Submitting a new device for electromagnetic compatibility (EMC) testing seems a little like showing up for the final exam after skipping all the lectures. You might get lucky and pass, but it really would have been smarter to take a few of the quizzes to see how things were going during the semester. Similarly, it would be nice to know you’re not making any boneheaded mistakes early in the design process, which is what this DIY TEM cell is all about.
We really like [Petteri Aimonen]’s explanation of what a TEM cell, or transverse electromagnetic cell, is: he describes it as “an expanded coaxial cable that is wide enough to put your device inside of.” It basically a cage made of conductive material that encloses a space for the device under test, along with a stripline going down its center. The outer cage is attached to the outer braid of a coaxial cable, while the stripline is connected to the center conductor. Any electric or magnetic field generated by the device inside the cage goes down the coax into your test instrument, typically a spectrum analyzer.
[Petteri]’s homebrew TEM is made from a common enough material: copper-clad FR4. You could use double-sided material, or even sheet copper if you’re rich, but PCB stock is easy to work with and gets the job done. His design is detailed in a second post, which goes through the process of designing the size and shapes of all the parts as well as CNC milling the sheets of material. [Petteri] tried to make the joints by milling part-way through the substrate and bending the sheet into shape, but sadly, the copper didn’t want to cooperate with his PCB origami. Luckily, copper foil tape and a little solder heal all wounds. He also incorporated a line impedance stabilization network (LISN) into the build to provide a consistent 50-ohm characteristic impedance.
How does it work? Pretty well, it seems; when connected to a TinySA spectrum analyzer, [Petteri] was able to find high-frequency conductive noise coming from the flyback section of a switch-mode power supply. All it took was a ferrite bead and cap to fix it early in the prototyping phase of the project. Sounds like a win to us.