Comtech EF Data CDM-550T - Manual (Page 107)

CDM-550T Satellite Modem Revision 3
Automatic Uplink Power Control (AUPC) MN/CDM500T.IOM
12–3
12.2.4 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)
• MAXIMUM (increase the TX Output Power to the maximum value permitted by the
parameter MAX RANGE)
The default setting is NOMINAL.
Note: If the local demod loses lock, the modem will automatically move its output power to the
nominal value.
12.3 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.
12.4 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.
CAUTION
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