Wideband tunable common‐mode suppression filter based on varactor‐loaded slot‐ring resonator for high‐speed differential signals

Abstract

In this study, a varactor-loaded slot-ring resonator (SRR) is investigated and applied in the design of tunable common-mode (CM) suppression filter for high-speed differential signals. Two tunable capacitors (realised by two kinds of varactors) with different values are symmetrically added to the SRR. Under the stepped-width structure of the SRR, the frequency-tuning principle is studied. It can be found that the fundamental resonant frequency f fund can be shifted up and down by properly controlling the respective values of the two loaded capacitors, i.e. bi-directional tuning. As a result, the tuning range of f fund can be greatly enlarged, approximately double that of the traditional tunable resonator. Meanwhile, the employed stepped-width SRR is useful not only for miniaturising the circuit size, but also further increasing the tuning range of f fund. By etching the proposed SRRs on the ground plane underneath the differential microstrip line, two wideband tunable CM-suppression filters using one and three SRR units, respectively, are designed, and their simulated and measured results showing good agreement are presented. It is found that the CM stopband frequency can be tuned from 0.78 to 1.52 GHz, while the differential signals still keep good signal integrity in eye-pattern observation.