NGT—Checks and adjustments
184 NGT Transceiver System Technical Service Manual
Adjusting the active bias
To adjust the active bias:
1 Connect a 50 Ω dummy load to the RF connector ( ) (see Figure 43 on page 183).
1 Connect a spectrum analyser via a 47 kΩ resistor to the RF connector (see
Figure 43).
This provides a low-level output for the spectrum analyser.
1 Select test channel 16.
1 Connect the signal generator to provide two-tone audio (700 Hz and 2300 Hz) via
the test unit to:
• the audio input on the 15-way GPIO connector on the junction box or 2012 RF
Unit (see Figure 40 on page 170), or
• the 2011 RF Unit via the NGT test handset (see Figure 41 on page 170)
1 Set up the spectrum analyser as follows:
1 Hold down PTT, then adjust the level of the two tones for compression.
1 Adjust the two tones displayed on the spectrum analyser for equal amplitude by
adjusting the balance control on the two-tone signal generator.
1 Adjust the sensitivity control on the spectrum analyser to set the two tones at the top
of the screen.
1 Adjust the ACT BIAS potentiometer (R241) for a minimum high order (> 9
th
)
intermodulation distortion level (see Figure 31 on page 162 or Figure 32 on
page 162).
1 Measure the intermodulation distortion levels relative to each tone.
Add 6 dB to the reading if referenced to PEP (see page 39, Transmitter
specifications for limits).
1 Release PTT.
ALTERNATIVE TEXT
Drawing 08-05869 (sheet 2) or 08-06461 (sheet 2)
NOTE
The active bias must be set before any intermodulation tests are
performed.
NOTE
The transceiver must be in Service mode to make this adjustment (see
page 154, Accessing Service mode).
Centre frequency 4.6 MHz
Frequency span 20 kHz (2 kHz per division)
Vertical level 10 dB per division
Video bandwidth 300 Hz
Sensitivity Depends on the signal level applied to the spectrum
analyser and requires adjustment when transmitting
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