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Balun Placement in Nucleo WL5JC Reference Schematics

Associate III

Hi All,

As an absolute beginner in schematics and PCB design, I have decided to build a breakout board for the STM32WLx MCU. For reference, I used the NUCLEO-WL5JC schematics.

I noticed that there are two different schematics for high-band boards:

  1. MB1389-WL55JC-highband-E02 Schematic Pack
  2. MB1389-WL55JC-highband-D04 Schematic Pack

However, when I compared their RF sections, they appeared to be similar. Could you please explain the difference between these two versions of schematics (E02 and D04)?

My second question also pertains to RF implementation. In the MCU documentation/manual (STM32WL55JC) Section 3.9.7, "IPDs for STM32WL and reference designs," it indicates that RF input and output pins connect to a balun (for example: BALFHB-WL-06D3 Balun for 2 Layer PCB), which is then connected to an RF switch.

But in the reference schematics, I noticed that there is no balun. However, there is an RF Wideband switch (BGS13S4N9) from Infineon, and there are also two NX3L1T3157 analog switches from NXP. Could you kindly explain the role of these analog DC switches from NXP and why there is no recommended balun from ST in these schematics?

Best regards