Keep emissions in the can

While working on a project that led to today's 100-Mbits/s LANs, Glen Chenier had a small equipment budget. To save money, he decided to build his own 250-Mbits/s bit-error-ratio (BER) tester for a fraction of the cost of buying one. The tester's data generator required a shield to prevent its electromagnetic emissions from interfering with other sensitive circuits.

Food cans shield sensitive circuits from EMI. Mount signal connectors on the unopened end of the can.

Use tin-plated cans only because you can solder them. Aluminum soft drink cans won't work because solder won't adhere to aluminum. Cans from juice, coffee, vegetables, or sardines work well.

Tin cans do a fine job of shielding radiated emissions—if you use them properly. For example, never use a can that has both ends removed. Mount your connectors on the intact flat end. If you need to shorten cable lengths, then mount connectors to the can's side wall.

After he inserted the circuit board into the can, Chenier closed the can by cutting a piece of copper PCB stock to match the can's diameter and soldering it to the can. A hole in the PCB stock provided access for power-supply wires. Chenier recommends that you keep power wires away from signal-carrying wires by routing them through the opposite end of the can.

Chenier also recommends that you solder the PCB material to the can in three or four places to get good shielding effectiveness. Ideally, you should solder around the full 360 where the PCB and can meet. You should also solder the can's walls to its rims to provide better shielding.

Editor's note: Like so many former telecom engineers, Glen Chenier now enjoys a different line of work. He designs and manufactures items made from wood.