3.5.2.5 Gauss Shaping/GFSK

To improve the spectral efficiency of the FSK transmission signal a digital Gauss-shaping filter can be enabled by setting FSEN.GAEN to ‘1’. The filter works with an oversampling ratio of 12 compared to the data rate. To achieve this value, an appropriate divider has to be configured in the GACDIVL and GACDIVH registers. The formula is available at the corresponding register description.

Four different bit time/bandwidth settings can be configured in FSFCR.BTSEL: BT = 2, BT = 1.5, BT = 1 and BT = 0.5. A smaller BT value reduces the occupied bandwidth of the spectrum but also gives rise to inter-symbol interference.

The activation of the Gauss-shaping filter introduces a jitter of approximately 8.5% on the symbol time. If both filters, preemphasis and Gauss-shaping are enabled, the combined jitter is still limited to 8.8%.