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SDM-300A Satellite Modem
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
Rev. 4
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 kbit/s of overhead. A 1544 kbit/s Rev.
3 IDR is programmed to 1640 kbit/s. 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.
Maintenance
5–13
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