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CiM-550 IP Enabled Satellite Modem Rev. 2
Theory of Operation CD/CIM550.IOM
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6.4.2 COMPENSATION RATE
As with any closed-loop control system, the loop parameters must be chosen to ensure
stability at all times. Several features have been incorporated to ensure that the AUPC
system does overshoot, or oscillate.
First, the rate at which corrections to the output power can be made is fixed at 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, the first correction made will be 80% of the calculated step. This
avoids the possibility of overshoot. 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.
6.4.3 MONITORING
The remote demodulator’s value of Eb/No can be monitored at all times, either from the
front panel (MONIT, AUPC) or via the remote control interface. The resolution of the
reading is 0.2 dB. For all values greater than or equal to 16 dB, the value 16.0 dB will be
displayed. As long as framing is enabled, the value will still be available, even though
AUPC may be disabled.
Also displayed is the current value of TX power increase. If Framing is enabled, but
AUPC is disabled, this will indicate 0.0 dB. This value is also available via the remote
control interface.
Comtech EF Data strongly cautions against the use of large
values of permitted power level increase under AUPC
control. Users should consider using the absolute minimum
range necessary to improve rain-fade margin.