High-selectivity dual-band bandpass filter using folded stepped-impedance resonators and asymmetric coupling lines

A compact microstrip line dual-band bandpass filter using folded stepped-impedance resonators (FSIRs) with high selectivity is proposed and implemented. The filter consists of two FSIRs and asymmetric coupling input/output lines. By utilizing resonance characteristics of the FSIRs, a compact filter is designed for WLAN (2.4 and 5.2 GHz). In the new design, asymmetric coupling input/output lines are applied not only to generate two more transmission zeros than the symmetric ones but also to tune the transmission zeros to improve the selectivity. These performances are demonstrated by simulation and measurement. Owing to asymmetric coupling input/output lines, five transmission zeros could be generated near the passband edges to improve the selectivity. Both experimental and simulated results are with good agreement. (C) 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:18661869, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26928