Regarding the AD9361 Tx third-order Butterworth BaseBand (BB) Low Pass Filter (LPF):

- UG-570's "BASEBAND Tx ANALOG FILTER CALIBRATION" allows for the derivation of the following calculation for maximum precision of nominal (theoretical) in-situ (actual) BB LPF 3dB cutoff frequency:
- BBBW_DESIRED,MHz // this is desired BaseBand BandWidth, which is "half the complex bandwidth"
- BBPLF_3dB_DESIRED,MHz = 1.6 * BBBW_DESIRED,MHz // "normally calibrated to 1.6x the BBBW"
- BBLPF_3dB_ACTUAL,MHz = BBPLL_ACTUAL,MHZ * ln(2) / (2 * pi * ceiling(BBPLL_ACTUAL,MHz * ln(2) / [2 * pi * BBPLF_3dB_DESIRED,MHz] )

Regarding the AD9361 Tx single-pole secondary LPF:

- this filter (which appears to be a single-pole RC filter with R values from UG-671 Table 37.) has no such formula (missing element appears to be the C values?)
- other forum posts refer to:
- [AD9361 RF Bandwidth ] an older AD9361 Calibration Guide which was supposedly merged into UG-570, but UG570 doesn't specify any secondary LPF formulas
- [AD9361 tx secondary filter resistor and capacitor ] refers to:
- The MATLAB Filter Design Wizard, which provides formulas for analog filter response (butter(...)), but they do not provide any insight into in-situ values for secondary LPF via AD9361 registers 0x0D0, 0x0D1, 0x0D2
- The No-OS ad9361_tx_bb_second_filter_calib() function, which calculates register 0x0D2 values but we still don't know what capacitor values they correspond to

- other forum posts refer to:

Can you 1) confirm the validity of the AD9361 Tx BB LPF 3dB cutoff frequency formula and 2) provide a similar formula (or enough information to derive it) for AD9361 Tx secondary LPF?

Why you are looking for calculating RC values?

The No-OS API does it and use Filter wizard to generate custom filters.

MATLAB Filter Design Wizard for AD9361 [Analog Devices Wiki]