Electromagnetic compatibility (EMC) is often described in terms of RF coupling phenomena. Usually the big four: Radiated and Conducted Emission and Radiated and Conducted Susceptibility/Immunity. These four elements cover all of the EMC issues in a very broad sense. Occasionally lightning and ESD will be divorced from susceptibility and described as if these two items were something other than just a high level transient susceptibility condition. That’s very understandable if you’ve witnessed a nearby lightning strike or been zapped on a cold winters day by a seemingly harmless doorknob. Sometimes in the clandestine world of communications security even the low level emissions are carefully examined for their content. It’s all a matter of interest ... the reality is that EMC simply means that electronic widgets can live together in peace and in harmony with one another and with the electromagnetic environment, and we get to define peace, harmony, and environment. It’s all a part of togetherness.
As I mentioned in the last post on MPG systems, as wavelengths (λ) approach the dimensions of boxes, cables, and grounding connections parasitic capacitive coupling tends to interconnect circuits having changing voltage potentials to other circuits and to ground references whether that coupling is desired or not. There are two primary ways of handling parasitic capacitance depending on whether a circuit to circuit connection is permitted. If the circuits must be kept isolated then shielding is used. The shielding can be a simple grounded fence between the cir-cuits or the circuits (one, both, or all) can be completely enclosed. For a peek at how this is done with a small system take a look inside a cell phone. Enclosure can be accomplished using PCB layers, small postage stamp sized boxes, or entire multistory buildings. Shielding will be covered later in the EMC-Zone.
The parasitic capacitance can be shorted if the circuits can be interconnected or connected directly to the ground reference. The intent is to keep the circuits at the same voltage potential. Unfortunately any conductor has inductance which results in voltages being developed from any changing currents through the inductance: V = L di/dt. In addition, the combination of the parasitic capacitance with an inductance may create either a parallel or series resonant circuit (only Murphy knows for sure) with the first resonance occurring at a frequency (F) = 1/ (2 π √(L C)). At this frequency the inductive reactance cancels the capacitive reactance leaving only the resistance of the conductor. Above and below the resonant frequency either the capacitance or the inductance will dominate. Consequently as the frequency increases, long individual wires cannot be used for grounding and bonding because their higher inductance (L) lowers the frequency where the grounding/bonding system fails..
The figure shows the migration from a wire to direct bonding as the frequency increases. The symbol L is used to honor Heinrich Lenz, even though Joseph Henry actually discovered both self and mutual inductance. However, L is also used by most people (especially the mechanical types) as the symbol for length (L). This turns out to be really appropriate because inductance (L) is primarily related to conductor length (L)!
Here’s a memory hook just to illustrate the point (Click to enlarge). Imagine that the figure shows a solid-flat bonding conductor with length (L), width (W), and thickness (t) where L >> W >> t. If the thickness (t) is increased by 10x the inductance is decreased approximately 10%. If the width (W) is increased by 10x the inductance is decreased approximately 30%. If the Length (L) is decreased by 10x (to 1/10th the original length) the inductance is decreased approximately 93%. That’s ninety-three percent! So keep conductors as short as possible, make them as wide as possible, and don’t worry too much about their thickness.
Before there is true togetherness, there are some other factors to be considered such as material types, surface finishes, and methods of connection. Stay tuned to EMC-Zone.com for the next piece on togetherness.
- Ron Brewer