STM32WB55RGV6 BLE design questions

[WLai.1]

Several BLE circuit design inquiries of the STM32WB55RG MCU.

The BLE impedance requirement of this MCU is 50 ohm, and it is recommended to use LPF (MLPF-WB55-01E3). For the detailed circuit diagram, please refer to the fourth page of the attached file (20220119_Main Board(3-8).pdf), component information Elements will appear by clicking on them with the mouse.

The PCB stack is 1oz CU / 0.1mm(FR4) / 1oz CU / 1.2mm (FR4) / 1oz CU / 0.1mm(FR4) / 1oz CU with a total thickness of 1.6mm.

1. With the current design, the BLE signal will first pass through the LPF, and a T-type matching circuit is designed between the LPF and the antenna. Is this design OK? I read the ST document (AN5165) that the filter is placed near the antenna. , the Pi-type matching circuit is placed between the filter and the MCU.

2. If it is feasible according to the current design, is there a tool to quickly calculate the component values and line width/length of the matching circuit?

3. Currently discussing the line impedance with the board factory, the board factory told the current design that the impedance cannot be measured during the production process, and the impedance must be measured on both sides of the RF trace (such as the white part in the attached figure), but this applies to my current design?

[Remy ISSALYS]

Hello,

1.Your design is OK. The LPF (MLPF-WB55-01E3) is for the STM32WB and the T-type matching circuit is for the antenna.

In this configuration, there is no LPF (MLPF-WB55-01E3) and it is replace by a Pi-type matching circuit following by a filter.

2.You can use the Freeware QUCS tool.

3.The RF trace must be fine tuned by the board factory with the parameters of the material used for the manufacturing of the board.

Best Regards,

[WLai.1]

Dear,

​

Thank you for your reply.

I used QucsStudio to simulate the impedance matching, documentation is attached.

You can try to open the attachment file, there have some question here:

1. In the schematic, I use a "Stripline step" to connect the T-circuit and the antenna (because it can set the length and width), is this reasonable?

2. In the actual PCB layout, there is no GND on both sides of the line connecting the T-shaped circuit to the antenna, so use the stripline as the simulation condition, is it OK?

​

[Remy ISSALYS]

Hello,

I do not use QucsStudio nut ADS, so I am not able to open your file.

If your line is on top of the PCB with no ground on both side, it is Microstrip and not Stripline.

Best Regards

[WLai.1]

Hello,

OK.

Attached is my simulation data (PDF file), can you help to check it?

[Remy ISSALYS]

Hello,

Sorry, but I do not use stub to match an antenna, only with SMD components.

Best Regards