NGT—Checks and adjustments
172 NGT Transceiver System Technical Service Manual
Adjusting the exciter output spurious balance
Adjustment of the exciter output spurious balance may be necessary only if the mixer
IC102 on the RF/IF PCB in the RF unit has been replaced.
To perform the adjustment:
1 Select test channel 28.
1 Unplug the coaxial cable between the PA PCB and the RF/IF PCB in the RF unit.
1 Connect a spectrum analyser to the Tx test point on the RF/IF PCB.
1 Set up the spectrum analyser as follows:
1 Using the transceiver test unit, apply an audio signal of 1 kHz at a level of
300 mV p–p 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 Hold down PTT.
1 Adjust the 24.9 MHz signal displayed on the spectrum analyser to the top of the
screen.
1 Reset the spectrum analyser centre frequency to 20.1 MHz.
1 Adjust R138 for a minimum 20.1 MHz signal.
The unwanted 20.1 MHz product should be greater that 60 dB below the 24.9 MHz
signal.
1 Release PTT.
ALTERNATIVE TEXT
Drawing 08-05261 (sheet 2) or 08-05889 (sheet 2)
NOTE If the PCB issue status is -06 or earlier, see drawing 08-05889 (sheet 2).
NOTE
The transceiver must be in Service mode to perform this check (see
page 154, Accessing Service mode).
Centre frequency 24.9 MHz
Frequency span 20 kHz
Vertical level 10 dB per division
Trigger Continuous sweep
Video bandwidth 1 kHz
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