2 RF Matching Network

The RF matching network consists of a balun and filter. The combination of the balun and filter also transforms the complex differential output impedance of the PA to 50 Ω. Furthermore, the PA is biased (powered) through the external RF network. To output 20 dBm, the PA is biased from VDDS to allow a large enough voltage swing.

To optimize the PA for 10 dBm operation, two changes must be done:

• First, the biasing is changed from VDDS to VDDR. This moves the DC operating point of the output stage and thus reduces the available output power to 10 dBm while maintaining high efficiency. Since VDDR is typically derived from the internal DCDC regulator that has very high efficiency, biasing from VDDR also reduces the overall current consumption for the radio.
• Second, update the component values in the RF matching network. Because the operating point of the PA is changed with different biasing and different output power, the optimum load impedance is also changed and thus the external matching must be adapted accordingly.

The resulting recommended schematic for 10 dBm PA operation is shown in Section A.1.

For this design, external load capacitors for the 48 MHz crystal should be used to reduce clock spurs.

The layout is based on the CC1352PEM-XD7793-XD24-PA24 design files. The top layer is equal to the top layer from the RF pins to the 50 Ω point, except for the biasing the RF network from VDDR instead of VDDS.