Stepped-Impedance Common-Mode Filter for Differential Lines Enhanced With Resonant Planes

Abstract

High-speed digital systems require differential lines for chip-to-chip links. A common concern in differential lines is the presence of common-mode noise, which is a major contributor to signal integrity problems in high-speed digital systems. Common-mode noise is typically excited by skew. It can occur due to timing mismatches at the driver, length mismatches due to routing constraints, or phase velocity mismatches due to inhomogeneous dielectrics with fiber-weave effect. Common-mode noise results in increased crosstalk and electromagnetic interference on packages and boards. This paper presents a methodology based on varying the common-mode impedance of a differential line, while keeping the differential-mode constant. The variation in impedance results in a filter response solely for the common-mode signals, improving the noise immunity of the differential pair. This design is applied on stripline-type differential lines and the performance is enhanced by adjusting the ground via pitch to create resonant planes with a high-impedance current return path for common-mode signals.