Comtech EF Data SNM-1002 - Manual (Page 111)

SNM-1002 LinkSync
 Modem
Revision 2
Maintenance MN/SNM1002.OM
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 directions provided in section A.3.
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 operating properly. 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 terminal 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 5-1. 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 5-3.
6. To check the frequency and phase modulation accuracy:
a. Set the modem to the continuous wave Normal mode by using the directions
in section A.3.1. 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 directions in
section A.3.1. This generates a single, upper side band suppressed carrier
signal. Ensure the carrier and side-band suppression is < 35 dBc.