AR 10037990, Operating And Service Manual, Page: 38 / 54

Model 250T6G18
18 Rev A
3.6 INITIAL TURN ON AND WARM-UP PROCEDURE
Install the TWTA as discussed in Section 3.2. Provide an RF generator to the RF input Type N connector A2J1.
Set RF generator level below -50dBm and set desired frequency in specified range. Connect a load suitable for
350 watts continuous operation to the output waveguide. The load VSWR should be less than 2.0:1 A power
meter and suitable attenuators may be connected to the RF sample port A2J2. (Refer to RF sample port calibration
factors on the rear of the unit or on the Info screen in MENU 2). These show the relation between the amplifier
output power and the RF sample port power as a function of frequency. When only the power of the fundamental
frequency is to be measured and when operating near rated power use; filters, a frequency selective receiver, or a
spectrum analyzer to reduce the harmonic content of the measured level.
Set keylock to LOCAL.
Switch on the MAIN POWER circuit breaker. The fan will operate. The front panel display will show several
identification messages and then the MENU 1 screen. The third line will indicate that the heater time delay is
active.
Allow the heater warm-up delay to expire. Line three will indicate OFF/READY.
Push S4 (MORE) three times to go to MENU 4. Verify that the heater voltage and current are near their nominal
levels. The values of these parameters at the time the TWTA left the factory are logged on the test data sheet.
Push the OPERATE push-button. You will now see the cathode and the collector voltages rise. Verify that the
grid, collector, and cathode voltages are near nominal. The helix current should be near the nominal value for no
RF drive. Then push MORE or EXIT to go back to MENU 1.
Set the TWTA gain to maximum. Adjust the RF generator to slowly increase the RF drive toward 0dBm to reach
the desired FWD PWR on the display and power meter (connected to sample port). The forward power bar graph
will become active, with maximum length when peak power output is achieved. Best performance is obtained
when the input RF drive is set at or just below the level that causes peak power output. Do not set input drive
above 0dBm (Input drive above +20dBm may damage the unit). The reverse power level should remain below
10% of the forward power or 20 watts (+43 dBm), assuming that the load is properly matched.
An alternate procedure is to pre-set the TWTA gain to minimum, set the RF generator to 0dBm and then slowly
increase the TWTA gain to set the desired RF output level.
Observe that the helix current is sensitive to the RF drive level of the TWT. It is at a minimum with no RF drive.
The helix current with no drive and with rated output power at mid-band are logged on the test data sheet. The
value of the helix current is a good qualitative indicator of RF drive present.
To shut the system down, turn down the RF generator level below -50dBm and press STANDBY. Allow the
TWTA to cool down until the TWT temperature drops below 70°C, then turn off main power.