Modulator Checkout - Comtech EF Data SLM-5650B Installation And Operation Manual

A.2.2

Modulator Checkout

Use the following procedure for modulator checkout:
Step
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.
Appendix A
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.
Task
A–3
SLM-5650B Satellite Modem
Revision 2
MN-SLM-5650B