Comtech EF Data SDM-300A - Manual (Page 342)

SDM-300A Satellite Modem Revision 6
Troubleshooting MN/SDM300A.IOM
18–6
18.2.2 Demodulator Checkout
Use the following procedure to test the demodulator.
1. Set up the equipment as shown in Figure 18-1.
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 19-4. 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.