested Applications.......................................................................................... 1
1.3 Specifications......................................................................................................... 1
1.4 Accessories ............................................................................................................ 1
1.5 Test Data Sheet ...................................................................................................... 2
2. THEORY OF OPERATION .............................................................................. 5
2.1 Design of the Amplifier ......................................................................................... 5
2.2 Description of the RF Circuit................................................................................. 5
2.3 Description of the Power Supply (A22826-352) ................................................... 6
3. OPERATION ....................................................................................................... 7
3.1 Warnings and Cautions.......................................................................................... 7
3.2 Installation ............................................................................................................. 7
3.2.1 Unpacking.............................................................................................................. 7
3.2.2 Mounting................................................................................................................ 8
3.2.3 Cooling Requirements ........................................................................................... 9
3.2.4 AC Line Power Connection................................................................................... 9
3.2.5 RF Output Connection........................................................................................... 9
3.2.6 External Interlock Connector............................................................................... 10
3.2.7 External IEEE-488 Connector ............................................................................. 10
3.2.8 RF Connector....................................................................................................... 10
3.3 Front Panel Features ............................................................................................ 11
3.4 Front Panel Display and Soft Keys...................................................................... 12
3.5 Rear Panel Features ............................................................................................. 16
3.6 Initial Turn On and Warm-up Procedure............................................................. 16
3.7 Emergency Bypass Operation.............................................................................. 17
3.8 Remote IEEE-488 Operation ............................................................................... 18
3.9 TWTA General Considerations ........................................................................... 23
4. MAINTENANCE............................................................................................... 25
4.1 Safety Warning .................................................................................................... 25
4.2 Unauthorized Repairs .......................................................................................... 25
4.3 Preventive Maintenance....................................................................................... 25
4.4 Troubleshooting................................................................................................... 26
4.5 Non-Repairable Modules..................................................................................... 27
5. TECHNICAL DOCUMENTATION ............................................................... 29
5.1 Top Level Build Tree........................................................................................... 30
5.2 Schematics ........................................................................................................... 31
5.3 Parts Lists............................................................................................................. 33
5.3.1 Parts List, HPA Logic and Control Module, A16485-000 ................................. 34
5.3.2 Parts List, HV Power Supply for TWT 6193A4/D4, A22826-352..................... 36
5.3.3 Parts List, Emergency Bypass Board, A24830-001............................................ 37
5.3.4 Parts List, HPA Interface Board (Plastic), A25444-000...................................... 38
5.3.5 Parts List, HPA 40W 26.5-40GHZ (AR) A28050-352....................................... 41

Model 40T26G40A
ii
5.3.6 Parts List, Microwave Power Assembly, KA, A28051-002................................42
5.3.7 Parts List, Wiring Kit, 40W K/KA Band, 28052-000 ..........................................43
5.5 Sample Program for IEEE-488 Communication..................................................45
FIGURES
3-1 Location of Supplementary Threaded Holes for Rear Panel Support ....................8
3-2 TWTA Front Panel...............................................................................................11
3-3 TWTA Front Panel Display Screens ....................................................................12
3-4 TWTA Rear Panel Features .................................................................................16
TABLES
3-1 TWTA Front Panel Features ................................................................................11
3-2 TWTA Rear Panel Features .................................................................................16
3-3 Catalog of IEEE-488 Commands .........................................................................18
3-4 Catalog of Status Codes .......................................................................................19
3-5 Catalog of Fault Code Returned by RDFLT Command.......................................20
3-6 Catalog of System State Codes ............................................................................21
3-7 *STA?; Response Codes ......................................................................................21
3-8 *STB?; Response Codes ......................................................................................22
3-9 Catalog of Heater Auto Off Time Delay Codes ...................................................22
4-1 Model 40T26G40A TWT Troubleshooting Guide...............................................26

