Return-Current Distribution in a PCB Microstrip Line Configuration, Part 2

September 1, 2023

Figure 1: CST Model – Two-layer PCB with an edge slot in the reference return plane

Reference Plane with Discontinuities

This is the second article of a two-article series devoted to the return current distribution in a PCB microstrip line configuration. The previous article [1] presented the CST simulation results in the case of a solid reference plane. This article addresses the case where the reference plane contains several discontinuities: edge slot, internal slot, slot holes, and via cutouts.

1. Baseline Results

In [1], we presented the baseline results for a solid reference plane and showed the return current path (forward current trace is hidden) flowing in the reference plane at different frequencies.

The results showed that at 10 Hz the return current spreads wide over the reference plane, flowing both under the top trace and directly from the load port to the source port. As the frequency increases to 100 Hz, more of the return current flows under the trace (with a narrower spread), and less of it flows directly from the load port to the source port. This trend continues as the frequency increases to 1 kHz. As the frequency increases beyond 10kHz, the return current path remains virtually unchanged, predominantly flowing beneath the forward trace. In other words, the return current path and current density no longer depend on frequency.

The results were confirmed by plotting the normalized current distributions.

2. PCB with Edge Slot

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Figure 1 shows the CST Studio model of a two-layer PCB with an edge slot in the reference plane.

Figure 2 shows the return current path (forward current trace is hidden) flowing in the reference plane at different frequencies.

Figure 2: Edge slot – return current path at different frequencies

The results show that the edge slot forces current to go around it and flow in a larger loop (higher inductance). The return current exhibits the frequency-dependent behavior similar to the solid reference plane case.

At lower frequencies (below 1 kHz), the return current spreads wide over the reference plane, flowing both under the top trace and directly from the load port to the source port. This trend continues as the frequency increases to 1 kHz. As the frequency increases beyond 10 kHz, the return current path remains virtually unchanged, predominantly flowing beneath the forward trace, the return current path and current density no longer depend on frequency.

This is confirmed by the normalized current distributions shown in Figure 3.

Figure 3: Edge slot – normalized current distributions at different frequencies

3. PCB with Internal Slot

Figure 4 shows the CST Studio model of a two-layer PCB with an internal slot in the reference plane.

Figure 4: Internal slot in the reference return plane – CST model

Figure 5 shows the return current path flowing in the reference plane at different frequencies.

Figure 5: Internal slot – return current path at different frequencies

The results show that current returns around both sides of the slot and thus does not ‘bulge’ as much as the edge slot case. The return current exhibits the frequency-dependent behavior similar to the edge slot and the solid reference pane case. As the frequency increases beyond 10 kHz, the return current path remains virtually unchanged, predominantly flowing beneath the forward trace, the return current path and current density no longer depend on frequency.

This is confirmed by the normalized current distributions shown in Figure 6.

Figure 6: Internal slot – normalized current distributions at different frequencies

4. PCB with Slot Holes

Figure 7 shows the CST Studio model of a two-layer PCB with slot holes in the reference plane.

Figure 7: Slot holes in the reference return plane – CST model

Figure 8 shows the return current path flowing in the reference plane at different frequencies.

Figure 8: Slot holes – return current path at different frequencies

The impact of the holes is minimal compared to the solid reference plane case. Return currents are permitted to flow between and around the holes and therefore the current does not bulge away from the return path out toward the middle of the PCB. The results are very similar to the previous cases: as the frequency increases beyond 10 kHz, the return current path remains virtually unchanged, predominantly flowing beneath the forward trace, the return current path and current density no longer depend on frequency.

This is confirmed by the normalized current distributions shown in Figure 9.

Figure 9: Slot holes – normalized current distributions at different frequencies

5. Ground Via Cutouts

Finally, we investigate the PCB with several ground via cutouts. Figure 10 shows the CST Studio model.

Figure 10: Two-sided PCB with via cutouts in the reference plane

Figure 11 shows the return current path on the reference plane at different frequencies.

Figure 11: Ground via cutouts – return current path at different frequencies

The results are similar to the previous cases and are confirmed by the normalized current distributions at different frequencies, shown in Figure 12.

Figure 12: Ground via cutouts – normalized current distributions at different frequencies

Conclusions

Ground plane discontinuities do affect the return current path to various degrees by increasing the loop area, and thus increasing the inductance. In all cases, however, the return current exhibits very similar frequency-dependent behavior. As the frequency increases beyond 10 kHz, the return current remains virtually independent of increasing frequency.