The presence of common-mode noise in differential lines 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 introduces the resonant-plane common-mode filter, a new differential-line design to suppress common-mode noise. This design is applicable for stripline-type differential lines and is based on adjusting the ground via pitch to create a high-impedance current return path for common-mode signals.