Ten Most-Important EMC Problems/Issues and How to Correct Them:
9. Non-continuous Return Paths

Most PCB (printed-circuit board) issues can be traced to non-continuous signal return paths. Today, this is more of an issue due to increasing clock frequencies.

Solutions:

• Best if the signal travels out a trace and returns immediately under that trace
• Unfortunately, the return path is often broken by a discontinuity: e.g. a gap or slot in the ground plane or the signal trace passes through a via and changes reference planes
• Carefully examine ground (and power) plane layers for gaps and slots
• Add extra vias for return currents when switching reference planes

Production of differential-mode emissions

 
Above: Ground loops when using through-hole components.

Differential Mode current flows in a loop. The loop radiates energy.

Examples of Non-continuous Return Paths

Slots in data-acquisition board signal return plane

Above image shows:

• Noisy IC straddling gap in ground plane
• Huge gap in ground plane
• Several more gaps

A temporary bridge with copper tape is used to reduce emissions by as much as 20 dB.

Non-continuous Return Paths (examples)

Above: Routing a trace over a slot in a plane can cause a large loop area.

Above: Routing a trace over an unrelated plane can cause several types of problems.

Trace passing through two planes with via

Above: Source / Termination. Signal Traces Adjacent to Different Planes. But ... where does the return signal current flow?