Skip to main content
Magal
Explorer II
August 5, 2026
Solved

STM32WBA62 (BLE 2.4 GHz): best approach for RF matching network + PCB antenna tuning?

  • August 5, 2026
  • 1 reply
  • 103 views

Context

I'm finalizing a 4-layer FR4 board with an STM32WBA62CIU6 (2.4 GHz BLE), and I'm not 100% sure about the best way to make the Bluetooth matching/tuning easier.

I first built a board with a PCB antenna identical to ST's (following the exact shape shown in application note AN5129) and a 5-element Pi network, as shown in one of the circuits in ST's application note AN5948:

1
​​​​​

I then used a VNA, connecting at the pad where the STM32 would sit, and tuned the Pi network components so that the Pi + antenna would present an impedance of 30 + j2.5 Ω — the output impedance of my chip at 10 dBm (the power I want to use):

2

My tuning workflow was: measure with the VNA → export the result → load it into the SimSmith "Load" → place components in the simulator to get a sense of which values to use in real life. However, when I placed the real components, the result was quite different from the simulator — so different that I ended up unable to complete the tuning.

Current setup / measurements

  • Stackup: 4-layer FR4. RF microstrip on Top Layer referenced to Layer 1 (inner ground), separation = prepreg PP-021, h = 0.1855 mm, bulk Dk = 4.74. Measured trace delay ≈ 6.07 ps/mm → microstrip ε_eff ≈ 3.3, VF ≈ 0.55. Trace 0.4 mm wide → Z0 ≈ 38 Ω (Altium field solver, uncontrolled).
  • Measurement: R&S FSH VNA with a U.FL pigtail < 50 mm. I'm seeing common-mode / cable-position sensitivity, and my discrete-model predictions in SimSmith don't track the board well after each component change (component parasitics at 2.4 GHz).

Here is the actual PCB:

3
4

Planned changes for the next revision

For the next PCB revision I'm considering:

  • Trace reference: cutting away the nearest ground plane and using the next ground layer as the reference, so I can route a wider trace with a properly calculated 50 Ω.
  • Antenna: making it slightly longer so I can trim it to better tune the resistive part of the impedance.
  • External antenna: adding a variant with a U.FL connector for an external antenna, to compare range.
  • Matching network: building the Pi network in one of these ways:

Option 1 — a simple 3-element matching network:

5
6

Option 2 — using the MLPF-WB-04D3 (ST integrated 2.4 GHz low-pass filter) or another filter together with a Pi network. I'm not sure whether to use a Pi both before and after the filter, or just one after it.

Option 3 — a "split" approach: a 3-element Pi to bring the chip + components to 50 Ω (measured with a spectrum analyzer) and another Pi to bring the antenna to 50 Ω (measured with a VNA), with a 0 Ω resistor separating the two halves.

Questions

  • Are these good ways to build this kind of PCB?
  • Should I really measure the chip output with a spectrum analyzer to tune it as well, or can I trust the output impedance that ST gives in the datasheet table?
    Best answer by Laurent LOUAZON

    Hello,

     

    Let me try to clarify.
     

    As you noticed, AN5948 provides impedance details : Tx impedance for various Tx output power and Rx impedance.
    You can use those impedance if you are focusing in a specific mode (optimizing Tx power or Rx sensitivity).
    If you want to have best trade off, you can simply consider a 50 Ohms impedance for Rx pin of STM32WBA.

     

    Then, what is the best method?
    I would say this depends on knowledge you have in RF design (seems you have :)), equipment (simulation tool and VNA) and size /cost of the PCB.

    in my opinion the option 3 is the good trade off.
    You simply implement the 3 elements PI described in AN5948 figure 26 right after STM32WBA RF pin (You should set it close from RF pin to ensure short current return path). This filter is 50 Ohms to 50 Ohms and just ensure Tx spurious rejection.
    You then add two or 3 discretes to match antenna.
    Both functions (STM32WBA matching/filtering and antenna matching) are then separated.
    If willing to do conducted measurements (certification), you then simply add SMA after STM32WBA matching/filtering

     

    Option2 is very similar.
    MLPF-WB-04D3 will simply replace the 3 elements PI described in AN5948 figure 26.
    You must respect the distance and impedance between STM32WBA RF pin and MLPF-WB-04D3 described in filter datasheet.

     

    Option 1 (like AN5948 figure 27) is of course the most efficient is size and cost but requires more expertise.
    You should measure antenna’s impedance (let’s imagine it is 25+j60 Ohms at 2440MHz).
    You must then use a power full simulation tool (having real discretes library model) to design a 3 elements filter that should ensure spurious rejection and impedance transfer from 50 Ohms to antenna’s conjugated impedance (25-j60 Ohm for example).
    You must then test it in anechoic chamber to confirm RF performances and then optimize it if needed.

    Hope that helps.

     

    Laurent

      

    1 reply

    Laurent LOUAZON
    Laurent LOUAZONBest answer
    ST Employee
    August 20, 2026

    Hello,

     

    Let me try to clarify.
     

    As you noticed, AN5948 provides impedance details : Tx impedance for various Tx output power and Rx impedance.
    You can use those impedance if you are focusing in a specific mode (optimizing Tx power or Rx sensitivity).
    If you want to have best trade off, you can simply consider a 50 Ohms impedance for Rx pin of STM32WBA.

     

    Then, what is the best method?
    I would say this depends on knowledge you have in RF design (seems you have :)), equipment (simulation tool and VNA) and size /cost of the PCB.

    in my opinion the option 3 is the good trade off.
    You simply implement the 3 elements PI described in AN5948 figure 26 right after STM32WBA RF pin (You should set it close from RF pin to ensure short current return path). This filter is 50 Ohms to 50 Ohms and just ensure Tx spurious rejection.
    You then add two or 3 discretes to match antenna.
    Both functions (STM32WBA matching/filtering and antenna matching) are then separated.
    If willing to do conducted measurements (certification), you then simply add SMA after STM32WBA matching/filtering

     

    Option2 is very similar.
    MLPF-WB-04D3 will simply replace the 3 elements PI described in AN5948 figure 26.
    You must respect the distance and impedance between STM32WBA RF pin and MLPF-WB-04D3 described in filter datasheet.

     

    Option 1 (like AN5948 figure 27) is of course the most efficient is size and cost but requires more expertise.
    You should measure antenna’s impedance (let’s imagine it is 25+j60 Ohms at 2440MHz).
    You must then use a power full simulation tool (having real discretes library model) to design a 3 elements filter that should ensure spurious rejection and impedance transfer from 50 Ohms to antenna’s conjugated impedance (25-j60 Ohm for example).
    You must then test it in anechoic chamber to confirm RF performances and then optimize it if needed.

    Hope that helps.

     

    Laurent

      

    Terms & ConditionsAccessibility statement