Direct synthesis of a bandpass filter with a controllable transmission zero

A novel bandpass filter with a controllable transmission zero located at the upper stopband is proposed. The circuit model and design equations are established. The location of the transmission zero can be adjusted by varying the shunt short-circuited stub of the resonators. Based on the coupled mechanism, the other transmission zero is created at the lower stopband by properly controlling the coupling strength of the coupled line. In this way, the proposed bandpass filter provides a better cutoff rate in the stopband and gives much improved selectivity. The illustrated filter with a center frequency of 2.4 GHz has an insertion loss of less than 2 dB and a return loss of more than 15 dB. The transmission zeros are located at 2.1 GHz and 3 GHz. A good agreement between the simulated and measured results is obtained.