3.5.2.6 Frequency Synthesizer

The RF carrier frequency can be selected with a resolution of f_XTO/2¹⁷⁽¹⁸⁾ = 92.7 Hz (185.43 Hz) for Low-Band (High-Band). The local oscillator frequency f_LO and the transmit frequency f_TX are generated by a fractional-N PLL (see Fractional-N PLL).

Two register sets, FFREQ1H/ FFREQ1M/ FFREQ1L and FFREQ2H/ FFREQ2M/ FFREQ2L, are available to define the fractional part of the two frequency settings in the DSP address space. The integer part has to be set in the FEMS front-end register.

The default frequency used corresponds to the FFREQ1 setting. To select the FFREQ2 setting, the select FSK mode bit (SFM) has to be set in the FSCR register and the modulation signal must be ‘1’, for example, by setting FSCR.TXMOD to ‘1’.

The required FFREQ settings for a certain RF frequency are determined by the configuration tool. Backwards calculation for the local oscillator frequency can be done by the formula given in Fractional-N Divider and SDM.