Optimal Impedance for STM32WB RF line

[APlo]

Hello everyone,

I am currently working on the hardware design of a STM32WB55 and there is an information I would like to clarify.

In the AN5165, it is explained that the RF track should be matched at 50 ohms. However, on page 43/48 - figure 36, there is an extraction of results with a test bench used by ST showing the optimal impedance for TX, which is a bit different than the 50 ohms specified (result is Zopt,TX = 42 - j19).

Which impedance should I target for my application ?

Regards.

[Remi QUINTIN · Accepted solution]

The RF output of a STM32WB microcontroller is not 50 Ohms.

Here the need to use matching network to ensure the antenna matching.

Moreover it does not only depends on the board stacking but this also depends on the package.

For a QFN48 package, the value is 43-j16 Ohm which is close to what you read in AN5165 which is indeed measure for a QFN package as mentioned on page 45.

For a WLCSP100 package, the value is 32-J10 Ohm

The complexe impedance of the RF output is the conjugate of those values.

Hence the proposal of integrated matching network (IPD) according to each package.