A Compact Narrow Band-Pass Filter Based on Composite Right-Left Handed Structures for WLAN Application

A new topology for the design of microwave narrow band-pass filters (NBPFs) with wide out of band rejection and sharp edges, based on the use of Composite Right-Left Handed (CRLH) Transmission Line (TL) concept is presented. The proposed NBPF has been realized by using two different sections. The left-handed section is implemented by etching spiral complementary split ring resonator (SCSRR) unit-cell in the ground plane as well as series interdigital capacitors in the upper metal level while the right-handed section is implemented by etching SCSRR unit-cell in the ground plane and conventional microstrip transmission line shorted via a hole in the upper metal level. Accordingly, CRLH transmission line concept is implemented by this configuration which acts as a narrow band-pass filter. Because of the smaller electrical size of spiral resonators, the total electrical size of the proposed NBPF can be reduced by using this proposed configuration. Compared with some other reported NBPFs, the presented NBPF has great improvements in size reduction and selectivity. Consequently, a compact NBPF is designed which exhibits extremely sharp rejection skirts around the target passband. The equivalent circuit model of the designed filter and full-wave simulation results of the NBPF are also developed. To validate the design concept, the proposed NBPF has been fabricated and tested. Experimental verification is provided and good agreement has been found between simulation and measurement. The proposed NBPF has a passband which covers 2.25 to 2.45 GHz and its measured 3 dB fractional bandwidth is about 8.5%. The total size of the proposed NBPF is 0.25 λg× 0.06 λg, where λg is the guided wavelength at the center passband.