Stepped impedance resonator bandpass filters with tunable transmission zeros and its application to wide stopband design

Bandpass filters with a very wide stopband are designed using parallel coupled stepped impedance resonators (SIR), which have advantageous resonance harmonic characteristics. The singly loaded Q (Q_si) of a tapped SIR is derived, so that the relation between Q_si and the position of tap point can be established. It is known that a single resonator with tapped input can create an extra zero. It is found that in this paper, with proper tapping at the first and the last SIR’s of a bandpass filter, two zeros can be created and tuned independently over a wide frequency range. By exploiting both the advantageous resonance characteristics of SIR and tapped-line input, a bandpass filter with high selectivity and/or wide stopband can be realized. The experimental results show a good agreement with the simulated prediction.