Comtech EF Data SDM-650B - Manual (Page 146)

Maintenance SDM-650B Satellite Modem
6–2 Rev. 6
The first step in turning up a carrier is to set the output frequency. This is done in the
Config menu on the front panel (refer to Chapter 4). The Config menu also allows the
operator to:
• Set the output level
• Turn the output ON or OFF
• Set the scrambler
• Differential
encoder
• Clock source
• Gives access to the test modes for system checkout
Available test modes are:
• RF loopback
• Baseband loopback
• CW
After the output frequency and level are set, the output is ready to be turned ON.
6.1.2 Demodulator
The demodulator supplies baseband data to the DATA I/O connector that is a result of the
QPSK modulated signal input at the IF input connector (CP2). Clock and data are
recovered, decoded, and descrambled from the input RF signal by the demodulator and
decoder cards.
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 6-2 and Table 6-1 to check
for the proper E
b
/N
0
level.
Refer to Figure 6-2 for an example of a 1/2 rate carrier operating at an E
b
/N
0
of 9.5 dB.
The (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
of Table 6-1. 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
• E
b
/N
0
These functions are not displayed until the demodulator is locked to a carrier.