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The CC2591 is in TX a highly non linear device hence S-Parameters are not accurately representing the device.
The CC2530_CC2591EM reference design is developed using load/source pull techniques while continuously monitoring overall performance such as output power, current consumption, sensitivity, harmonic content, spurious emissions and more. The reference design is the circuitry that currently is identified to provide the best compromise of these parameters.
In the CC2530_CC2591EM rev 3.2.1 reference design, the effective load impedance at CC2530ANT pin is approximately 13Ω at 2440MHz The impedance is measured by removing CC2591 and measuring the impedance on solder pad CC2591 pin 11. Note, this impedance is not necessary the only optimal impedance as impedance on pin AVDD_PA2, AVDD_LNA and others also influence the effective load/source impedance in normal operation. This means that i.e. changed AVDD_PA2 impedance can generate different optimal ANT pin impedance measured on pin 11.
Remember that impedances at other frequencies also influence circuit operation. Especially the filtering characteristic holds dramatic influence on overall circuit performance.
The differential impedance between RF_N and RF_P when CC2591 is terminated as in reference design is approximately (105+J30)Ω at 2440MHz
Optimal CC2530 load impedance for CC2530 is approximately (69+J29)Ω
Given the complexity of dependencies in CC2591 designs we recommend copying a reference design as closely as possible to minimize the number of design iterations necessary to get acceptable overall performance.
