A compact bandpass filter using lumped coupling elements between input port and output port

A compact T resonator filter with the lumped coupling elements between two ports is presented to have not only two transmission zeros near its passband, but also a controllable transmission zero in the upper stopband. The lumped coupling elements between two ports make it possible for the filter to generate three transmission Zps, and the position of the zero in the upper stopband is controllable by adjusting the capacitance value and the position of interdigital capacitor. At the same time, the centre frequency and bandwidth are both not changed. Then, the bandwidth of filter can be just adjusted by the different length of short-circuited branch. A compact filter operating at 3 GHz with a fractional bandwidth of 26% and minimal insertion loss of 0.7 dB and three Zps has been designed and fabricated. Good skirt selectivity and controllable stopband characteristics can be achieved simultaneously.