What is the output impedance of the STM32WB55?

[0x34]

I check the datasheet and it say that the output impedance os the STM32WB55 is 50 ohms , so i don't must need impedance matching but on the reference design have a pi-match circuit and a RF filter. Really the MCU have a 50 ohms impedance output or the pi-match is to adapt the impedance to the input of the RF filter?

[Remi QUINTIN]

​The STM32WB RF output impedance is not 50 ohms. This is why an IPD (integrated passive device) is added on the RF path to ensure the 50 ohms matching.

[0x34]

Ok, if the output impedance is not 50 Ohms what is the value? To make the impedance matching you need to know the impedance output of the MCU and the output necessary impedance, in this case 50 ohms.

[Remi QUINTIN]

The output impedance is adapted to 50 ohms thanks to the various IPDs proposed by ST depending on the STM32WB lines and packages. These IPDs have been developed to cope with this adaptation constraints and remove this burden for customers.

For example, the MLPF-WB55-01E3 IPD integrates an impedance matching network and harmonics filter. The matching impedance network has been tailored to maximize the RF performance of STM32WB (Integrated impedance matching to STM32WB55Cx/Rx, STM32WB50Cx, STM32WB35Cx and STM32WB30Cx). This device uses STMicroelectronics IPD technology on non-conductive glass substrate which optimizes RF performance.

You can find other IPDs for other packages and lines on the www.st.com web site => https://www.st.com/en/emi-filtering-and-signal-conditioning/rf-filters.html

[NHand]

Do I still need the band pass filter after this IPD? Or is it sufficent to route a 50 ohm trace to the antenna? I guess a pcb antenna still needs a pi filter.

[Remi QUINTIN]

The low-pass filter is expected to be 50 ohm at its output but it​ not always the case for the antenna. So you should add another matching network in between the filter and the antenna.

[alexRossRBR]

@Remi QUINTIN  It is nice to have the option of the MLPF-WB55-01E3 but what is the output impedance of the STM32WB55? 

Is there a document? 

[Remi QUINTIN · Accepted solution]

The output impedance depends on the package;

WB5x :

for QFN48 package, the RF output impedance is 42 + j 19

for QFN88 package, the RF output impedance is 50 + j 19

for WLCSP100 package, the RF output impedance is 32 + j 10

for BGA129 package, the RF output impedance is 32 + j 8

WB1x :

for QFN48 package, the RF output impedance is 42 + j 19

for WLCSP49 package, the RF output impedance is 32 + j 10

[rbenfichera]

Very helpful Remi,  but where did you find this information?  Is it provided in the data sheet?

[Remi QUINTIN]

These values are the results of our own measurement using an anechoic chamber and performed in TX mode..

These values are not included in the datasheet. 

Moreover, you also have to consider the RX mode for the sensitivity. The RF input impedance is generally not the same as the internal circuitry is not the same for the RX path and the TX internal paths.

Please have a look at AN5165  that explains how to develop RF hardware using STM32WB MCUs.