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CDM-600L Satellite Modem
BUC FSK Communications
17.4
BUC Output Power Leveling
The BUC's capability to report the detected high power BUC output carrier level
via the FSK link with the modem makes possible an automatic BUC output power
leveling scheme. In this scheme, the CDM-600L M&C continually monitors the
reported output power level from the BUC, and adjusts the modem transmit output
carrier level to maintain the BUC output level at a target tracking value.
This process is enabled when the FSK LEVELING is turned ON. When
LEVELING changes from OFF to ON, the CDM-600L M&C reads the output
power level reported by the BUC, and establishes that current output level as the
target tracking level. Then, the M&C continues to monitor the BUC output level
at 10-second intervals, and adjusts the modem output carrier level if needed to
keep the BUC output level at the established target value. With LEVELING
turned ON, the BUC output power level is held constant within the stability of the
BUC output power detector.
BUC output power leveling cannot be used in conjunction with AUPC since the
two processes conflict directly with each other. Therefore, BUC output power
leveling cannot be turned ON if AUPC is ON, and vice versa. Further, BUC
output power leveling cannot be turned ON if the BUC output is turned off by FSK
command, or if the BUC PLL is faulted. In these cases, there is no carrier output
from the BUC so a valid tracking level cannot be established. Once leveling loop
operation is established, however, the loop will "freeze" if :
1. A PLL fault in the BUC causes an automatic shut down of the output
2. FSK communication fails
3. FSK is turned OFF
4. Modem IF output is turned OFF
5. FSK command turns OFF the BUC output.
Any of these events opens the feedback loop preventing valid level correction.
Once the stimulus causing the loop to freeze is removed, the leveling loop will
resume normal operation using the previously established target tracking level. A
convenient way to verify that the leveling loop does indeed function is to first
establish leveling loop operation, then turn off FSK and change the modem
transmit output power level, and finally turn FSK back ON. Observe either the
modem output power level in the CONFIGURATION: TX: POWER menu, or the
monitored BUC output power level in the MONITOR: BUC menu. The levels
will incrementally change at 10-second intervals to bring the BUC output back to
the target tracking level.
power amplifier
17–4
Revision 2
MN/CDM600L.IOM
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