Emc engineer definition12/28/2023 Keeping these two concepts in mind just reinforces the importance of designing transmission lines (power and signal traces with return path directly adjacent), rather than just simple circuit trace routing. However, we also need to consider the fact that the energy of the signal is not only the current flow, but an electromagnetic wave front moving through the dielectric, or a “field theory” point of view. Most of us were taught the “circuit theory” point of view and it is important when we visualize how return currents want to flow back to the source. If it is interrupted in any way, the propagating electromagnetic wave will “leak” all around inside the PC board dielectric layers and cause electromagnetic coupling and “common mode” currents to form, which then couple to other signals (cross-coupling) or to “antenna-like structures”, such as I/O cables or slots/apertures in shielded enclosures. The important thing is that this combination of conduction and displacement current must have an uninterrupted path back to the source. Refer to References 1, 2, and 3 for further details.įigure 1 – A digital signal propagating along a microstrip with currents shown. The actual physical mechanism of near light speed propagation is due to a “kink” in the E-field, which propagates along the molecules of copper. The signal’s wave front travels at some fraction of the speed of light, as determined by the dielectric constant of the material, while the conduction current is comprised of a high density of free electrons moving at about 1 cm/ second. This is the same principle for capacitors and Maxwell called this effect “displacement current”. Of course, this conduction current cannot flow through the PC board dielectric, but the charge at the wave front repels a like charge on the return plane, which “appears” as if current is flowing. As shown in Figure 1, a high frequency signal propagates along a microstrip transmission line (circuit trace over return plane, for example), and the wave front induces a conduction current in the copper trace and back along the return plane. That’s why the use of solid return planes under high frequency signals and then segregating digital, power, and analog circuitry (keeping them separate) on your board is so important.Īt frequencies greater than DC, digital signals start to propagate as electromagnetic waves in transmission lines. Just be aware of the importance of designing defined signal and power supply return paths. We’ll discuss PC board design in the next article. Where some board designs go wrong is when high dV/ dt return signals, such as those from low frequency DCDC switch mode converters or high di/dt return signals get comingled with I/O circuit return currents or sensitive analog return currents. For frequencies above this, the return current tends to follow directly under the signal trace and back to the source (path of least impedance). For frequencies less than this, the return current will tend to follow the shortest path back to the source (path of least resistance). So what is “high frequency”? Basically, anything higher than 50 to 100 kHz. If you think about it, we don’t even draw these return paths on the schematic diagram – just showing it as a series of various “ground” symbols. The problem we circuit designers miss is defining the return path back to the source. These two concepts are closely related and coupled to one another. Digital signals create the propagating field, which induces the convection current to flow in the copper traces/planes. The two concepts are related because they are intertwined together. Understanding EMC is all about two important concepts: (1) all currents flow in loops and (2) high frequency signals are propagated as electromagnetic waves in transmission lines and the field energy travels through the dielectric.
0 Comments
Leave a Reply.AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |