Novel compact bandpass filter using sextuple-mode ceramic cavity resonator

This letter proposes a novel compact ceramic cavity bandpass filter with surface silver coating and air coupling apertures. It is constituted of a sextuple-mode ceramic cavity as the main resonator, two single-mode cavities to connect the sextuple modes transversely with source and load, and two single cavities to suppress higher harmonics. This proposed filter has the advantages of compact size, wide band, low insertion loss, high selectivity, wide spurious-free window, as well as coplanar source and load interfaces. There are no blind holes in the filter, which is beneficial for improving the machining accuracy. One prototype filter operating at 3.5 GHz is fabricated and measured. The minimum insertion loss in the passband is 0.65 dB. The inherent spurious mode of the sextuple-mode resonator at around 1.2f(C) (f(C) center frequency) is suppressed under -30 dB. The measured and simulated results are in good agreement.