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Dealing with EMI in PCB design

15 Apr 2015  | Minoru Ishikawa

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With ever-faster interfaces on PC boards, managing electromagnetic interference (EMI) is one of the most serious challenges designers face. There are many possible causes of unwanted emissions. Here are a few examples that can cause EMI issues:

 • High-speed trace crossing a reference plane gap
 • High-speed trace routed near a plane edge
 • Reference plane changes for a high-speed trace

Using design rule checks is a quick way to locate potential sources of EMI. Though rule-based checking is not perfect, it provides many user benefits. 3-D EM simulation is commonly used to simulate EMI phenomena. However, the simulation results do not identify the cause of radiation, they only show you how the EM fields are behaving. The source of the radiation usually cannot be found even through close examination of the simulation results.

Rule checking to eliminate EMI sources
Let's look at an example of using rule checks to locate sources of EMI. The PCB in figure 1 has three FPGAs, four DDR3 memory devices, and four DDR2 devices. One of the most common sources of EMI is a high-speed trace crossing a gap.

Figure 1: This PCB has a high-speed trace crossing a gap and can cause impedance changes.

Microstripline is widely used for high-speed traces. It has a reference plane in the adjacent layer of the trace. The return current goes just beneath the signal current (figure 2). This trace will have a very stable impedance.

Figure 2: With microstripline, the reference is directly below the signal trace, resulting in stable impedance and no EMI concern.

When a trace crosses a gap of reference plane, as in the PCB in figure 1, the return current needs to detour around the gap. When the gap is too big or wide, the signal is reflected at the gap (figure 3).

Figure 3: When a gap results, the resultant trace forms a loop antenna, radiating unwanted RF energy.

The looped current forms a loop antenna, which is, unfortunately, an efficient radiator.

To find the traces crossing gaps, a rule can be written and executed using a tool such as Mentor Graphics HyperLynx DRC. The rules are simple to write and eliminate the need to prepare more complex IBIS models. In addition to increased performance of the rule-based method, it is much easier for less experienced users to use.

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