PSM-500/500L/500LT SCPC Satellite Modem
Maintenance
PSM-500/500L/500LT - Rev. 0.91
Page 4-5
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.
Symptom
: The Modem acquires a carrier but loses lock intermittently.
Possible Cause
: Receive acquisition range set too narrow. When the carrier drifts
outside of the acquisition range the demodulator looses lock until the carrier returns
inside the acquisition range.
Action:
Check the actual receive carrier frequency and the receive offset at the front
panel and set the acquisition range appropriately.
Possible Cause
: Receive level varying out of AGC range.
Action:
Check the actual receive input level at the front panel. Change the carrier input
level to within the correct range. In Ku–Band systems, rain fade can cause significant
receive level variance.
Possible Cause
: Transmit or Receive Converter equipment noisy.
Action:
The dependence up good phase noise in converter equipment is especially
noticeable at low data rates and when using QPSK modulation. Very low frequency
phase noise on the converter oscillators is very difficult to see or measure, but is
detrimental to proper low data rate performance. Substitution of another modem will
verify the correct modem functioning.
Symptom:
The Modem output data is corrupted.
Possible Cause
: Receive data or clock inverted.
Action:
Check the current state of the Demod Clock and Data Phase. Try inverting the
phase.
Possible Cause
: Receive Carrier signal Eb/No is too low resulting in poor BER
performance.
Action:
Ensure that the transmit end is properly set and that the receive subsystems are
all operating correctly. In a small station ensure that the antenna is “peaked” on the
satellite. In a Ku-Band station, intense rain can cause poor receive performance.
Possible Cause
: Transmit and Receive scrambler or differential encoder options do not
match or not enabled. Note – The differential encoder in the PSM-500 is under processor
control only, but this cause could apply to a mixed system linked to another modem.
Action:
Check the current state of the Scrambler and differential encoder. In all operating
systems the differential encoder/decoder and one of the available scramblers must be
enabled.
Symptom:
The Modem receive FIFO buffer indicates “Slip”.
Possible Cause
: The FIFO automatically re-centers when an overrun or under-run
condition occurs.
Action:
Check that the proper clocking options are used and the FIFO buffer is set large
enough to handle the expected satellite Doppler shift over a 24 hour period. No amount
of buffering will correct for different clocks on the input and output of the FIFO.
Symptom:
Receive DTE equipment indicates “clock slip” or “sync lost”.
