Dear ST community,
I am developing a PCB design for wireless communication utilizing STM32WLE5CCU6. I have a very basic grasp of RF knowledge, impedance matching for example. I read thoroughly the documents: AN5457 and AN5407 of designing RF layout. I also took a glance on the Nucleo PCB design files. I noticed some discrepancies and I would like to ask you for your insight.
1. In document AN5407 section 9 Table 5 there is example of recommended discontinuity management. It is designed for RF trace width matching the width of SMD components. I checked the STM32WL Nucleo design and noticed that for RFO_LP and RFO_HP signals the traces are quite thick between the matching components. I extracted the trace design properties (width, height, gap, Er) and put them into Saturn PCB calculator. The Zo impedance doesnt match 50 Ohms as shown below:
The Nucleo layout doesn't follow the design rules, as you can see on picture below (I marked a concerning layout with a green rectangle):
The "NETC15_1" trace doesn't have 50 Ohm impedance, and there is a polygon above this net. From the same calculator, the width of trace Matching 50 Ohms impedance should be around 0.155mm. I do understand the fact that the Nucleo design works and was probably a subject of multiple evaluations and checks.
My questions are:
1.how should I deal with discontinuitues between SMD component pad size and trace width (0402 Pad width > trace width), knowing that a rectangular polygon of abstract size related to SMD pads' size works?
2. If I route traces following the guidelines (impedance matched traces expanding their width to match SMD pads) can I still use the C and L values of matching network, or was there some Smith Chart magic included on Nucleo design?
I will highly appreciate any designing tips, I am thankful in advance.
Best regards
Marek Jaworski.
