A new book I ran across recently made me realize how important it is to be aware of the electromagnetic interference (EMI) problem all wireless and other electronic engineers face in new designs. The book is Radio Frequency Interference Pocket Guide by Kenneth Wyatt and Michael Gruber, from Scitech Publishing. Radio Frequency Interference (RFI) is basically the same as EMI, but relates more to communications rather than other non-communications. Let’s face it: We live in a massive fog of radio signals and other electromagnetic fields produced by all the electrical and electronic gear we use. Some of those signals can potentially interfere with existing electronic devices. Your job as an engineer is to see that this does not happen.
EMI is basically any signal that can disrupt the operation of any other circuit or device. It may create data errors, garble voice or video, cause intermittent operation, or mask desired signals. EMI is generated by virtually any circuit. It comes from local oscillators or VCOs in receivers, synthesizers in transmitters, and clock oscillators in computers and other digital equipment. Particularly offensive sources are switch-mode power supplies like switching regulators, dc-dc converters, and dc-to-ac inverters. Compact fluorescent lamps (CFLs) are nasty sources. Digital signals with fast rise and fall times and all the harmonics are a major nuisance. And don’t forget noise and load switching transients on the ac power line. Then there are the natural sources of EMI such as lightning and other atmospheric noise in addition to cosmic noise from the sun and stars.
EMI gets into “victim” devices by direct connection or radiation. Conducted EMI gets transferred by the ac mains, shared dc power buses, and cable connections. Radiated EMI is passed like a radio signal or by mutual inductive or capacitive coupling.
The best way to minimize EMI is to keep the generators and victims as far apart as possible. Since radio signals and electromagnetic fields decrease by a power of two or three with distance, separation is a good first step in both locating equipment and laying out a PC board. Next, keep all wiring short. Do not run cables in parallel for any distance. Use shielded twisted pair or coax to minimize cross-talk. If that doesn’t work, try the holy trinity of EMI mitigation: filtering, grounding, and shielding.
For example, try an ac line low-pass filter if that fits the problem. Select the filter type by the nature of the EMI signal. Filters work well in reducing EMI, but add cost and can attenuate or distort the desired signal. Then there is grounding. Some EMI is caused by high-resistance grounds or ground loops. A heavy single-point ground is best. If all else fails, try a metallic shield around an offending circuit or device.
The best way to find EMI is with an oscilloscope and/or a spectrum analyzer. Most of the major test equipment manufacturers offer EMI packages with antennas or probes to help you sniff out the offending signals.
If you want to know more, the book I mentioned earlier is a good summary and starting point. Also get familiar with FCC requirements for EMI control in Parts 15 and 18 of the Code of Federal Regulations (CFR) 47.