Dual-Band Balanced Bandpass Filter With Common-Mode Suppression Based on Electrically Small Planar Resonators

The design of fully planar dual-band balanced bandpass filters with common-mode noise suppression is reported. The proposed filters are based on electrically small resonators coupled through admittance inverters. For design purposes, the circuit models of the considered resonators are reported. The key aspect for selective mode suppression (i.e., common-mode rejection in the differential-mode pass bands) is related to symmetry properties. Thus, for the differential-mode the symmetry plane is an electric wall, and the equivalent circuit for that mode provides dual-band functionality. Conversely, for the common-mode the symmetry plane is a magnetic wall, and the equivalent circuit exhibits a rejection band. As a proof of concept, the design of an order-2 Chebyshev dual-band balanced bandpass filter with center frequencies
$f_{1} = 1.8$
GHz (GSM band) and
$f_{2}= 2.4$
GHz (Wi-Fi band), fractional bandwidth
$FBW = 7 \%$
, and ripple level
$L_{Ar}= 0.01$
dB is reported.