Rev A 1
1. DESCRIPTION AND SPECIFICATIONS
This manual provides operating, interfacing and selected service information pertinent to the AR
Worldwide Model 40T26G40A Broadband Microwave Amplifier. The Model 40T26G40A is a 40-watt
Ka-band Traveling-Wave Tube Amplifier (TWTA). Refer to the Model Configurations on the data
sheet to determine the applicable features of this unit.
1.1 TWTA DESCRIPTION
The amplifier uses a broadband millimeter-wave traveling-wave tube (TWT) to provide 40 watts minimum
output over the TWT amplifier’s full bandwidth. The amplifier is well suited for susceptibility and general
laboratory testing where instantaneous bandwidth and high gain are required.
The amplifier is completely self-contained and packaged for standard 19-inch rack mounting or for bench-top
use. The front panel of the rack mountable amplifier is 5.25 inches high, and the overall unit is 25.25 inches
deep, excluding the rear-panel connectors. For bench-top use, the amplifier is supplied in an enclosure with
integral carrying handles.
Primary power is 99–260 volts, 50–60 Hz, single phase. An efficient switching power supply design provides
minimum power consumption. A fast regulation control loop and a high degree of filtering ensure
performance within specifications over a wide range of operating conditions. The amplifier is fully enclosed,
and the upper and lower panels of the rack-mountable amplifier are interlocked to reduce the likelihood of
accidental contact with high voltage.
1.2 SUGGESTED APPLICATIONS
•RF Susceptibility testing
•Antenna and component testing
•Equipment calibration
•General laboratory instrumentation
1.3 SPECIFICATIONS
Refer to the AR Worldwide Data Sheet at the end of this section for detailed specifications.
1.4 ACCESSORIES
AR Worldwide offers a number of accessories for use with this amplifier, including:
•Directional coupler
Antenna
Flexible transmission line
Refer to a current AR Worldwide catalogue for Microwave Accessories.

Model 40T26G40A
2 Rev A
1.5 TEST DATA SHEET
A Test Data Sheet for a specific unit is prepared at the time of manufacture and is included with the unit’s
copy of this manual.

and RF components allow for easy access and
repair. Use of a switching mode power supply
results in significant weight reduction.
Housed in a stylish contemporary cabinet, this
unit is designed for benchtop use but can be
removed from the cabinet for rack mounting.
The Model 40T26G40A provides readily availa-
ble RF power for a variety of applications in Test
and Measurement, (including EMC RF suscepti-
bility testing), Industrial and University Research
and Development, and Service applications.
These sub-octave amplifier features moderate
harmonic content.
The export classification for this equipment is
EAR99. These commodities, technology or soft-
ware are controlled for export in accordance
with the U.S. Export Administration Regulations.
Diversion contrary to U.S. law is prohibited.
Refer to Model Configuration Chart for alterna-
tive configurations and special features.
The Model 40T26G40A is a self contained,
forced air cooled, broadband traveling wave
tube (TWT) microwave amplifier designed for
applications where wide instantaneous band-
width, high gain and moderate power output are
required. A reliable TWT provides a conservative
40 watts minimum at the amplifier output con-
nector. Stated power specifications are at the
fundamental frequency.
The amplifier’s front panel digital display shows
forward and reflected output plus extensive sys-
tem status information accessed through a series
of menus via soft keys. Status indicators include
power on, warm-up, standby, operate, faults,
excess reflected power warning and remote.
Standard features include a built-in IEEE-488
(GPIB) interface, 0dBm input, VSWR protection,
gain control, RF output sample port, auto sleep,
plus monitoring of TWT helix current, cathode
voltage, collector voltage, heater current, heater
voltage, baseplate temperature and cabinet tem-
perature. Modular design of the power supply
Features
Specifications
40T26G40A
 M1–M12
 40 Watts CW
 26.5GHz–40GHz
AR RF/Microwave
Instrumentation
160 School House Rd
Souderton, PA 18964
215-723-8181
For an applications engi-
neer call:800.933.8181
www.arworld.us

