Comtech EF Data SDM-100A - Manual (Page 144)

Maintenance SDM-100A Satellite Modem
5–2 Rev. 0
5.1.2 Modulator Checkout
The first step in turning up a carrier is to set the output frequency. This is done in the
CONFIGURATION menu on the front panel (refer to Chapter 3).
The CONFIGURATION menu also allows the operator to set the output level, turn the
output on or off, set the scrambler, differential encoder, clock source, and gives access to
the test modes for system check-out.
Available test modes are RF loopback, IF loopback, Baseband loopback, and CW mode.
After the output frequency and level are set, the output must be turned on.
A typical output spectrum is shown in Figure 5-1.
5.1.3 Demodulator Checkout
The input to the demodulator card must be set within the proper frequency and power
level for the demodulator to lock to the signal.
Refer to Figure 5-2 and Table 5-1 to check for proper E
b
/N
0
level. Figure 5-2 is an
example of a 1/2 rate carrier operating at an E
b
/N
0
of 5.0 dB. (S+N)/N is measured by
taking the average level of the noise and the average level of the top of the modem
spectrum, as shown. 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 3). These functions are
not displayed until the demodulator is locked to a carrier.
Typical constellations with noise and without noise are shown in Figure 5-3. To view the
constellation, the oscilloscope must be in the X-Y Mode. Connect the oscilloscope to test
points TP 37 and TP 38 on the Demodulator/M&C/Interface board.