Day: November 2, 2023

Homebrew TEM Cell Lets You EMC Test Your Own Devices

November 2, 2023 by 14 Comments

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.

A Network Adapter Thinks It’s A CD-ROM. Restore Its True Calling!

November 2, 2023 by 31 Comments

A mildly annoying trend over recent years has been for USB hardware devices to expose a CD-ROM drive containing their drivers for Windows users. Of course there’s no real CD in there, instead the software lives on a piece of flash memory. It’s usually not a problem as they also appear on the USB bus as their true calling, but not always.

[Martijn Braam] found himself the lucky owner of a USB network adapter which seemed to see its only purpose in life to be such a drive, and since he  wasn’t anxious to make another piece of e-waste, he broke it open to see if the fake CD drive could be disabled.

Inside the flimsy case he found a CoreChips SR9700 Ethernet controller, a chip for which there seems to be very little data in the wild. On the underside of the PCB was a flash chip, and as expected disabling this caused the CD drive to disappear to be replaced by the expected network card.

It’s a simple but useful hack, but there’s a little bonus for those unaware in the write-up. There’s a piece of software called USB_modeswitch that can perform this task on many cards, which is worth storing away in the event that it’s needed.