July 2013

“I’ve all ready read the books on EMC and visited a lot of home pages... But all these references did not mention anything about the physical phenomenon that causes common mode currents... Are common mode emissions inherent in any physical system? Can I model them?” Overheard on the ‘Net

It’s by no means a trivial question. And, in spite of decades of hand waving by authors and consultants, the principal mechanism by which common mode currents are created in digital devices was not well understood until the decade of the 90s. In this article, we’ll explore the physics behind the creation of common mode currents, and perform some experiments to verify our understanding.

Every EMI (electromagnetic interference) problem ultimately starts or ends at an electronic circuit. And since electronic components are the building blocks of circuits, it only makes sense to pay attention to the EMI impact of those individual components.

Probably the most important thing to remember about electronic components is that nothing is ideal. Components change values with frequency, current, voltage, and even physical size. And those changes may be nonlinear, adding a new level of complexity. Like a pilot, you need to know the limits so you stay within the envelope of safe performance.

In this article, a time-domain EMI measurement system for the frequency range from 10 Hz to 40 GHz is presented. Signals with a frequency of up to 1.1 GHz are sampled by an ultra-fast floating-point analog-to-digital-converter (ADC) and processed in real-time on a field-programmable-gatearray (FPGA). An ultra-broadband multi-stage down-converter allows for the measurement of signals with frequencies up to 40 GHz. Measurement times can be reduced by several orders of magnitude compared to traditional EMI-receivers that work in frequency-domain.