CC26xx Optimal Load Impedance
The CC26xx supports several front end configurations, both differential and single-ended operation with either internal or external bias. The different RF front-end configurations are presented in more details in CC26xx RF Frontends and Antennas. TI offers several reference designs for CC26xx showing recommendations for the different RF front-end configuration. Note that the different RF front-end configurations from the different reference designs do not follow the package sizes (7x7 mm, 5x5 mm and 4x4 mm) and can be mixed as wanted.
The CC26xx impedance change with the state of the chip (TX/ RX) and the output / input signal level. When operated in receive, the LNA gain is adjusted according to the input signal level and is thus not constant. This results in that the LNA operates with different configuration for the different gain setting. The PA impedance is further different than the receive impedance, it changes with the output power level chosen and is not linear. The term output impedance is used for linear amplifiers or amplifiers that can be approximated by a linear equivalent. Output impedance is normally used to design complex conjugate impedance matching between amplifier and load. For linear amplifiers this is sufficient to secure optimum power transfer. This method is thus not valid for the CC26xx series. CC26xx operations are heavily dependent upon filter-balun impedance up to at least the third harmonic. Matching load impedance only at fundamental frequency could easily result in high current consumption, low output power and high spurious/harmonics.
To get the optimal performance with a CC26xx design, we highly recommend customers to follow the reference designs (schematic, layout and stack-up). TI have found the recommended balun and matching circuits through simulations and load- and source-pull measurements over the full operational range. The RF circuits are designed to give best overall TX and RX performance (output power, sensitivity, current consumption, and harmonic and spurious emission). For more tips and recommendations when making a new design, we also recommend customers to take a look at the design tips found in the HW section under Desing Resources on the BLE wiki.
Simulated values for the optimum load impedances for CC26xx are:
Differential External Bias
Differential Internal Bias
Single-ended Internal Bias
Note that impedance calculated based on analyzing datasheet and reference design schematic often deviate from the optimum load impedance given by TI. PCB parasitic and component imperfections generally accounts for these differences. When operating at high frequencies, PCB traces has to be modeled to account for phase shift, skin effect increased resistance, inductive and capacitive effects. Manufacturers of passives normally provide linear LCR models and/or S-parameter models representing their components at higher frequencies. Be aware to check the valid frequency range for the models, and only use them within this range. Simulators often extrapolate the model data without warnings and simulation results become invalid. Remember that valid frequency range is the range up to the highest significant frequency component within your circuit.