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SLM-3650 Satellite Modem
Maintenance
5.2.3.1
Modulator Faults
Fault/Alarm
IF SYNTHESIZER
DATA CLOCK SYN
I CHANNEL
Q CHANNEL
AGC LEVEL
INTERNAL SCT SYN
EXT REF ACT
MODULE
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Possible Problem and Action
Modulator IF synthesizer fault.
This is considered a major alarm, and will turn off the modulator output.
Return the modem for repair.
Transmit data clock synthesizer fault.
This fault indicates that the internal clock VCO has not locked to the
incoming data clock, or the internal clock synthesizer has not locked to the
internal reference. This is considered a major alarm, and will turn off the
modulator output. Ensure the proper data rate has been set up and
selected, and the incoming data rate matches the modem selections.
In the IDR type configuration, the data rate must reflect any overhead bits
that are added. In the IBS type, the internal reference is changed to
account for the IBS overhead of 16/15. A standard IDR type configuration
operating at Rev. 2 would be programmed to the input data rate. An IDR
type operating at Rev. 3 would be programmed to reflect the 96 kbps of
overhead. A 1544 kbps Rev. 3 IDR is programmed to 1640 kbps. An IBS
type would be programmed for the input data rate to the channel unit. The
modem accounts for the overhead because of the change in internal
reference. Use interface loopback for isolating the problem. Verify the
frequency of the input data clock to be within the lock range of 100 PPM.
Activity alarm for the I channel digital filter.
This alarm is considered a major alarm, and will turn off the modulator IF
output. An alarm in this position indicates either a fault in the scrambler, or
if the scrambler is disabled, the alarm indicates a loss of incoming data. If
the fault is active with the scrambler turned off, check for input data at the
DATA I/O connector.
Activity alarm for the Q channel digital filter.
Use the I channel procedure.
Output power AGC level fault.
Indicates the level at the modulator output is not the programmed level.
Internal TX data clock synthesizer fault.
The SCT has failed to lock to the internal reference.
External reference activity fault.
Indicates clock reference not detected.
Modulator module fault.
Typically indicates that the modulator module is missing or will not
program.
5–13
Revision 3
MN/SLM3650.IOM
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