Comtech EF Data CDM-625A, Installation And Operation Manual

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CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix B Revision 4
B–21
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. BPSK
uses a second-order non-linearity; QPSK uses a fourth-order non-linearity. When you use data at
a certain symbol rate 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 you use FEC decoding, the decoder synchronization state can be used to
verify the correct lock point has been achieved, and to reject the false locks.
However, if you use uncoded operation, 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.