Flexible control of tranmission zeros using doublet for cavity bandpass filter design

A variety of multiple-mode narrow-band bandpasss filer is designed, analyzed, and fabricated based on doublet and its properties. Three resonant modes: TE101, TE011 and TM110 modes in a single cavity are excited. The proposed filters use a microstrip line as a feeder then couple to a slot on one side of a single rectangular cavity. By rotating and offsetting two slots at the input port and output port of the rectangular cavity, three fundamental modes are implemented without any cross-coupling. The proposed structure also create two transmission zeros (TZs), by adjusting the positions of the two TZs near the passband, e.g., at lower or upper stopband, high rejection is achieved. Due to emergence of the three resonant modes and realization of a pair of TZs, a high attenuation and sharp filtering selectivity is achieved. Good agreement between measurement and simulation is achieved.

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