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Maintenance
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.
4.2 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 not results then test equipment
may be necessary to help isolate the trouble. A spectrum analyzer is invaluable. So is a Bit Error
Rate Test set (BERT). The PSM-500 has a built in BERT function.
In most cases the first attempt at isolating a problem suspected of being within the modem is to
place the modem in the Self-Test Mode. Since this the vast majority of internal circuitry then a
pass on this test means that the user should probably concentrate on parameter settings and
external equipment and connections.
One obvious and time-honored method of isolating problems is to substitute suspect equipment
with known good equipment. Assuming that 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.
Symptom: The Modem will not acquire the incoming carrier:
Possible Cause: Improper receive input to modem.
Action: Check that the receive cabling is correct.
Possible Cause: Receive carrier level too low.
Action: Check that the receive cabling is correct, that the downconverter 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
Page 4-4
PSM-500/500L/500LT SCPC Satellite Modem
PSM-500/500L/500LT - Rev. 0.91
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