lcamtuf :verified: :verified: :verified: (@[email protected])

We all know that electricity flows mostly through conductors. Electrons marching through a wire single-file and all that. Of course, electronic circuits require energy to nudge the electrons or stop them in their tracks. A lot of this energy is ultimately dissipated as heat, emitted as light, and so forth. But before that happens, in a working circuit, where does this energy temporarily reside? If your answer is "in the wires", you are largely wrong. The actual answer is "mostly in the surrounding space". This may be the electric field inside the non-conductive film of a capacitor, the magnetic field in the ceramic-like core of an inductor.... but it's also the fields inside the epoxy material of the PCB and in the surrounding air! The last two materials only store a tiny bit of energy; it follows that the creation and collapse of the associated electromagnetic fields happens quickly and produces only a very slight lag for electronic signals. In a low-frequency circuit, you usually don't even notice that it's happening. But if you're toggling bits at 100+ MHz speeds, these momentary movement penalties become a big deal. All of sudden, you're wasting a lot of energy fighting the dielectric medium, and before you know it, your square-wave signals start looking more like sines. The energy you're pumping out isn't just dissipated as heat, either: it couples to nearby signal lines or causes "return currents" that flow in a way that conforms to the alternating field, instead of taking what you think is the most logical route. That's where the design of your board begins to matter. You might need to keep the traces short and spaced far apart. You might need a ground plane that provides a "target" for AC coupling through the PCB material. And you might end up growing some gray hair.