Also, if a signal is not referenced to its own voltage, i.e.

This is usually done with a stitching via, but if the planes are different voltages, then the stitching must be done using a capacitor, or group of capacitors.

This can cause a break in the return path if the different reference planes are not connected together. Deviation of return current for trace crossing a slot in the reference plane (shown in HyperLynx 3D)Īn equally common occurrence on a PCB is a signal changing references as it passes through a via. In this case, the deviation in return current is much smaller, but some radiation will occur, more-so for larger slots.įigure 1. This problem can be somewhat mitigated by placing a stitching capacitor close to the crossing, but because a mounted capacitor is a narrowband structure, it does not completely eliminate the problem.Īnother, less severe instance of this is when the signal net crosses a hole or slot in the reference plane. The wider the deviation, the more high-frequency content is radiated as EMI.

When a signal net crosses a plane split, the return path is completely broken, and current will need to traverse an alternate path (usually made by some capacitance on the board) to form a closed loop. The most severe case of a broken return path is a signal net crossing a gap in a reference plane (see Figure 1). On a PCB, part of that loop is created by electromagnetic fields coupled between the signal trace and it reference plane, and any break in that path will create an antenna and cause radiated emissions. Any time a high-frequency signal does not flow in a closed loop, an antenna is created, whether intentional or not. The most common cause of radiation from a PCB is a deviation or break in the return current path for a signal. 224 Gb/s Per Lane: Options and Challenges

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