A compact stepped-impedance pseudo-interdigital bandpass filter with controllable transmission zero and wide stopband range

A compact pseudo-interdigital bandpass filter with controllable transmission zero is presented by using open-circuited stepped-impedance resonators (SIRs) and step-impedance parallel-line couplers (SIC). When the filter is working at the odd resonance mode of the SIR, it has a virtual ground at the mid-point of the resonator such that the filter looks similar an interdigital topology. Therefore, the high band-selectivity of the interdigital topology is preserved. Particularly, the frequency of the close-in transmission zero can be controlled above or below the passband simply by changing the inter-coupling magnitude between the adjacent SIRs. Additionally, the wide stopband range is achieved by the appropriate impedance and line ratios of the SIR and the SIC. The simulation and measurement results are presented to demonstrate the control of the transmission zero and wide stopband range.