Comtech EF Data CDM-550T - Manual (Page 81)

CDM-550T Satellite Modem Revision 3
Forward Error Correction Options MN/CDM550T.IOM
NOTE: Whenever uncoded operation is selected, the modem automatically uses the differential
encoder/decoder appropriate for the modulation type. It cannot be disabled.
The second problem inherent in PSK demodulators is that of ‘data false locking’. In order to
accomplish the task of carrier recovery, the demodulator must use a non-linear process. A second-
order non-linearity is used for BPSK, and a fourth-order non-linearity is used for QPSK. When
data at a certain symbol rate is used to modulate the carrier, the demodulator can lock at incorrect
frequencies, spaced at intervals of one-quarter of the symbol rate away from the carrier.
Fortunately, when FEC decoding is used, the decoder synchronization state can be used to verify
the correct lock point has been achieved, and to reject the false locks.
However, if uncoded operation is used, there is no way to recognize a data false lock. The
demodulator will indicate that it is correctly locked, but the data out will not be correct.
CAUTION
Comtech EF Data strongly cautions users when using uncoded operation. If the
acquisition sweep width exceeds one quarter of the symbol rate, there is a very
high probability that the demodulator will false lock.
As an example, if 64 kbps QPSK uncoded is used, the symbol rate will be half of this rate, or 32
ksymbols/second. One quarter of this equals 8 kHz. Therefore, the absolute maximum acquisition
sweep range which should be considered is
± 8 kHz. If there is any frequency uncertainty on the
incoming carrier, this should be subtracted from the sweep width. The problem becomes
progressively better with increasing symbol rate.
IMPORTANT
COMTECH EF DATA SHALL NOT BE HELD RESPONSIBLE FOR
INCORRECT OPERATION IF THE USER DOES NOT ADHERE TO THESE
GUIDELINES WHEN USING UNCODED OPERATION.
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