Comtech EF Data SNM-1000 - Manual (Page 138)

SNM-1000 Node Control Modem Revision 3
Maintenance MN/SNM1000.IOM
5–8
5.1.3.1.1 Modulator Checkout
Use the following procedure to check out the modulator:
1. Set up the equipment. Refer to Chapter 4 for modulator specifications.
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 (Chapter 4).
4. Measure
the Eb/N0 with a receiver that is known to be properly operating. Refer
to Table 5-2.
5. Check for proper Eb/N0 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.
6. Read across the page to find the S/N and Eb/N0 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 Eb/N0. Refer to Chapter 1 for
examples of BER performance curves.
7. 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 5-1 and Figure 5-2.
8. 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 (Chapter 3). 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 (Chapter 3). This generates a single, upper
side-band-suppressed carrier signal. Ensure the carrier and side-band
suppression is < -35 dBc.