Comtech EF Data CDM-570A - Manual (Page 578)

CDM-570A/570A Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix M Revision 2
M–2
• The recommended minimum carrier spacing equals (1 + Txα) x symbol rate,
assuming equal symbol rates for all carriers and zero frequency error between carriers.
• At lower symbol rates, particular care needs to be taken with the frequency accuracy and
stability of the RF carriers. It is your responsibility to determine if sufficient guard band is
built into the frequency plan to avoid carriers moving towards each other and overlapping
at the edges.
Now that you can choose between six different values of Txα, it now becomes
important to match the Rx Alpha (Rxα) accordingly. Comtech EF Data has analyzed
the effect of mismatching Txα and Rxα, and has concluded that there is little benefit
in providing six values of Rxα. It has been determined that just two values of Rxα –
0.35 and 0.12 – are required to match all six Tx cases.
In order to correctly set Rxα – this assumes that you have knowledge of the Txα being
transmitted – follow these guidelines:
• For a Txα of 0.05, 0.10, 0.15, or 0.20, use Rxα = 0.12.
• For a Txα of 0.25 or 0.35, use Rxα = 0.35.
If these rules are followed, then the amount of degradation due to mismatch will be < 0.05 dB.
If these rules are not followed, then the amount of degradation due to mismatch can be > 0.3
dB.
In Chapter 6. FRONT PANEL OPERATION, see:
• Sect. 6.3.2.3.11 (Config: Tx) α (Tx Alpha Filter Rolloff Factor)
• Sect. 6.3.2.4.11 (Config: Rx) α (Rx Alpha Filter Rolloff Factor)