Understanding how matching impedance networks work (SMT32WB55 and BLE) - Page 1
Forum section: electronics / rf-microwave

author: guscrown
author_rank: Regular Contributor
timestamp: June 22, 2024, 09:11:10 pm
content: I'm just getting into RF design since I inherited a board that uses an STM32WB55C for BLE. For now I want to start getting more familiar with the RF concepts since I don't have a lot of experience.As I was going through ST's RF Design guide here:[How to develop RF HW with the STM32WB]https://www.st.com/resource/en/application_note/an5165-how-to-develop-rf-hardware-using-stm32wb-microcontrollers-stmicroelectronics.pdfAnd also found this post on their community forums:[Characteristic impedance of the RFO pin in the WB55]https://community.st.com/t5/stm32-mcus-wireless/what-is-the-output-impedance-of-the-stm32wb55/td-p/162181Alright, so let me tell you what I'm struggling with:I now know that the characteristic impedance of the RF pin in the QFN48 is 42 + j19, hence why they are using either an IPD or a matching network filter.In that guide you can see the following values for the matching network:But I ran those numbers via a S-Paramter Calculator and I end up with a characteristic impedance of 62 + j22.I then used the following calculator to get the values for a matching network for a 50Ohm loadhttps://www.electronics-tutorials.ws/tools/pi-pad-impedance-calculator.htmlAnd it gives me the following results:And they don't match the STM guide. If I run the numbers from pi-pad on my Smith calculator I get a proper 50Ohm impedance.Can anybody help me understand what's going on with the Application Note, and what I am doing wrong?I then tried SimSmith on MacOS and I noticed something. If I put the target on the left side, and plug in the values from STM32's AN then I do see a value closer to the reported 42 + j19:You can see here right under C2 an impedance of 42.46 + j13.28Now I am thinking I may be doing something wrong, or I have misunderstood how a matching network actually works.Edit:I just realized that the order of the components in the pi filter is not the same in that last calculation I did, so it may just be a coincidence that it calculated a number close to 42 + j19.

author: guscrown
author_rank: Regular Contributor
timestamp: June 22, 2024, 10:24:15 pm
content: I just realized something: In SimSmith if i set the load impedance to be the WB55 pin impedance and load the values I got from pi-pad I get a source impedance of 50 Ohm. And if I set the load impedance to be 50 Ohm, re-order the capacitors in the pi-filter I get a source impedance that is the conjugate of the WB55 pin impedance (42-j19).Either way, it seems that the values from the pi-pad calculator are working as intended. I still cannot make sense of the values that STM has in their application note.What am I missing?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: June 24, 2024, 01:37:55 am
content: All the SMT components have parasitic elements (for example the capacitors might have 0.5 nH of inductance) and the inductors might have 0.2 pF of shunt capacitance. Throws off all the simulations and further, exactly how they are mounted will throw it off more. And vias have a few hundred pH of inductance depending on board stackup and the inductors SMT pads have maybe 50-100 fF or shunt capacitance. So if you are not using a library of SMT parts those values wont' be in your simluilation.This was part of the EE142 lab I helped write for UC Berkeley. Lab 1 design a BPF filter using SMT parts. Lab 2 measure the filter (they were all 50-70% too low); Lab 3 measure all the parts individually. Lab 4, redesign the filter and test it again, using the full model.