Cascading Common Mode Filter for High-Speed Differential Traces
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Solution Overview
Problem
High-speed differential signal traces in server/storage products experience interference radiation due to common mode energy, which disrupts signal transmission and increases noise levels, especially at multiple frequency bands.
Innovation Solution
A cascading common mode filter is implemented on a printed circuit board with a U-shaped and H-shaped void section in the ground plane layer, designed to reduce common mode energy by creating a return current path that cancels noise at specific frequencies, using the constraints L1=LX−L4X, 2L4X≥L2≥L4X, L3≤L4X, and L4=L2X to determine the lengths of the void sections for effective noise cancellation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a traditional ground plane is used for differential traces, then the structure is simple and manufacturing is easy, but common mode energy radiates through holes in the chassis causing interference and noise
Solution Approach 1:
The ground plane is segmented by introducing U-shaped and H-shaped void sections that divide the continuous ground plane into separate regions. This segmentation creates controlled return current paths that prevent common mode energy from radiating through chassis holes, directly addressing the interference problem while maintaining a manageable structural complexity
Solution Approach 2:
The void sections act as intermediary elements between the differential traces and the chassis holes. By positioning these void sections strategically, they mediate the electromagnetic field distribution and guide return currents away from radiation paths, reducing interference without requiring complete redesign of the entire grounding system
2Object-affected harmful factors
If the ground plane is modified with void sections to reduce radiation, then interference is reduced, but the manufacturing precision requirements increase
Solution Approach 1:
The invention specifies particular dimensional relationships for the void sections (such as length ratios and positioning relative to differential traces) that optimize radiation reduction. By establishing these parameter relationships, the design achieves effective common mode energy reduction while providing clear manufacturing guidelines that balance precision requirements with fabrication capabilities
3Speed
If high-speed differential traces are used to transmit data at different frequencies, then data transmission capability is improved, but interference and noise increase at different transmitted frequencies
Solution Approach 1:
The U-shaped and H-shaped void section configuration serves multiple frequency bands simultaneously. The geometric design creates broadband electromagnetic shielding effects that address interference across the entire spectrum of high-speed differential signals, allowing the same structure to protect against noise at various transmission frequencies without requiring frequency-specific modifications
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
The cascading common mode filter significantly reduces interference radiation across multiple frequency bands, enhancing signal integrity and meeting the lower noise requirements of modern Class A-AV devices by effectively canceling common mode energy and increasing coupling terms.
Implementation Method 1
A current return path in a ground plane layer underlying the differential traces and having a U-shaped current return path pattern... The second current return path is designed to reduce signal radiation at a second frequency that is a harmonic of the first frequency
Implementation Method 2
determining a length of a second void section based on a first target radiation frequency... determining a length of a first part of a fifth void section based on a second target radiation frequency, the second target radiation frequency has a harmonic relationship to the first target radiation frequency
Data Source
AI summary
A high-speed circuit includes a printed circuit board, a ground plane layer, a pair of first and second differential traces, and a cascading common mode filter. The printed circuit board has a first surface and an opposite second surface. The ground plane layer has a first surface in contact with the second surface of the printed circuit board. The pair of first and second differential traces are on the first surface of the printed circuit board. The first and second differential traces carry an electrical signal. The cascading common mode filter includes an outer and an inner common mode filter. The outer common mode filter includes a U-shaped void section on the first surface of the ground plane layer. The inner common mode filter includes an H-shaped void section on the first surface of the ground plane layer. The H-shaped void section is located proximate to the U-shaped void section.