POWER (fundamental), CW, @ OUTPUT CONNECT-
OR: Nominal, 45 watts; Minimum, 40 watts;
Linear @ 1dB Compression 10 watts minimum
FLATNESS: ± 8 dB
FREQUENCY RESPONSE: 26.5–40 GHz instantane-
ously
INPUT FOR RATED OUTPUT: 1.0 milliwatt maximum
GAIN (at maximum setting): 46 dB minimum
GAIN ADJUSTMENT (continuous range): 35 dB mini-
mum
INPUT IMPEDANCE: 50 ohms, VSWR 2.0:1 maximum
OUTPUT IMPEDANCE: 50 ohms, VSWR 2.5:1 typical
MISMATCH TOLERANCE: Output power foldback
protection at reflected power exceeding 10 watts. Will
operate without damage or oscillation with any magni-
tude and phase of source and load impedance. May
oscillate with unshielded open due to coupling to in-
put. Should not be tested with connector off.
MODULATION CAPABILITY: Will faithfully reproduce
AM, FM, or pulse modulation appearing on the input
signal. AM peak envelope power limited to specified
power.
VIDEO PULSE CAPABILITY (S8V OPTION)
Pulse Width: 0.1 microseconds min.
Pulse Rate (PRF): 10 kHz max
RF Rise and Fall: 30 ns max. (10% to 90%)
Delay: 300 ns max. (pulse input to 90% RF)
PW Distortion: ± 30 ns max.
NOISE POWER DENSITY: Minus 60 dBm/Hz maxi-
mum); Minus 70 dBm/Hz (typical)
HARMONIC DISTORTION: Minus 20 dBc maximum;
Minus 28 dBc typical
PRIMARY POWER: See Model Configurations
CONNECTORS:
RF input: Type K female, rear panel
RF output Type WR-28 waveguide
flange, rear panel
RF output sample port Type K female, rear panel
GPIB IEEE-488, rear panel
Interlock DB-15 female, rear panel
Video Pulse Input (S8V Option): Type BNC female,
rear panel
COOLING: Forced air (self contained fans), air entry
and exit in rear.
WEIGHT: 30 kg, 65 lbs
SIZE (WxHxD): 50.3 x 16.5 x 68.6 cm, 19.8 x 6.5 x 27
in
EXPORT CLASSIFICATION: EAR99
Page 2
Specifications
40T26G40A
 M1–M12
 40 Watts CW
 26.5GHz–40GHz

Page 3
40T26G40A
 M1–M12
 40 Watts CW
 26.5GHz–40GHz
EPackage Alternatives. May select an alternative
from the following [E1C or (E1C and E2S)
and/or E3H]:
E1C Cabinet: Without outer enclosure for rack
mounting, size (W x H x D) 48.3 x 13.3 (3U) x
68.6 cm, 19.0 x 5.25 (3U) x 27 in, Subtract
approximately 7 kg, 15 lbs, for removal of
outer enclosure.
E2S Slides: slides installed, add approximately 2
kg, 5 lbs.
E3H Handles: Front pull handles installed.
P Primary Power must select one primary power
from the following options [P1 or P2]:
P1 99-260 VAC, 50/60 Hz, single phase, 850VA
max.
P2 400V Europe 360-435 VAC, 3 phase, WYE (5
wire) 50/60 Hz, 850 VA max. CE marked to
comply with EMC European Directive 89/336/
EEC for operation inside a shielded room.
Model Configurations
SSpecial Features: May select a special feature
(extra cost) from the following [(S1R or S3F)
and/or S2F and/or S5F and/or S4F]:
S1R Reflected Power Port: Type K female connector
on rear panel. Forward and reflected sample
port calibration data supplied on disk in Excel
format at 51 points, evenly spaced over speci-
fied frequency response.
S2F Flatness: Flatness ± 6 dB max at rated power.
S3F Reflected power port: type K female connector
on front panel. Forward and reflected sample
port calibration data supplied on disk in Excel
format at 51 points, evenly spaced over speci-
fied frequency response.
S4F RF input connector: On front panel, not on
rear panel.
S5F Forward output sample port: On front panel,
not on rear panel.
S6F RF output connector: on front panel.
S7E Ethernet Remote Interface: removes IEEE-488
interface; RJ-45 connector on rear panel.
S8V Video Pulse Capability
To order AR Products, call 215.723.8181. For an applications engineer call:800.933.8181. Direct to Service call: 215.723.0275 or
email: [email protected] For Faxing Orders:866.859.0582 (Orders Only Please) [email protected]
Approved for public release by AR RF/Microwave Instrumentation 072216
Model Number
Features
EPS
40T26G40A Base model P1 –
M1 E1C P1 –
M2 E1C & E2S &
E3H
P1 –
M3 See individual Specification Sheet
M4 E1C P1 S2F
M5 - P1 S1R
M6 E1C P1 S1R
M7 E1C & E2S &
E3H
P1 S1R
M8 E3H P1 S3F, S4F, S5F
M9 E1C & E2S &
E3H
P2 S3F, S5F, S6F
M10 E1C & E2S P1 S1R, S2F
M11 E3H P1 S2F
M12 E1C, E3H P1 S7E, S1R,
S8V
Example: Model number example: Model 40T26G40AM1 would have option
E1C, no outer enclosure.

