The Art of Stifling Interference on a Printed Circuit Board Assembly

摘要

A RAGING FRATERNITY party with thumping house music can be annoying as the morning hours approach. Noise suppression ordinances to the rescue! The partiers have two choices: quiet down or get shut down. In that sense, the fraternity party is like building an electronic circuit. If our machines make too much "noise" in any part of the spectrum, it's game over. Just like kids can stop trampling everyone's lawn and come inside, shut the doors, windows, shutters and even the fireplace flue, we can also contain unwanted spectral emissions. Left unchecked, a printed circuit is an antenna for transmitting and receiving energy from within and outside the board. Even a well-designed PCB has compromises. Our goal is to be ready to react to spurious emissions that take us beyond the allowable threshold. We start with a power budget, and a noise floor, and our electronics must comply with those design criteria while meeting regulatory requirements. As we bring different functions aboard, the problems in the near field multiply. We think of high-speed transmission lines as the focus of our EMI abatement efforts. They are seen as the cause of the problem, owing to their signal rise times or a harmonic of that momentary event. When the root cause is identified, the solution often comes in the form of an additional filter of some kind. Sometimes, that's not enough, and a more extensive re-spin of the board is needed. Just as good fences make good neighbors, so does metallic shielding around various circuits on the PCB.