3.5.2.3 ASK Shaping/Power Ramping

A digital ASK shaping filter can be enabled by setting FSEN.ASEN to ‘1’. The filter implements a smooth up- and down-ramping of the PA output power in seven equally spaced steps. This improves the spectral efficiency of the RF transmission signal. If the ASK shaping is enabled, the power amplifier is not completely switched off at a data ‘0’. The transmitted power is still approximately 27 dB below the active value.

The rise and fall time of the power ramp can be configured by the FSFCR.ASDIV value. 2^ASDIV represents the number of XTO clock cycles between two steps.

If the power amplifier is switched off in FSCR.PAOER while the ASK shaping filter is enabled, the hardware design ensures that the power amplifier has an overrun until the ramp-down is completed. The status must be checked by polling the FSCR.PAON bit before switching off the bias of the power amplifier in the FEEN2.PAEN front-end register or even switching off the complete analog front end.

The ASK shaping filter can also be activated in FSK mode to apply ramping on the initial carrier switch-on and final carrier switch-off operation.