Filtered FSK

As with ASK modulation, it is possible to control the spectral occupancy of CPFSK with a pulse-shaping filter prior to the modulator. In the case of FSK, however, there is not a one-to-one mapping between the pulse-shaped spectrum and the modulated spectrum owing to the non-linear modulation process (for example, the VCO).

 

The effect of pulse shaping on the FSK spectrum is shown here for the unfiltered and filtered case. The filter commonly used is an unusual low pass filter called a Gaussian filter, which is particularly well suited to achieving low spectral occupancy outside the FSK main lobe. A modern example of the use of this type of FSK, commonly termed Gaussian Minimum Shift Keying (GMSK), is within the European and North American (GSM-based) digital cellular systems.