A.2.2 Modulator Checkout - Comtech EF Data SLM-5650A Installation And Operation Manual

SLM-5650A Satellite Modem
Appendix A

A.2.2 Modulator Checkout

Use the following procedure for modulator checkout:
Step Task
1 Set up the equipment as shown in Figure A-1 using 70/140 MHz IF interface. Refer to Chapter 1.
INTRODUCTION for the modulator specifications.
2 Set up the modem for IF loopback operation by using TEST: Loopback  IF from the front panel
menu, or use an external IF loop.
3 Clear all TX faults, Stored Faults, and Alarms using MONITOR: Event-Log  Clear-All from the
front panel menu.
4 Measure the E b /N 0 with a receiver that is known to be properly operating:
a.
b.
c.
5 Connect a spectrum analyzer to the modem as shown in Figure A-1. Ensure the IF output meets
the appropriate mask and spurious specifications. Measure the power output at different levels and
frequencies.
6 Check the frequency and phase modulation accuracy as follows:
a.
b.
7 Repeat steps 2-6 using the L-Band IF interface.
Refer to Table A-1 and Figure A-2 to check for proper E b /N 0 level. The (S+N)/N is measured
by taking the average level of the noise and the average level of the modem spectrum top.
Use this measurement for the first column on Table A-1.
Read across the page to find the S/N and E b /N 0 for the specific code rate.
Once the demodulator has locked to the incoming signal, using MONITOR: Rx-Params
from the front panel menu displays frequency offset, signal level, BERT data (if enabled),
buffer status, E b /N 0, and corrected BER.
A typical output spectrum is shown in Figure A-3.
Set the modem to the continuous wave mode by using TEST: Carrier  Tx-CW from the front
panel menu. This sets the Carrier modulation in the OFF condition. A pure carrier should now be
present at the IF output. This should only be used for frequency measurements. Spurious and
power measurements should be taken with the modulation on.
Set the modem to the continuous wave Offset mode by using TEST: Carrier  Tx-1,0 from
the front panel menu. This generates a single upper side band and suppressed carrier signal.
Ensure the carrier and side-band suppression are < -30 dBc.
A–3
Revision 8
MN-SLM5650A