Analysis and Modeling of Wideband Common-Mode Absorption With Lossy Complementary Split-Ring Resonator Chain in Resistor-Free Differential Microstrip Lines

Novel resistor-free differential microstrip lines prototype loaded with lossy metamaterial is proposed to achieve wideband common-mode (CM) absorption. The metamaterial is constructed by a chain of lossy complementary split-ring resonators (CSRRs), which are etched on the ground plane. Different from the CM absorption realized by lumped resistors, this work utilizes the inherent dielectric and conductor losses introduced by the differential microstrip lines and CSRRs to absorb CM energy. Furthermore, CSRRs are designed with gradual perturbation to broaden the CM absorption band. The lossy even-mode equivalent circuit models are established and analyzed to reveal the CM absorption mechanism. In order to verify the proposed strategy to design resistor-free differential microstrip lines with CM absorption, a sample was fabricated and measured. The measured
$S_{\mathrm {cc21}}$
and
$S_{\mathrm {cc11}}$
are kept below −10 dB simultaneously from 4.3 to 5.65 GHz with 27.4% fractional bandwidth, where the CM energy is absorbed over 80% by the dielectric and conductor losses. Meanwhile, the low insertion loss of
$S_{\mathrm {dd21}}$
below 6 GHz as well as the good differential-mode eye diagrams demonstrates that good signal integrity of differential signals is kept.