Simple and Compact Balanced Bandpass Filters Based on Magnetically Coupled Resonators

A simple strategy is proposed to design differential-mode bandpass filters with good common-mode (CM) rejection using simple resonators. Specifically, the CM rejection is enhanced by using conventional open-loop resonators as well as folded stepped-impedance resonators without the addition of printed or lumped elements along the symmetry plane of the filter or the use of defected ground solutions. The novelty of the present proposal is that a good CM rejection is achieved by the use of magnetic coupling instead of the more commonly employed electrical coupling. Magnetic coupling inherently yields poorer CM transmission as requested by good differential filters. The resonators, due to their geometrical simplicity, can easily be cascaded to implement high-order filters. The use of simple geometries also simplifies the design methodology and makes final tuning based on electromagnetic simulation simpler or unnecessary.

I. Introduction

The use of balanced differential circuit architectures [1] for both digital high-speed electronics and analog microwave circuits has become very popular in recent years due to their high degree of immunity to environmental noise and electromagnetic interference, as well as their good electromagnetic compatibility performance when compared with single-ended configurations. This trend has pushed on the research on differential versions of classical single-ended passive components. Some examples of this kind of research are power dividers and combiners [2]–[5], diplexers [6], or passive equalizers [7]. Nevertheless, common-mode bandstop filters (CM-BSFs) and differential-mode bandpass filters (DM-BPFs) are by far the components that have received more attention [8] in this frame. A few typical examples of CM-BPFs implemented in printed circuit technology can be found, for instance, in [8]–[13]. These structures are designed to reject the common-mode (CM) signal over a wide frequency band, whereas the insertion loss (IL) for the differential signal is kept as low as possible (through good matching and low losses) from dc to the highest frequency of interest. Our attention in this work will be focused on DM-BPFs. These components are designed so that they behave as standard single/multiple BPFs for the differential signal (low IL at the desired frequency bands and strong rejection in the out-of-band regions) while also showing strong rejection of the CM signal over the differential passband(s) and around them.