Comtech EF Data CDM-625A, Installation And Operation Manual

Page: 585 / 756

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix C Revision 4
C–3
C.2.3 Alarm
This defines modem behavior if the maximum power limit is reached while under AUPC control.
The two choices are:
•
NONE (no action) – default setting .
•
Tx ALARM (generate a Tx alarm).
C.2.4 Demod Unlock
This defines the action the modem will take if the remote demodulator loses lock. The two
choices are:
• NOMINAL (reduce the Tx Output Power to the nominal value) – default setting .
• MAXIMUM (increase the Tx Output Power to the maximum value permitted by the
parameter
MAX RANGE
).
If the local demod loses lock, then the modem automatically moves its output power
to the nominal value.
C.3 Compensation Rate
As with any closed-loop control system, you must choose loop parameters that ensure stability
at all times. Several features ensure that the AUPC system does overshoot, or oscillate:
• First, corrections for which the output power can be made are fixed at the rate of once
every 4 seconds. This takes into account the round trip delay over the satellite link, the time
taken for a power change to be reflected in the remote demodulator’s value of Eb/No, and
other processing delays in the modems.
•
Second, if the comparison of actual and target Eb/No yields a result that requires a change
in output power, to avoid the possibility of overshoot the first correction made will be at
80% of the calculated step. Subsequent corrections are made until the difference is less than
0.5 dB. At this point, the output power is only changed in increments of 0.1 dB to avoid
‘hunting’ around the correct set point.