Digital modems of minimum frequency shift keying signals and their characteristics

Background. The need to describe the features of the formation of minimum frequency shift keying signals, reveal the advantages of this type of modulation, provide circuits for digital demodulators of minimum frequency shift keying signals that provide the best reception noise immunity, and indicate ways to improve the spectral efficiency of minimum frequency shift keying signal modems. Aim. Various options for constructing digital modems for minimum frequency shift keying signals, features of modem circuits for even and odd numbers of half-periods on segments of the duration of transmitted messages, as well as the specifics of the formation of reference oscillations in coherent demodulators are considered. Methods. The principle of operation of modulators and demodulators of minimum frequency shift keying signals is explained by the timing diagrams of signals at various points in the paths for the formation and detection of minimum frequency shift keying signals. This representation ensures the reliability and clarity of the operation of modems of minimum frequency shift keying signals. Results. It is shown that coherent demodulators of minimum frequency shift keying signals provide the same noise immunity as opposite signals of relative phase shift keying, but at the same time they have a constant signal level and a smaller spectrum width. A dual-channel minimum frequency shift keying signal modem doubles the spectral efficiency of a single-channel modem, but the signal level becomes inconsistent. Conclusion. Information is given explaining the reason for the widespread use of minimum frequency shift keying signals in cellular communication systems, in space communication systems and in cordless phones, as well as ways to further improve digital modems of minimum frequency shift keying signals.