## 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). |

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#### 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.