STM32WBA antenna matching

[dryet]

Hello,

I am working on a new design using the STM32WBA52CGU6. Is it possible for me to use the MLPF-WB55-01E3 to match the antenna impedance? Or what is the output impedance of this MCU?

Thank you in advance.

[dryet · Accepted solution]

UPDATE:

For anyone interested I got an answer from ST. 

My MCU (STM32WBA52) has the output matched to 50 ohms. So there is only one matching network needed to get the antenna from it's off value to 50 ohms and then it can be directly fed to the RF pin.

STM32WB MCUs are NOT matched to 50 ohms so they need the MLPF chips to be correctly matched for your 50 trace.

[hatone]

STM32WBA52 RF output not  matched to 50 ohms, so they need one matching network to be correctly matched for your 50 trace.  What is the output impedance of STM32WBA52 RF output? No reference materials found

[dryet]

I have also not found any reference material but I got this answer from support:

Let me clarify this matter and you will immediately understand what impedance you should get to adjust the antenna.

This is because the WBA52 has a matched output of 50 OHM. Further in the RF path is an RF filter, which is mainly used for selectivity purpose and suppression of harmonic frequencies. It is an LP filter, but it can also be a BP filter. If you are going to use a chip, i.e. an integrated component, it is usually designed to 50 OHM on both ports, but even so, I recommend checking and possibly correcting the impedance using discrete components. The second option is a discrete filter that you build from L and C Low-ESR and High-Q components. Basically, a 5-order LP filter should be enough.

The antenna is usually not exactly set to 50 OHM (it is also impacted by surrounding objects), so you will probably need to insert a matching circuit just before the antenna port. For Bluetooth, a fairly narrow (in terms of bandwidth) matching network should be sufficient, so a PI or T structure, but often only 2 components are sufficient.

If you use a discrete filter instead of an RF ceramic filter (suppressing harmonics) and at the same time this filter is close to the matching circuit of the antenna, they can be combined into one RF filter, where you will have an impedance of 50 OHM at the input and the complex conjugated value of the antenna at the output. For long RF line I do not recommend this approach.

The MLPF filter is intended for the WB series (not the WBA) for the reason that the WB output is not exactly in 50 OHM and therefore the MLPF filter input is not in 50 OHM either. This filter contains part of the matching network, which is needed to adapt the RF output of the WB transmitter. The second part of the matching network is the section of the RF line between the MCU and the MLPF. This clearly shows that this filter is not completely suitable for WBA. Although I believe it could be quite easily adjusted to 50 OHM at the input and could be used as well. The fact is that then you will lose the price advantage and you will not save space either.

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