AR 10027190, Operating And Service Manual

Page: 27 / 62

Models 200T18G26z5A/200T26z5G40A
Rev – 17
3.6 INITIAL TURN ON AND WARM-UP PROCEDURE
Install the TWTA as discussed in section 3.2. Connect a suitable RF generator to the RF input Type K
connector. Set RF generator level below -50 dBm and set desired frequency in specified range. Connect a
load suitable for 300 watts continuous operation to the output waveguide flange. The load VSWR should be
less than 2.0:1
A power meter and suitable attenuators may be connected to the RF forward sample port. (Refer to RF
sample port calibration factors on the rear of the unit or on the
Info screen in MENU 2 or in the test data
sheet provided with this manual). 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 fans 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 collector and cathode voltages are near nominal. The helix current should be close to 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 0 dBm to
reach the desired FWD PWR on the display and power meter (connected to sample port). The forward power
indication will become active, with maximum reading when the 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 0 dBm (Input drive above +13 dBm may damage the unit). The reverse
power level should remain below 10% of the forward power, assuming that the load is properly matched.
An alternate procedure is to pre-set the TWTA gain to minimum, set the RF generator to 0 dBm 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 currents with no drive and with rated RF output 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 -50 dBm and press STANDBY. Allow the
TWTA to cool down until the TWT temperature drops below 70°C, then turn off main power