A Compact Symmetrical Single-Cell Bidirectional Absorption Common-Mode Filter

Abstract

A novel circuit with two mirror symmetry planes is proposed to synthesize bidirectional absorption common-mode filters (A-CMFs). In contrast to the literature, this proposed circuit first realizes bidirectional A-CMFs in only one stage (also called a single cell) featuring a compact size. Based on the symmetry property, bidirectional common-mode (CM) noise absorption can be achieved in the proposed single-cell circuit. Analytical design formulas have been found; thus, all circuit parameters can be straightforwardly determined. To validate the proposed circuit, two configurations are developed for CM noise absorption in the 2.4-GHz industrial, scientific, and medical (ISM) band. One uses a meander differential pair with a slotline, and the other applies a straight differential pair with a meander asymmetrical coplanar strip (MACPS). To obtain a better differential-mode (DM) signal integrity and 43&#x0025; size reduction, the latter one, MACPS-type A-CMF, is fabricated and measured. The measurement results show that <inline-formula> <tex-math notation="LaTeX">$\vert S_{\text {cc21}}\vert $ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$\vert S_{\text {cc12}}\vert $ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$\vert S_{\text {cc11}}\vert $ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$\vert S_{\text {cc22}}\vert $ </tex-math></inline-formula> are less than &#x2212;10 dB from 2.37 to 2.76 GHz to have the bidirectional CM absorption efficiency of more than 80&#x0025;, and the &#x2212;3-dB bandwidth of DM insertion loss reaches 7.83 GHz. In the time domain, the measured eye diagram of the proposed A-CMF is identical to that of a reference board for data rates up to 20 Gb/s. Above all, having a cost advantage, a compact size of <inline-formula> <tex-math notation="LaTeX">$0.0317\lambda _{\text {g}}^{{2}}$ </tex-math></inline-formula> in a two-layer printed circuit board (PCB) is presented.