Understanding impedance matching in RF PCB applications (STM32WB55)

Question

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

And also found this post on their community forums:

Characteristic impedance of the RFO pin in the WB55

Alright, 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 instead of the desired 50 Ohm:

I then used the following calculator to get the values for a matching network for a 50Ohm load

This one here.

And 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.

I tried a different tool: 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 + j13.28

Now I am thinking I may be doing something wrong, or I have misunderstood how a matching network actually works.

Can anybody help me understand what's going on with the Application Note, and what I am doing wrong?

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.

Answer 1

Technically a shunt capacity of 0.6pF (or an equivalent, short 50 Ohm transmission line stub) would already be enough to make the output impedance 50 Ohm:

The component values for the matching network are only valid for the PCB layout shown in the design guide. ST probably assumes an additional 0.4pF parasitic capacity to ground at the input and output of the matching network:

Another simple matching solution would be to load the output with a short 50 Ohm transmission line in series with a 2.7pF capacitor: