14.6.4.2 External RF Front End Control

When using an external RF front-end including a power amplifier (PA), it may be required to adjust the setup time of the external PA relative to the internal building blocks to optimize the overall power spectral density (PSD) mask.

The start-up sequence of the individual building blocks of the internal transmitter is shown in the figure below where transmission is actually initiated by the rising edge of pin 11 (SLP_TR). The radio transceiver state changes from PLL_ON to BUSY_TX and the PLL settles to the transmit frequency within one symbol period. The modulation starts one symbol period after the rising edge of SLP_TR. During this time, the internal PA is initialized.

The control of the external PA is done via the differential pin pair DIG3/DIG4. DIG3 = H / DIG4 = L indicates that the transmission starts and can be used to enable the external PA. The timing of pins DIG3/DIG4 can be adjusted relative to the start of the frame using the PA_LT bits in the RF_CTRL_0 register (RF_CTRL_0.PA_LT).