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Use of EBG Void Shapes in Reference Planes to Suppress Common Mode Noise on Differential Signals

IP.com Disclosure Number: IPCOM000186558D
Original Publication Date: 2009-Aug-26
Included in the Prior Art Database: 2009-Aug-26
Document File: 3 page(s) / 272K

Publishing Venue

IBM

Abstract

Use of EBG void shapes in reference planes to suppress common mode noise on differential signal traces.

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Use of EBG Void Shapes in Reference Planes to Suppress Common Mode Noise on Differential Signals

Disclosed is a solution which uses defects in the signal reference plane to reject a certain amount of the common mode noise without impacting the differential mode signal. The defects consist of Electromagnetic Band Gap structures (EBGs) voids patterned unto the reference plane. There is no bill-of-materials cost associated with this solution, it works to very high frequencies beyond 10 GHz, it does not consume board real estate, and it does not force any layer transitions that would impact the differential signal. Furthermore, being a one dimensional EBG structure, this approach is very practical and therefore, more attractive for microwave designs as is it simple to fabricate.

Common mode noise on differential data buses is a detrimental factor for signal integrity and a major cause of EMC problems. A known solution for reducing common mode noise is the use of common mode filters. The limitations of this solution are components cost as well as board real estate and limited operating bandwidth. In addition, use of common mode filters can adversely impact the differential signal due to via transitions between input and output of the filter. EBGs are known for their inherent ability to suppress electromagnetic waves propagation within particular frequency regions also known as 'forbidden regions'. At these given frequencies, the transmission coefficient of any propagating wave on the material surface is close to zero. Only tangential field components may exist within these bands. While there is a vast amount of literature on research activities exploiting this unique characteristic of EBGs in various aspects of the microwave discipline, the disclosure herein is first to report on the use of EBGs as a novel means of suppressing common mode noise on differential data buses. This novel technique addresses the shortfalls of commonly used filters method.

Defects in the reference plane provide a barrier for common mode currents. The impedance of this barrier varies over frequency and has natural resonances that depend on the shape and size of the defect. While a simple, rectangular, void slot in the reference plane will resonate sharply at a frequency corresponding to its length, through use of EBGs, it is possible to realize a broader suppression band depending upon its shape. For example, making the slot have varying width, like a bowtie shape, widens the range of reso...