The TRKRFFREQ is the frequency tracking register which stores the output from the Automatic Frequency Control (AFC) unit.
The AFC calculates the deviation of the receiver frequency to the carrier frequency and allows for the receiver PLL to track the frequency error mostly due to aging and temperature drift of the TCXO/XTAL (at both RX and TX).
Details on the TRKRFFREQ register are provided in the AX5043 Programming Manual from page 47.
Note: TRKRFFREQ is updated by the outer frequency tracking loop, while the TRKFREQ register is used for the inner tracking loop (used for PSK only).
The actual frequency deviation in Hz can be calculated from TRKRFFREQ register from the following:
The frequency deviation is calculated following the equation:
The optimal value for the AFC can be easily calculated directly in the AX-RadioLab GUI under the PHY panel. A detailed explanation on the best practices for setting the AFC loop can be found from page 16 on the AX-RadioLab User Manual.