Compact narrow band-pass filter based on alternate right-left handed transmission line concept

A new topology for the design of microwave narrow band-pass filters (NBPFs) based on the alternate right-left handed transmission line (TL) concept is proposed. The proposed NBPF is a three-stage structure which two LH stages and one RH stage are used. The proposed LH stage is implemented by a spiral complementary split ring resonator (spiral CSRRs) and an interdigital capacitor while for implementing the RH stage, the conventional TL shorted by via and the spiral CSRR are used. The spiral CSRRs have been etched in the ground plane. To improve frequency selectivity and out-of-band rejection, the proposed RH stage has been cascaded with two LH stages. Accordingly, a compact NBPF is designed which exhibits extremely sharp rejection skirts around the target passband. The equivalent circuit model and full-wave simulation results of the proposed NBPF are developed. To validate the design concept, the designed filter has been fabricated and tested. Experimental verification is provided and a good agreement has been found between the simulation and measurement. The proposed NBPF has a passband covering 2.3-2.45 GHz and its measured 3-dB fractional bandwidth is about 7.5%. The total size of the proposed NBPF is 0.18 x 0.15 lambda g2. Compared with some other reported NBPFs, the presented NBPF has significant improvements on size reduction and selectivity.