Training OPERATION of TWTA Products part 1 ~ RAHMAT' BLOG'S

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Amplifier Block Diagram

Operating Mode

* Filament Time Delay

* Standby

* Heater Standby

* Beam ON

* Fault

Filament Time Delay Mode ( FTD )

When power is first applied to the amplifier, voltage is applied to the TWT's filament. The TWT's filament must be at its operating temperature for the tube to function properly. The typical warm-up period is three minutes. Duringthis warm-up period, the FTD indicator (amber) will be illuminated. When the delay cycle is complete, the FTD indicator will be dark.

If there is a power failure, highvoltage will be OFF for a period proportional to prime power off-time. At the end of the recovery period, if high voltage is ON when the prime power interruption occurs, high voltage is automatically restored.

Standby and HV ON Mode

* Standby Mode

In this mode, there are no faults and the TWT is ready for high voltage to be applied.

* High Voltage ON Mode

When High Voltage ON is selected, an internal relay is latched ON. This enables High Voltage ON.

Heater Standby Mode

The purpose of heater standby is to extend the life to the tube when the high voltage is off.

The temperature of the cathode is the primary determinant of tube life.The emission of electrons from the surface of a cathode has a cooling effect on the cathode surface.Therefore, when high voltage is turned off the cathode surface temperature increases shortening the life of the tube. This is a special concern when the amplifier is in standby mode for extended periods of time; that is, when it is the standby unit.

Heater Standby reduces the heater voltage when high voltage is off. The temperature of the cathode is reduced to a temperature slightly below thenormal operating temperature. When high voltage is turned on, it overrides the heater standby command and returns the heater voltage to its normal setting.

When the high voltage is turned off again the heater voltage is reduced.When the amplifier has been online and then placed in the heater standby mode, there is no time delay when high voltage is turned on.

Fault Mode

This mode exists anytime a condition detrimental to the TWT is detected by the power supply fault detection circuitry. Faults are indicated on the front panel by red LEDs. Any of the following conditions trip the fault detection circuitry:

Excessive Helix Current Fault—(Latched) When the power supply detects a Helix Current fault, it turns OFF high voltage and then tries to turn the high voltage ON. If the fault persists for three OFF/ON cycles, the fault circuit latches and the power supply stops automatically recycling.

Voltage Fault—A Voltage fault will latch immediately when a fault occurs. A Voltage Fault indicates a problem in activation of the high voltage system.

TWT Over-Temperature Fault—A TWT over-temperature fault will clear itself once the temperature of the TWT's collector is reduced to a safe operating level.High voltage will automatically be enabled when the overtemperature fault clears (if high voltage ON has been selected).

Initial Turn On Procedure

Prepower Check

1.Prime power voltage is within the specified limits of the amplifier.

2.The RF Output is terminated with a load capable of dissipating rated RF output power and the load has a maximum VSWR of 1.3 : 1.

3.The RF drive signal is within the frequency range of the amplifier and +5 dBm or less with the HPA set for minimum gain ( controll fully counterclockwise ).

Turn ON Sequence

1.Set the AC Power Switch ON.

2.Within two minutes after setting AC Power ON, press the ENABLE and HV OFF keys on the front panel of the amplifier at the same time. (This action assures that high voltage will be OFF when the Filament Time Delay Cycle is over.)

3.Press and hold the [Enable] key while rotating the Gain Control to minimum gain (fully counter-clockwise).

4.Connect a RF monitor to the output load assembly.

5.Connect a RF test generator or the operational RF drive to the RF Input on the Rear Panel.

6.With gain set to minimum value, set the RF drive source for a maximum output of +5 dBm.

7.Wait until the Filament Time Delay cycle is complete (approximately three minutes). When the Filament Time Delay cycle is complete, turn the RF drive off. The Standby indicator should light (green); the Htr Standby indicator should light (Amber) and the FTD indicator should go from amber to off.

8.Press the [Enable] key and the [HV ON] key at the same time. The high voltage ON indicator should light (green). This indicates that high voltage is ON and applied to the TWT.

9.Turn ON the Rf drive signal. Slowly increase the RF drive to the TWT by rotating the Gain Control clockwise. Monitor the RF output power.

Normal Turn ON

The following procedure assumes that the Initial Turn-on procedure was successfully completed, the AC Power Switch has been set to OFF (O), and the TWTA is properly connected to the operational system.

1.Set the AC Power switch (Rear Panel) to ON (|). The AC Power indicator should light (green) and the FTD indicator should light (amber).

2.Wait until the Filament Time Delay cycle is complete (approximately three minutes). When the Filament Time Delay cycle is complete, the Standby indicator should light (green) and the FTD indicator should go dark.

3.Press the [Enable] key and the [HV ON] key at the same time. The high voltageON indicator should light (green). This indicates that high voltage is ON and applied to the TWT. The Standby and Htr Standby Indicators should turn OFF.

4.Select Local or Remote operation. The TWTA is on-line.

Normal Turn Off

When the amplifier is going to be off for an extended period turn the High Voltage OFF.Then set the AC Power Switch to OFF (O). This will extend the life of the TWT. For short periods of off time

(one hour or less), it is best to switch to the HV OFF

(in Heater Standby mode.)

Operation Indicator