Misleading (but Often-Repeated) EMC Phrases
Perhaps I've become somewhat of an EMC curmudgeon, but it really bothers me when somebody says their microcontroller is "radiating." I have the same response if they say a trace is radiating, or that a gap in a return plane behaves like a slot antenna. In my opinion, statements like this mislead well-meaning product developers and encourage poor design practices. And it's not just the misuse of the word radiating that bothers me. There are several phrases commonly repeated in EMC publications and presentations that bother me much more than they probably should. Here are some of the things EMC people often repeat that make me cringe.
The microprocessor is radiating!
Is it? Perhaps that's true, but not enough to matter in an EMC radiated emissions test. I think it's important for EMC engineers to distinguish between electromagnetic radiation and field-coupling. They are very different coupling mechanisms. There is almost no overlap between guidelines for preventing radiated coupling compared to guidelines for dealing with electric- or magnetic-field coupling. Microprocessors are often the source of radiated emissions, but they are not good antennas. To radiate enough to exceed a radiated emissions specification, power must be coupled out of the processor and drive external objects that make reasonably efficient antennas. The dominant coupling mechanism that carries power out from the processor is usually conducted coupling, but E-field and H-field coupling can also be important in some cases. Things that virtually never radiate enough to matter include microstrip or stripline traces, integrated circuits, and short gaps in a return plane.
My product is 6 dB(μV/m) over the limit.
I doubt it. Do you mean the emissions were 46 dB(μV/m) and the limit is 40 dB(μV/m)? In that case, your product is 6 dB over the limit. 6 dB(μV/m) corresponds to a field strength of 2 μV/m. If the limit was 40 dB(μV/m), which is 100 μV/m, and you were really 6 dB(μV/m) over the limit, the emissions would be 102 μV/m. That's 0.17 dB over the limit, and I'm pretty sure that's not what you meant.
Ground is an equipotential surface.
No. There are at least three types of "ground" that are relevant to EMC engineers and product designers: safety ground, EMC ground, and circuit current returns. None of them is necessarily an equipotential surface. This statement is troublesome because it's often used to make the argument that ground doesn't exist at high frequencies. In fact, all three types of grounding listed above work at frequencies where the grounding conductors are not electrically small.
Ground is a path for returning currents to their source.
Perhaps that's an apt definition for signal ground, but safety ground and EMC ground are not intentional current return paths. Safety and EMC grounds are not allowed to carry "objectionable" current. For safety grounds, an objectionable current is typically a current backed by a voltage greater than about 50 volts. For EMC ground, an objectionable current is typically any current at MHz frequencies or higher, driven by 1 mV or more. Of course, these are just examples, and the amount of acceptable current will depend on the application. The main reason the statement above makes me cringe is that the two types of ground that often matter the most in EMC should never return signal or power currents to their source.
Ground is a place where carrots and potatoes thrive.
True perhaps, but not helpful. I'll admit, I smiled the first time I heard this. However, this phrase is often used to suggest that the concept of a reference ground is not important. That place where carrots and potatoes thrive may not be important, but identifying and utilizing the local reference ground is an essential part of designing for EMC compliance.
Radiation is proportional to loop area.
Ok, this one doesn't actually make me cringe. But it's worth pointing out that this statement only applies to the radiation from electrically small loops, and electrically small loops are virtually never the source of significant radiated emissions. Yes, it's a good idea to minimize the area of circuit loops. This helps to minimize the chances of inadvertently coupling power from the circuit to other circuits or structures that might be efficient antennas.
Inductance is proportional to loop area.
It's not.
They don't teach this in the schools ...
I'm not going to claim that everything of any importance is part of an undergraduate electrical engineering curriculum. Still, virtually every time I've heard this phrase in an EMC-related presentation, the speaker has gone on to cite something that is not correct or something that is definitely part of any ABET-accredited electrical engineering program.
I could go on, but perhaps I should stop here. I don't want people to get the impression that I'm some kind of EMC grammar policeman. And the truth is that the presenters who use these phrases often have some good advice to offer. So, the next time you're in the presence of a speaker who utters one of these phrases, don't immediately dismiss everything they have to say. It might be better to flash a slight knowing smile and move on.