Deviation Error
For an M-FSK system, the average deviation error is defined as the average spread magnitude of the FSK demodulated symbol spaced waveform around the ideal symbol (frequency) locations.
Mathematically, the deviation error is defined as
where M is the FSK modulation format, ƒideal,i is the ideal symbol location at the FSK frequency corresponding to location i, and <ƒactual,i> is the mean value of the demodulated symbols at location i.
In the preceding figure, the horizontal lines represent the actual FSK demodulated symbol locations, and the vertical lines represent the ideal symbol locations for a 4-FSK format.