Differential Pair Ground Overlap for Reflection Noise Suppression
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Solution Overview
Problem
In differential signaling, reflection noise occurs due to impedance changes at boundaries created when partial metal traces overlap with clearances and ground metal, disrupting the continuity of characteristic impedance along the traces.
Innovation Solution
Extending the metal ground to fully overlap with the differential pair traces, maintaining impedance continuity and eliminating reflection noise by configuring the ground plane to cover the entire trace length before connecting to plated through holes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the ground plane is configured with clearances (non-ground areas), then routing flexibility and component placement are improved, but impedance continuity is disrupted and reflection noise occurs
Solution Approach 1:
The patent applies local quality by creating different ground plane configurations in different regions: solid ground areas under plated through holes to maintain impedance continuity, and clearances in other areas to provide routing flexibility. This spatial variation of ground plane properties resolves the contradiction between adaptability and reliability.
Solution Approach 2:
The ground plane is segmented into discrete solid areas and clearance regions. Each plated through hole is surrounded by its own solid ground area, creating localized impedance-matched zones that maintain signal integrity while allowing clearances elsewhere for routing flexibility.
2Reliability
If the metal ground is extended to fully overlap with differential pair traces, then impedance continuity is maintained and reflection noise is eliminated, but manufacturing complexity increases
Solution Approach 1:
The patent applies partial action by extending the ground plane overlap only where necessary - specifically under the differential pair traces and at critical transition points - rather than covering the entire board. This provides sufficient impedance continuity to eliminate reflection noise while avoiding the excessive manufacturing complexity of a fully continuous ground plane.
3Ease of manufacture
If clearances are positioned to overlap with plated through holes, then manufacturing alignment is simplified, but impedance discontinuity occurs and reflection noise increases
Solution Approach 1:
The patent applies preliminary action by pre-positioning solid ground areas around plated through holes in the ground plane layout, ensuring that impedance continuity is maintained at critical points before final assembly. This preliminary configuration of ground planes around PTHs prevents impedance discontinuity while maintaining reasonable alignment tolerances.
Data Source
AI summary
A circuit board comprises a substrate with opposite first and second sides. A pair of plated through holes (PTHs) extends along z-axis. A pair of signal traces are made on the first side of the substrate and electrically coupled to the pair of the PTHs respectively to form a differential pair. A ground metal is made on the second side of the substrate, the ground metal has a clearance made therein. The ground metal extends fully overlapping with the full signal traces to eliminate reflection noise caused by a boundary between the clearance and the metal ground.