8.5 AUTOMATIC LEVEL CONTROL (ALC)
The unit has Automatic Level Control feature (ALC) that will keep the output level at a previously
set value. Normally the unit should work with 10 dB less gain (set by operator) than the maximum
gain, in ALC mode and at an output power 3 to 15 dB back-off from P1dB. The 10 dB less gain set
is necessary as a reserve, in order to allow the ALC system to ensure a constant output power in
case the input power dropped.
The output power is monitored via an output power detector that sends to the Monitor & Control
Board an analogue voltage signal proportional with the actual output power. This signal is
converted into a digital value and stored in a calibration table putting in correspondence digital
values with actual output power levels. After a certain output power is set (by setting a certain gain
level to the unit) the M&C is able to maintain this set output power level by increasing or
decreasing the attenuation command signal that is sent to the attenuator for gain control. An ALC
control loop is so created ensuring the maintaining of the output power at the set level when the
input power varies.
The ALC command may be followed by a value of the needed output power level and in this case
the target power will be set to that value. If the ALC command is not followed by any value, the
current value of the output power level will be considered as target value.
Note that if the ALC feature is ON, the gain control is not available for the operator.
The ALC “high” alarm is issued when the input power is too high and the attenuation reached the
maximum allowable value (the attenuator cannot ensure enough attenuation of the input power).
The ALC “low” alarm is issued when the input power is too low and the attenuation reached the
minimum allowable value (even with 0 dB attenuation, at maximum gain set, the input power level
is too low for maintaining the target output level).
MONITOR
AND
CONTROL
GAIN CONTROL
Up-CONVERTER
OUTPUT
POWER
DETECTOR
30 dB
AMPLIFIER
ALC LOOP
33
