Return path discontinuities and EMI: Understand the relationship

-June 11, 2015

It’s conventional wisdom that a solid, continuous return path provides a better result in electromagnetic compatibility (EMC). This article discusses the relationship between return path discontinuities and EMC.

A quality signal channel has a nice, uniform trace and a continuous return path from driver to receiver. Disruption to the return path introduces noise, and is typically caused by:
  • Changing the reference plane(s) along the signal path
  • Discontinuities within the reference plane
There are two modes of high-frequency current flow, illustrated in figure 1.

Figure 1: Normal mode (top) and common mode (bottom) high-frequency current flow.

Normal mode: This is the simpler mode. Current goes along a closed circuit loop, so the total current along the loop becomes zero. The loop is small/narrow enough, so the radiation from the incident current is canceled by the return current. The return path of via microstrip or stripline is just beneath/above plane(s) of the trace signal. Thus, when a trace crosses over a plane gap it breaks this condition.

Common mode: Noise power goes through both of the traces and, lacking an appropriate, closely spaced plane, something like the enclosure can become the return path. The noise induced by the currents on the signal traces is not canceled by a nearby return current, so strong radiation could occur. This physically larger circuit can act as antenna, so it may cause EMI as well as an EMS (electromagnetic suseptability) issue. The common-mode noise source could be the reference plane discontinuity mentioned in Normal mode.

Figure 2 is a simulation result of a microstrip signal trace that crosses a slot on the ground plane. The driver is on the left side of the slot, connecting to both the signal trace and the ground plane with matched impedance. The current goes from the driver to the left end of the trace and returns on the ground plane. Due to the slot, the return current on the plane spreads along the gap, and some current is shown at the edge of the board. As can be seen, the discontinuity affects the normal mode current flow.

Figure 2: Return current around a plane slot (modeled with Mentor’s Nimbic nWave).

A differential pair and its currents
For higher-frequency signal traces, a differential pair is used. Differential pairs carry the signal and the opposite phase of the signal so that they can combat common mode noise or induced noise. All of the SerDes signals, such as PCI Express or Serial ATA, use differential pairs.

Many differential pairs use a differential impedance of 80 to 100 ohms. Since the paired signals are coupled tightly, a differential pair is far more tolerant of reference plane discontinuities.

Let’s look at two types of differential pairs: Figure 3a has narrow spacing between traces and thinner dielectric layer, resulting in 73 ohms impedance for each trace. In Figure 3b, each trace has an impedance of about 50 ohms. Both circuits have 100 ohms differential impedance, but the one in Figure 3b has stronger coupling between the traces. Figure 3a has stronger coupling to the reference plane. A typical design will be somewhere between these two extreme cases.

Figure 3: In (a), the trace impedance is about 73 ohms;  in (b) it is 50 ohms.

Figure 4 shows current on the differential pair and on the plane of the wider trace spaced example. Though the traces are a differential pair, it acts like two single-ended traces that have current flowing in opposite directions to each other.

Figure 4: Differential pair crossing a gap.

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