Rev A 5
2. THEORY OF OPERATION
2.1 DESIGN OF THE AMPLIFIER
The Model 40T26G40A TWT amplifier consists of four principal subsystems. Two of these subsystems, the
microwave power assembly (A28051-002) and the TWT power supply (A22826-352), 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 (A25425-001), 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 (A27742-001). The cooling system utilizes a 400 Hz fan
with a dedicated 400 Hz power supply (A23692-000).
2.2 DESCRIPTION OF THE RF CIRCUIT
The TWTA consists of two stages of RF amplification: a solid-state preamplifier assembly (E02020-000) and
a traveling-wave tube amplifier (E01887-000).
The Type K RF input connector is located on the rear panel. The RF input is fed to the solid-state
preamplifier via an input isolator (E20375-000). The solid-state preamplifier’s output passes through a
voltage-controlled attenuator (E20369-000) to drive the RF input of the TWT. Some units are provided with
an RF equalizer (E01462-000) adjusted to achieve the 40T26G40A flatness specification. The RF output of
the TWT is WRD-180 waveguide flange. After a transition to WR-28 the output is directed through a
waveguide assembly provided with a -30/-30 dB dual-directional coupler. The output WR-28 waveguide
flange of the coupler protrudes through the rear panel of the TWTA to function as the TWTA’ 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.

Model 40T26G40A
6 Rev A
Amplifier gain is set by the voltage-controlled variable gain 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 attenuator’s loss.
The emergency bypass board (A24830-001) is mounted behind the front panel. It is provided with a circuit
that increases the attenuation so that reflected power is limited to a level on the order of 10 watts. In
emergency bypass operation (see section 3.7), the gain control signal is provided locally by means of a
potentiometer on the emergency bypass board. The foldback circuits remain on line during emergency bypass
operation.
2.3 DESCRIPTION OF THE POWER SUPPLY (A22826-352)
The TWT’s power supply is of modular construction. Low-voltage power for logic and control of the entire
power supply assembly is provided by the low-voltage power supply module (A23687-001). In addition, this
module provides Direct Current (DC) power for the HPA interface and emergency bypass boards. Control
logic and TWT protection circuits are contained in the HPA Logic and Control Assembly (A16485-488).
The Heater Power Supply Module (A25963-000) powers the TWT DC heater. In addition, the millimeter
wave TWT has a grid modulator (A23684-352).
The high-voltage power supply consists of the following: the Power Factor Correction Module (A23683-100)
converts line voltage to DC for the high-voltage switching supply. Switching transistors are on the Power
Board Assembly (A23710-000), and switching is controlled by the Pulse Width Modulation (PWM) Board
(A10017-488). The high-voltage transformer and rectifiers are contained in the High-Voltage
Diode/Capacitor Assembly (A23707-488). The high-voltage DC is filtered in the HV Filter Assembly
(A21461-488).
Low-voltage interconnects between the power supply modules are through a motherboard. It is installed in a
finned heat sink assembly to which the modules are bolted. The heat sink is cooled by the incoming cabinet
air. The Heat Sink/Motherboard Assembly is A23709-000.