The spectrum of the CPFSK signal also changes as a function of the
frequency spacing between the two symbol states and how the phase trajectory is controlled
when changing from one frequency to another. A detailed mathematical description (see
) of the spectral response for CPFSK can be found in Proakis (1989), and we shall
consider only two special cases known as Sunde's FSK
and Minimum Shift Keying (MSK).
Sunde's FSK arises when the spacing between the two symbol frequencies is made
exactly equal to the symbol rate. For this case, the spectrum uniquely contains two
discrete spectral lines at the two symbol frequencies, in addition to a broad spectral
spread. These spectral lines can be used in a coherent FSK
detector as the source of the carrier references, often extracted using a phase-locked loop. |
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