RF Board (2020, Age 15)
RF engineering in general is often considered by electrical engineers to be black magic. This reputation is not unwarranted. The assumptions one may make about how electricity behaves begin to break down, destroying all abstractions that are used to make the wrangling of electricity more tolerable, leaving a pile of unpleasant math and pure physics in its wake.
Sounds like fun.
The task sounded simple; design and produce a breakout for the nRF24L01+ 2.4GHz transceiver. And to an extent, it was. Plenty of prior art exists, with this component in particular being a common part used with Arduinos for radio communication. Nordic Semiconductor even provides a very helpful application example, complete with their BOM. Simple, right? I wish.
As it turns out, they leave out a single entry on their example: the antenna. Including a master’s degree in electrical engineering might’ve helped too. As it turns out, and quite to my naive surprise, antennas are weird. I ended up opting for a microstrip inverted-F antenna, but if I were to do this again, I would instead leave an SMA connector for an external antenna¹.
At the heart of the breakout is the nRF24. Most of the pins on the oh-so-annoying QFN20 package are dedicated to communication or power, except for 12 and 13; the antenna pins. That is two pins, differential. We need single-ended 50Ω. It’s time for a balun! Since I didn’t want to bother designing (or borrowing) that, I simply opted for a COTS balun designed for the nRF24¹². A very short 50Ω transmission line to the antenna, and it is (theoretically) done.
I had the 4 layer PCB made with JLCPCB, with the parts bought from DigiKey. Assembly was done by hand with a hot air station. I only injured myself once. Unsurprisingly, it did not work. I suspect a short between a number of the pins of the nRF24, via the thermal pad in the center. It is probably a good thing it did not work, as I made numerous severe and vaguely embarrassing design errors. I can count three in the right-hand picture, and even more in the half I was too embarrassed to show. How many can you count?
Hey, I learned something though!
¹Thorny problem? Just outsource it to another engineer!