Mocomtech SLM-5650 - Manual (Page 105)

SLM-5650 Satellite Modem Revision 1
Maintenance MN/SLM5650.IOM
5–3
5.1.3 Modulator Checkout
Use the following procedure to check out the modulator:
Step Procedure
1 Set up the equipment as shown in Figure 5-1 using 70/140 MHz IF
interface. Refer to Chapter 1 for the modulator specifications.
2 Set up the modem for IF loopback operation by using the
Test\Loopback\IF from the front panel menu, or use an external IF
loop.
3 Clear all TX faults, Stored Faults, and Alarms using the clear
Monitor\Event-Log: from the front panel menu.
4 Measure the Eb/N0 with a receiver that is known to be properly
operating.
a. Refer to Table 5-1 and Figure 5-2 to check for proper Eb/N0 level.
The (S+N)/N is measured by taking the average level of the noise
and the average level of the modem spectrum top.
b. Use this measurement for the first column on Table 5-1.
c. Read across the page to find the S/N and Eb/N0 for the specific
code rate.
Note: Once the demodulator has locked to the incoming signal, the
Monitor\Rx-Params: menu will display frequency offset, signal level,
BERT data if enabled, buffer status, Eb/N0, and corrected BER.
6 Connect a spectrum analyzer to the modem as shown in Figure 5-1.
Ensure the IF output meets the appropriate mask and spurious
specifications. Measure the power output at different levels and
frequencies.
Note: A typical output spectrum is shown in Figure 5-3.
7 Check the frequency and phase modulation accuracy as follows:
a. Set the modem to the continuous wave mode by using the
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.
b. Set the modem to the continuous wave Offset mode by using the
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.
8 Repeat steps 2-7 using the L-Band IF interface.