Realization of transmission zeros in combline filters using an auxiliary inductively coupled ground plane

The out-band transmission zeros of the multilayer low-temperature co-fired ceramic bandpass filter are studied. The design target of this filter, with passband between 2.3-2.65 GHz (the true application is 2.4-2.483 GHz), is to reject the local oscillator and image signals and, in the mean time, to suppress the harmonic frequency. This filter is a modified three-pole combline filter, which can easily achieve a high rejection rate and low insertion loss. The source-load coupling capacitor incorporation with cross-coupling grounding inductors can generate three transmission zeros. By properly adjusting the inductance values of the inductors between three resonators and ground the transmission zero can easily be manipulated at a high-side skirt to suppress the harmonic frequency. The measured results match well with the electromagnetic simulation that provides evidence for the feasibility of the proposed design.