TRAVALLING WAVE TUBE (TWT)

TWTA

CMPA:
The new Compact Medium Power Amplifier series is designed for satellite communication earth stations, satellite news gathering vehicles and fly away applications, operating in the C-band and Ku-bands. RF power up to 400 W is available in this series.

The CMPA series has been specially designed for enhanced performance and ease of operation. Implementation of a compact, light weight, wide band TWT (Travelling Wave Tube) permits continuous, efficient use across the entire frequency band.

Design of the CMPA is also based on the extensive use of LRUS (Line Replacable Limits). Comprehensive diagnostic procedures allow field personal to quickly isolate a faulty LRU.

Major Subsystems and their functions:
The RF section includes the TWT, SSIPA (Solid State Intermediate Power Amplifier) input/output isolation circuits, input attenuatior, RF detectors and output filter.

The power supply section includes the power factor correction power processor and high voltasge regulation circuitry as well as monitor and control circuitry.

Micro Processor's circuit provides automatic sequencing to control both CMPA operation and continuous monitoring of critical parameters.

CMPA RF Block Diagram





Technical Desicription:
A low level RF signal input is applied to the CMPA via a type N connector (isolator) located at the rear of the enclosure. The RF input is then routed to the attenuatior. The attenuatior controlled via the front panel has a control range of a nominal 25dB with quick response and excellent linearity.

The output of the attenuator is connected to the input port of the SSIPA. The SSIPA is designed to be transparent to final amplifier. RF parameters and its temperature compensated to minimize drift. The output of the SSIPA is connected to an isolator that protects the SSIPA from TWT failure. The SSIPA and TWT provide a combined subsystem gain atleast 75dB at maximum rated power for C-Band.

TWT's featuring compact conduction cooling dual depressed collectors for efficent operation and periodic permanent magnet focused Helix design.

The output waveguide assembly interfaces to the TWT and protects the tube from abnormal conditions that could pernanently damage the TWT. This assembly consists of an isolator, Harmonic filter, receive reject filter and 3 port directional coupler.

TWT Overview Opertions:
The CMPA is controlled and monitored using the front panel, located here are a digital LED display, push button switches, monitoring LEDs, RF setting control and the main power switch.

Operation of CMPA is performed in the following sequence starting from turning the main power ON.
1.POST: Power on self test.
2.HTD: Heater Time delay up to 3 minutes
3.Standby LED lights after HTD count down is completed.
4.Transmit/standby push button has to press to initiate beam on sequence.
5.Transmit LED lights indicate that the CMPA is capable of transmitting RF.

Two different faults may occur during operation:
Beam-off malfunction state: CMPA will recycle to the state prior to the fault, clear the fault source as indicated by the LED detail behind the front panel door, then press reset to return the CMPA to standby.

Front panel description:
The following front panel components- Push buttons, LED's and other controls are described below
Push Buttons:
TRANSMIT /STANDBY Toggles beam state between beam on and beam off. When beam is on ,as indicated by a solidly lit Transmit LED.The CMPA will provide RF output.
RESET: Resets the unit after faults conditions (like recycled fault LED).
LOCAL/REMOTE/CIF: Toggles between the three control points.
METER SELECT: Toggles through all valid meter selections.
LAMP TEST: Lights all front panel indicators( located behind front panel access door)
STATUS LEDS:
LOCAL- Lit when valid control point is the front panel.
REMOTE- Lit when valid control point is remote port.
CIF- Lit when valid control point is the cif port.
HTD-Lit during heater time delay.(HTD)
(If unit transitions to fault during HTD, the HTD countdown will continue)
TX SELECT-Lit when Transmit push button pressed during HTD, CMPA will transitions to Transmit at the conclusion of HTD.
STANDBY- Lits during standby state.
TRANSMIT-Flashes during beam on sequence, lit during transmit state.
RECYCLED FAULT- Flashes after the unit successfully recycles from faults.
FAULT-Flashes when unit is in fault condition.
Displaying Meter Readings:
Up to six different meter readings can be selected for viewing on the digital LED display located on the front panel. To view a meter reading, press the meter select push button repeatedly until the LED lights next to the desired item to be displayed.
The following metering items can be displayed a indicatd by a lit LED.
RF OUTPUT (dBm) : Indicates RF output power in dBm, if CMPA is equipped with forward power metering option (LED flashes for either low or high RF alarm).
RF OUTPUT (watts) : Indicates RF output power in watts.
ATTENUATOR SETTING (volts) : Indicates attenuator control voltage setting. Valid only when unit is equipped with PIN diode attenuator option. (LED flashes for RF inhibit).
REFLECTED RF (watts) : Indicates reflected RF power in watts.
HELIX I (mA) : Indicates helix current in milliamps.
HELIX V (kv) : Indicates helix voltage in kilovolts.

