Comtech EF Data SDM-300A - Manual (Page 349)

SDM-300A Satellite Modem Revision 6
Troubleshooting MN/SDM300A.IOM
18–13
18.3.3.1 Modulator Faults
Fault/Alarm Possible Problem and Action
IF SYNTHESIZER Modulator IF synthesizer fault.
This is considered a major alarm, and will turn off the modulator output. Return the modem for repair.
DATA CLOCK SYN 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.
I CHANNEL 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.
Q CHANNEL Activity alarm for the Q channel digital filter.
Use the I channel procedure.
AGC LEVEL Output power AGC level fault.
This is a major alarm and will turn off the modulator IF output.
Indicates the level at the modulator output is not the programmed level.
INTERNAL SCT SYN Internal TX data clock synthesizer fault. The SCT has failed to lock to the internal reference.
EXT REF ACT External reference activity fault. Indicates clock reference not detected.
MODULE Modulator module fault.
Typically indicates that the modulator module is missing or will not program.