Comtech EF Data SDM-9000 - Manual (Page 161)

SDM-9000 Satellite Modem Maintenance
Rev. 4 6–3
6.1.2 Modulator
Use the following procedure and the test setup shown in Figure 6-1 to check out the
modulator PCB. Refer to Chapter 5 for the modulator specifications. Section 6.1.4.2 lists
the modulator test points and signal names. Figure 6-6 shows the test point locations.
1. Set up the modem for IF loopback operation by using the Configuration
Demodulator front panel menu, or use an external IF loop with attenuation. Refer
to Chapter 4 for a block diagram of the IF loopback operation.
2. Clear all TX faults by correct use of data and clock selection (refer to
Chapter 4).
3. Measure the E
b
/N
0
with a receiver that is known to be properly operating. Refer
to Figure 6-2 and the following tables to check for proper E
b
/N
0
level:
Modulation Type Table
QPSK 6-1
8PSK 6-2
16QAM 6-3
Figure 6-2 is an example of a 1/2 rate carrier operating at an E
b
/N
0
of 7.8 dB.
The (S+N)/N is measured by taking the average level of the noise and the
average level of the modem spectrum top as shown. Use this measurement for
the first column on the appropriate table as listed above. 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
.
4. Connect a spectrum analyzer to the modem as shown in Figure 6-1. 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 6-3.
5. 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 (refer to Chapter 4). This sets the Carrier mode in
the OFF condition. A pure carrier should now be present at the IF output.
This should be used for frequency measurement only. 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 (refer to Chapter 4). This generates a single upper
side-band suppressed carrier signal. Ensure the carrier and sideband
suppression is less than -30 dBc.