Issue link: https://iconnect007.uberflip.com/i/1512857
DECEMBER 2023 I DESIGN007 MAGAZINE 33 which add inductance. is keeps the loop area small and reduces radiation. As the stackup layer count increases, so does the number of possible combinations of the structure. But if one sticks to the basic rules then the best-per- forming configurations are obvious. Figure 2 shows the electromagnetic fields emanating from signal traces in a microstrip configuration. Electric fields terminate when they come into contact with a solid plane but will radiate into the surrounding volume while magnetic fields tend to radiate unless they are shielded by the planes. But in a stripline con- figuration, the fringing fields still radiate from the board edges. e return current of a high-speed, fast-rise time digital signal will always follow the path of least inductance, which is directly beneath the signal path. However, discontinuities tend to divert the return current increasing the loop area, inductance, and delay, which is not desirable. e best way to identify the discontinuities is to follow the signal path and imagine the return path closely coupled on the nearest plane. If there are multiple planes present in the layer stack, then the dis- placement current will still take the path of least inductance and follow closely coupled to the signal trace. If a discontinuity inter- rupts this return flow, then the return cur- rent will be forced into a distant plane where it has a clear run, creating increased induc- tance. A via that provides the connection between signal traces, referenced to different planes, creates discontinuities. In other words, the return current has to jump between the planes to close the current loop, which in turn increases the inductance of the current loop, affecting the signal integrity. is return cur- rent also excites the parallel plate mode, caus- ing significant EMI. If the reference planes are at the same DC potential, then they can be connected by stitching vias near the signal via transition to provide shorter paths for return currents. However, if the planes are at differ- ent DC potentials, then decoupling capacitors must be connected across the planes at these Figure 2: HFSS simulation of electromagnetic energy in microstrip. (Source: Juliano Mologni)