A compact balanced filter with quadruple-mode resonators for UWB application

Abstract

A compact ultra-wideband (UWB) balanced bandpass filter (BPF) with quadruple-mode resonators on a microstrip line is proposed. The circuit of the proposed filter is analyzed, and it is shown that the differential-mode circuit performs as a UWB BPF, while the common-mode circuit performs as a UWB bandstop filter (BSF). The differential-mode circuit consists of two short-circuited stubs that are properly attached to the initial multiple-mode resonator (MMR) to form a quadruple-mode UWB BPF. Having a highly-attenuated wide upper-stopband in the frequency response of the differential-mode operation is a significant feature for this circuit. By installing two pairs of open-circuited stubs in the symmetrical line, the differential-mode frequency response remains unchanged but its common-mode frequency response is reshaped to achieve the stopband in the overall UWB. Finally, a microstrip UWB balanced BPF is designed, simulated and measured. Simulated and measured results are in good agreement with each other, showing that the proposed filter has compact size, UWB differential passband (3dB fractional bandwidth 113%) and UWB common-mode suppression (12dB attenuation across 3.3–12.4 GHz).