Comtech EF Data SDM-9000 - Manual (Page 166)

Maintenance SDM-9000 Satellite Modem
6–8 Rev. 4
6.1.3 Demodulator
Use the following procedure and the test setup in Figure 6-1 to check out the
demodulator PCB. Refer to Chapter 5 for the demodulator specifications. Section 6.1.4.3
lists the demodulator test points and signal names. Figure 6-7 shows the test point
locations.
1. Set up the modem with an external IF loop, and level. Use a modulator that is
known to be properly operating, and ensure power levels, data rates, code rates,
etc., are compatible.
2. Allow the modem to lock up. When the green carrier detect LED is on and the
DEMUX lock fault has been cleared (where applicable), the modem should 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 30 kHz.
Ensure the modem still runs within the specified error rate.
3. 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 6-4.