Comtech EF Data CDM-625 Installation And Operation Manual page 293

CDM-625 Advanced Satellite Modem
Forward Error Correction Options
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.
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Revision 13
MN-CDM625