SLM-3650 Satellite Modem
Revision 3
Maintenance
MN/SLM3650.IOM
5–3
5.1.2 Modulator
Checkout
Use the following procedure to check out the modulator:
1. Set up the equipment as shown in Figure 5-1. Refer to Chapter 4 for modulator
specifications.
2. Set up the modem for operation by using the Configuration Modulator and
Demodulator front panel menus.
3. Clear all TX faults by correct use of data and clock selection (Chapter 4).
4. Measure the E
b
/N
0
with a receiver that is known to be properly operating. Refer
to Table 5-1 and Figure 5-2 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 5-1.
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, the Monitor menu will
display signal level, raw BER, corrected BER, and E
b
/N
0
. Refer to Chapter 1 for
examples of BER performance curves.
5. Connect a spectrum analyzer to the modem as shown in Figure 5-2. Ensure the IF
output meets the appropriate mask and spurious specifications. Measure the
power output at different levels and frequencies.
A typical output spectrum is shown in Figure 5-3.
6. To check the frequency and phase modulation accuracy:
a. Set the modem to the continuous wave Normal mode by using the Carrier
Mode front panel menu (Chapter 3). This sets the Carrier mode in the off
condition. A pure carrier should now be present at the IF output. This should
only be used for frequency measurements. In this mode, spurious and power
measurements will be inaccurate.
b. Set the modem to the continuous wave Offset mode by using the Carrier
Mode front panel menu (Chapter 3). This generates a single, upper
side-band-suppressed carrier signal. Ensure the carrier and side-band
suppression is < -35 dBc.
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