Maintenance
M7/M7L/M7LT Modular Satellite Modem
4-6
M7/M7L/M7LT - Rev. 0.05
In the loop-back diagram shown above, the BER test sets are physically between the satellite and
terrestrial loop-back functions. Thus data traversing through either of these loop-backs does not
involve the BER test sets, even if enabled. If the built-
in BERT is being used the local “satellite loop-
back” function should not be enabled.
The transmit output from the built-
in BERT always faces toward the modem’s transmit or satellite
side, while the receive comes from the modem’s receive side. It is not designed to transmit and
receive signals from the terrestrial side of the data connection.
All of these settings and test results are also available via the remote control interface, allowing for
automated and periodic testing of units not in service.
4.4.
Troubleshooting
The following is a list of possible problems that could be caused by failures of the modem or improper
setup and configuration for the type of service. The list is arranged by possible symptoms exhibited
by the modem. When simple solutions yield no results then test equipment may be necessary to help
isolate the trouble. A spectrum analyzer and an external Bit Error Rate Test (BERT) function are
invaluable. The modem has a built in BERT function that can be used as a substitute for the external
BERT function.
In most cases the first attempt at isolating a problem suspected of being within the modem is to
substitute the suspect modem with known good equipment assuring the configuration parameters are
correct. If the configuration setting of the equipment is not the source of the problem, this method will
quickly eliminate items from the potential source list. The two drawbacks to this method are the
availability of extra equipment and the possibility of interaction between two or more equipment items.
4.4.1.
Symptom: The Modem will not acquire the incoming carrier:
Possible Cause:
Improper receive input to modem.
Action:
Verify all receive cabling is correct.
Possible Cause:
Receive carrier level too low.
Action:
Verify all receive cabling is correct, that the down converter is properly set and that the
LNA is turned on. If a spectrum analyzer is available, locate and measure the receive level, which
should not be below
–60 dBm absolute. At lower data rates the input level may be as low as –84
dBm.
Possible Cause: Receive carrier frequency outside of acquisition range.
Action: Check the receive acquisition range is adequate for the possible system offsets. Setting
the value to 30 kHz is a standard value encompassing all normal offsets. After acquisition, the
actual receive frequency can be read from the front panel.
Possible Cause:
Transmit carrier incompatible.
Action:
Check the receive parameter settings and ensure that they match those on the
modulator.
Possible Cause:
Modem is in test mode.
Action:
Check the modem front panel for yellow warning LEDs indicating a test mode is enabled.
Self-Test or IF Loop-back disconnects the Demodulator from the IF receive input connector.
Possible Cause:
Interference on the satellite.
Action:
The interference can take many forms. The most common are an adjacent large carrier,
antenna feed polarization off resulting in carrier interference in opposite polarization,
intermodulation products. Most of these cases can more easily be determined with a spectrum
analyzer. If possible move to another operating frequency to see if that resolves the problem.
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