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Reading the STM32WB15CC datasheet, 3.6.1 states that:

Thanks to an internal transformer at RF pins, the circuit directly interfaces the antenna (single ended connection, impedance close to 50 Ω). The natural bandpass behavior of the internal transformer, simplifies outside circuitry aimed for harmonic filtering and out of band interferer rejection.

This, to me, suggests that you can connect the RF pin directly to a 50 ohms antenna (e.g. an PCB SMA connector).

However, ST AN5165 (How to develop RF hardware using STM32WB microcontrollers) shows the use of an external RF filter (which appears to be a bandpass followed by a low pass).

It also seems to be conventional (i.e. for most 2.4 GHz/Bluetooth MCUs) to use a bandpass filter, either by creating your own out of passive components, or by using an IPD (e.g. TDK DEA202450BT-1213C1 or Murata LFB182G45BG2D280).

The ST AN5165 document has a filter design which uses passive components, with a--somewhat rarely stocked--low pass component:

schematic

simulate this circuit – Schematic created using CircuitLab

For comparison, I found an example of a 2.4 GHz bandpass filter by Phil's Lab, but this is for an ESP32 (I'm not sure if this has an internal transformer for the RF pin):

schematic

simulate this circuit

So, if the IC has an internal transformer on the RF pin, do you still need the external filter?

Note: I would actually like to use the Texas AN043 SWRA117D antenna PCB design; would using this instead of a shielded PCB SMA connector introduce the need for the external filter?

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    \$\begingroup\$ look at figure 3 in the STM32WB15CC datasheet \$\endgroup\$
    – jsotola
    Commented May 5, 2023 at 16:01
  • \$\begingroup\$ Yes, I noticed that. Based on this, I am proceeding with the presumption that "the circuit directly interfaces the antenna" actually elaborates to "the circuit directly interfaces the antenna, through an RF filter". My thinking is that the PCB may introduce noise into the antenna, especially if it's a PCB antenna. \$\endgroup\$ Commented May 5, 2023 at 19:12

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As well as Figure 3 in the STM32WB15CC datasheet indicating an external filter, the ST Nucleo WB55RG schematic shows several external filter options, indicating that the external filter. This indicates that it is needed.

RF section of ST Nucleo WB55RG schematic

Curiously, they omit the 1.2 pF on the IC side of the antenna matching network. They also have different filter values for the SMA antenna.

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