Comtech EF Data SDM-300A - Manual (Page 339)

SDM-300A Satellite Modem Revision 6
Troubleshooting MN/SDM300A.IOM
18–3
18.2.1 Modulator Checkout
Use the following procedure to check out the modulator:
1. Set up the equipment as shown in Figure 18-1.
2. Set up the modem for operation by using the Configuration: Modulator and
Configuration: Demodulator front panel menus.
3. Clear all TX faults by correct use of data and clock selection.
4. Measure
the E
b
/N
0
with a receiver that is known to be properly operating. Refer to
Table 18-1 and Figure 18-2to 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 18-1. 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 18-1. 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 18-3.
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 5). 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 5). This generates a single, upper side-band-suppressed
carrier signal. Ensure the carrier and side-band suppression is < -35 dBc.