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Background

I'm trying to design a device with an STM32WL series MCU using the UFBGA-73 package. There is only one reference circuit given for the RF portion of the MCU in an application note (pg. 6) and the examples given in the document use the same package, so the pins are identical.

There are 4 RF signal I/O pins, 2 for RX and 2 for TX:

  • RFI_P: Positive differential input (RX+, 100Ω).
  • RFI_N: Negative differential input (RX-, 100Ω).
  • RFO_HP: Low power output (TX, 50Ω).
  • RFO_LP: High power output (TX, 50Ω).

There is an internal "power amplifier" supplying from pin VR_PA which itself is supplied from external input to the VDDPA pin. The external supply for VDDPA is required to be full voltage (e.g. 3.3V) when using high power (RFO_HP) but when using low power VDDPA must be connected to a different supply (SMPS feedback, 1.55V).

The VR_PA pin appears to be connected via an "inductor" (no value ever given in any document) to either RFO_HP or RFO_LP depending on the chosen TX power level, but another application note (pg. 8) shows a configuration for using both power levels is shown in this image:

both power levels

The sole reference image of the schematic for the RF circuit used in an unreleased MB1552B reference board can be seen below. This board uses an RF switch for choosing either TX or RX to the antenna connector from an external input and is configured for using RFO_HP but can be switched to RFO_LP via "permanent" solder connections, assumed to be for testing purposes.

RF matching circuit

Questions

I have two issues, one about the power amplifier connection to the TX line and one about the general content of the reference circuit.

1. Component Function

Most of the components don't have values, so it feels like an educated guessing game to figure out what they're for. I've included a rough interpretation of what I think they might be for. Reference interpretation

2. PA connection to TX (choke?)

I plan to target 915 MHz operation, so if the inductor L3 in the reference is acting as a choke to block noise from the PA I should theoretically be able to use the same inductor for both of the two transmission lines because the blocking properties would not be related to power level (assuming it can handle the power). I am using an RF switch for selecting the power level as shown below, using the same signal as changing the two other switches (i.e. control input at each "LP" into a switch in the image). Can I use a single inductor to connect the currently active PA to the currently active RFO?.

As a side note I have seen guidelines for the choke impedance to be 10x the impedance to 10x50 = 500Ω, please let me know if this is appropriate/correct. Possible Option

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Quick partial answer with docs that have become available since your wrote your question:

ST has published a pretty good app note AN5457 on the matching network design: https://www.st.com/resource/en/application_note/dm00690797-rf-matching-network-design-guide-for-stm32wl-series-stmicroelectronics.pdf

You can also find specific component values in the BOM for the Nucleo WL55JC docs: https://www.st.com/content/st_com/en/products/evaluation-tools/product-evaluation-tools/mcu-mpu-eval-tools/stm32-mcu-mpu-eval-tools/stm32-nucleo-boards/nucleo-wl55jc.html and there's a schematic for a not-yet-published reference board on the ST forum: https://community.st.com/s/question/0D53W00000FzAd6SAF/mb1552b-board-stm32wl-reference-layout-bom-and-schematic-

Finally, there are sx1262 docs and reference layouts on Semtech's site.

I still found it a mess to track down all the details 'cause the docs are scattered, but it's not a wild guessing game anymore.

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