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SLM-3650 Satellite Modem
Maintenance
5.1.3
Demodulator Checkout
Use the following procedure to test the demodulator.
1. Set up the equipment as shown in Figure 5-1. Refer to Chapter 4 for the
demodulator specifications.
2. Set up the modem with an external IF loop and level. Use a properly operating
modulator, and ensure that power levels, data rates, code rates, etc., are
compatible.
3. Allow the modem to lock up. Depending on the data rate and overhead type,
lock up may take several seconds. When the green carrier detect LED is on and
the DEMUX lock fault has been cleared (where applicable), the modem will run
at the specified error rate. Run the TX power level (input amplitude) over the full
range, and offset the TX frequency from the RX frequency by 35 kHz. Ensure
the modem still runs within the specified error rate.
4. Set up the modem to check the constellation patterns with an oscilloscope that is
set in the X-Y mode. Typical constellation patterns with noise and without noise
are shown in Figure 5-4. These test points are available on the auxiliary
connector (J9, pins 6 and 8). It is not necessary to open the modem to look at
these test points.
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5–6
Revision 3
MN/SLM3650.IOM
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