Comtech EF Data SNM-1001L - Manual (Page 193)

SNM-1001L Satellite Modem Revision 1
System Checkout MN/SNM1001L.IOM
8–3
8.1.2 Modulator Checkout
Use the following procedure to check out the modulator:
1. Set up the equipment as shown in Figure 8-1.
2. Set up the modem for operation by using the Configuration Modulator and
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 8-1 and Figure 8-2 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 8-1.
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
.
5. Connect a spectrum analyzer to the modem. 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 8-3.
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. 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. This generates a single, upper
side-band-suppressed carrier signal. Ensure the carrier and side-band
suppression is < -35 dBc.