Operating procedures:
Perform the CMPA normal operating procedures in the following order :
1. Switch ON the main power switch located on the CMPA's front panel.
(Power on self tests will start at this time as indicated by the front panel display “post”. The CMPA should be in local mode as indicated by the local LED being it.)
2. Verify that HTD (Heater Time Delay) counts down from 3:00 to 0:00 minutes. [This count down is displayed on the digital LED display, the HTD LED will be lit during this period].
3. Verify that the STAND BY LED lights and that the HTD LED goes off after HTD is complete.
4. Press TRANSMIT/STANDBY push button. The transmit LED flashes during the beam on sequence, then lights continuously. The CMPA is now capable of transmitting.
5. Ensure that an RF input is being applied to the CMPA.
6. Confirm that RF output power has been set to the desired level.
7. To end transmission, press the TRANSMIT/STANDBY push button.
Setting the RF output level:
1. If equipped with optional forward power metering, press the meter select push button repeatedly until the LED lights next to the desired setting to be displayed.
2. To change the value of the setting, rotate the control panel RF setting adjustment control until the desired level is displayed on the digital LED display or seperate user provided power meter.
The following RF settings cn be displayed and changed when equipped with optional forward power metering and PIN Diode attenuator.
RF OUTPUT :- Can be displayed in watts (W) or in dBm.
ATTENUATOR SETTING :- the attenuator may be set at any time of the PIN diode attenuator is used, the attenuator setting will indicate the control voltage applied to the attenuator. If the standard manual attenuator used, use the dial counter and record the desired attenuation settings.
CMPA SHUT DOWN :
When transmission is completed no other transmission is planned for an hour or two, the CMPA's AC power should be switched OFF to conserve electricity and prolong the life of the TWT (Travelling Wave Tube).
1. Press transmit/standby push button to place the CMPA is stand by mode.
2. Wait approximately 5 min. to allow the TWT to cool off.
3. Switch OFF the main power switch.
MAINTENANCE OF CMPA ;
To function safely and effectively, the CMPA requires periodic maintenance. This consists of inspection, cleaning, testing and caliberation of the CMPA's subsystems at monthly to 6 month intervals.
Preventive Maintenance Points :
1.The air filter at the rear of the amplifier should be checked monthly and cleaned or replaced.
2.Atleast once a month, check that all the front panel status indicators (LED's) are working properly by depressing the LAMP TEST push button which we found after opening the front panel access door.
3.The CMPA should be visually inspected for physical defects. The daily equipment readngs log is a good source for clues that could indicate the marginal condition. Any increase in reflected power or unexpected fault conditions could point to a deteriorating condition and may be found during through visual inspection.
4.Inspect all waveguides for discoloration, cracks, loose connections and improper seating. Tighten or replace wave guides as required.
5.The TWT collector air flow system must be checked to ensure it is properly transferring the heat into the air stream and away from the TWT.
6.Check the operation of rear cooling fan located at the rear of the CMPA and another fan that cools the collector assembly is located inside the CMPA. Follows the 5th and 6th steps monthly to inspect the fans.
7.The following parts of the CMPA require routine cleaning every 3 month, or more frequently in unusually dirty environments. Ensure that the main powr has been switched off, any time it is necessary to clean inside the CMPA.
Clean all transformers, coils using a small vaccum cleaner at low pressure or a lint free cloth. Clean the front panel using a dry clean cloth.