Nautel XR6, Operation And Maintenance Manual

Page: 76 / 83

XR6/XR3 Operations and Maintenance Manual Test and adjustment
Issue 3.1 2013-03-14 Page 5-5
Exciter Gain
Equalize the PDM pulse train between exciters A and B so that no change in RF output level occurs
during exciter changeover, as follows:
1. Select exciter A as the active exciter.
2. Turn on the XR6/XR3 and select the highest preset RF power level and verify that the RF
output, displayed on the
RF KILOWATTS
meter, is the desired forward power level.
3. Select exciter B as the active exciter.
4. Adjust exciter B's interphase PDM driver PWB's
GAIN TRIM
potentiometer (R31) until the
RF output is exactly the same as in Step 2 .
Modulation Peak Limiting
Modulation peaks must be limited to reduce RF power amplifier stress current when the RF output's
modulation envelope contains low frequency (long duration), high amplitude peaks. Verify the
modulation peaks are limited as follows:
1. Connect an oscilloscope between TP7 and ground on the remote interface PWB.
2. Connect an audio signal generator between TB2-1 (+) and TB2-3 (-) on the remote interface
PWB. Preset its output to 30 Hz at a zero output level (turned off).
3. Select exciter A as the active exciter.
4. Turn on the XR6/XR3 and select the highest preset RF power level and verify that the RF
output, displayed on the
RF KILOWATTS
meter, is the desired forward power level.
5. Increase the output level of the audio signal generator until a modulation depth of 95% is
attained, as indicated on the oscilloscope.
6. Adjust the remote interface PWB's
MOD PROT THRESHOLD
potentiometer (R165) until the
trailing edge of each 30 Hz cycle is just being clipped (see Figure 5.1 on page 5-6 ).
7. Adjust the
MOD PROT THRESHOLD
potentiometer until the oscilloscope indication does not
clip, but is on the edge of the clipping threshold.