STRF Shows How to Do RF Design with an STM32-Based Project
KiCad schematic and PCB layout techniques for a design with high-speed digital and RF.
The STRF by Philip Salmony is a relatively simple board with a USB port on one side, an SMA connector on the other, and an STM32 microcontroller in the middle. There is, quite frankly, nothing extremely exciting or unique about the design. (Although, the guard band around the RF section is pretty snazzy.) Many hobbyist projects and actual products involve three key features: USB, a 32-bit microcontroller, and an RF stage. Instead, this project caught our attention because of the excellent instructional video that Salmony created to accompany it.
"With these videos I am trying to give the information in a condensed form and in one place, that I would have liked to have had starting out when designing STM32-based hardware (as well as PCB routing in general!)."
His explanation provides both a high-level overview while also touching on many of the crucial details related to high-speed digital designs and proper RF layout techniques. It starts by explaining each of the blocks in the STRF schematic and highlights a few design decisions that should be considered by any designer. For example, on the 3.3 volt regulator, he mentions why he picked 10 uF capacitors. They limit the in-rush current on the USB 5 volt rail.
The layout section of the video contains many RF nuggets. Salmony explains the rationale behind STRF's component placements, which pieces to route first, how to design impedance matched traces, and a good explanation of proper four-layer stack-up for RF design. In particular, he correctly shows how to pour ground planes under the antenna matching section. (And when not to!)
While he is using KiCad for the STRF design, it is not necessarily a KiCad tutorial. The techniques and tips he provides are universal to RF board design. But, of course, specific features like differential trace routing are specific to KiCad.
Before diving and re-creating the project based on just the video, you might want to check the STRF GitHub repo first. There were a few minor improvements made to the design after others had a chance to review it. Also, we recommend subscribing to his YouTube channel, as there are likely more good tutorials to come.