Differential PCB Trace Return Path for Common-Mode Radiation
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Solution Overview
Problem
High speed differential signal traces in server/storage products experience interference due to common mode energy radiation, leading to noise and signal disruption, which existing technologies have not adequately addressed while maintaining differential signal integrity.
Innovation Solution
A high speed differential trace design with a return path in the ground plane layer, utilizing a U-shaped or ladder-shaped void pattern to increase coupling terms and create destructive interference, thereby reducing common mode energy at specific frequencies by determining the length of the return path based on target radiation frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a conventional ground plane return path is used, then the trace structure is simple, but common mode energy radiates through connectors causing noise and interference
Solution Approach 1:
The ground plane return path is segmented into discrete sections rather than being continuous. The void pattern divides the return path into multiple segments that are separated by controlled gaps, transforming the uniform ground plane into a structured segmented path that reduces common mode radiation while maintaining return current flow.
Solution Approach 2:
The return path is given different local properties through the void pattern. Specific regions of the ground plane are modified with voids of particular sizes, shapes, and distributions to create localized coupling terms that generate destructive interference at target frequencies, while other regions maintain standard ground plane characteristics.
2Object-affected harmful factors
If the return path length is increased to reduce common mode energy, then interference reduction improves, but the trace design becomes more complex and harder to manufacture
Solution Approach 1:
The void pattern parameters (size, shape, spacing, distribution) are systematically varied to achieve the desired interference reduction effect. By adjusting these geometric parameters, the return path length and coupling terms are optimized to create destructive interference at specific frequencies without requiring excessive path length or complex structures.
Solution Approach 2:
Rather than modifying the entire ground plane uniformly, the void pattern applies partial modifications to specific regions. The voids are strategically placed in certain areas of the return path while leaving other areas intact, achieving the necessary interference reduction with minimal structural changes and maintained manufacturability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Significantly reduces common mode noise at target frequencies, such as 8 GHz, by canceling interference and maintaining differential signal energy, as demonstrated by the drop in return current and noise levels in the proposed trace configurations.
Implementation Method 1
utilizing a U-shaped or ladder-shaped void pattern to increase coupling terms and create destructive interference, thereby reducing common mode energy at specific frequencies
Data Source
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AI summary
A high speed differential trace structure is disclosed. The differential trace structure includes a first trace 210 and a parallel second trace 212. A printed circuit board layer 202 has a top surface 204 and an opposite bottom surface 206. The traces 210, 212 are formed on the top surface 204. The structure includes a ground plane layer 208 having a top surface 220 in contact with the opposite bottom surface 206 of the circuit board. A first void section 224 is formed in the ground plane layer to one side of the first trace 210. A second void section 226 is formed in the ground plane layer to one side of the second trace 212. The length of the second void section is determined based on a target radiation frequency. A third void section 228 is formed in the ground plane layer that joins the first void section and the second void section.