Compact Lowpass and Dual-Band Bandpass Filter With Controllable Transmission Zero/Center Frequencies/Passband Bandwidth

Abstract

A simple and compact filter structure for lowpass and dual-band bandpass response with controllable transmission zeros/center frequencies/3-dB bandwidth is proposed in this brief. The proposed filter structure uses two-unit cells, which contains ${{\lambda }}$ /5 length of folded coupled line and open circuited L-shape strip. A lowpass and a bandpass response has been realized by a unit cell due to formation of LC series and shunt tank circuit. Another bandpass response has been achieved by connecting two-unit cells with a strip. In literature, there are very few designs which can achieve two different type of filtering responses. The proposed design is compact as compared to the state-of-the-art and also the two filtering responses have been achieved with improved selectivity and impedance matching due to formation of additional tank circuit. The size of the filter excluding feed line is $0.13{{\lambda }}_{g}\times 0.06{{\lambda }} _{g}$ (15.5 mm $\times $ 7.2 mm) at the 3-dB cut-off frequency (1.05 GHz) of the lowpass filter. The position of transmission zeros/center frequencies and 3-dB bandwidth characteristics can be separately controlled by varying the physical parameter of designed filter structure. The filter performance has been validated through electromagnetic simulation and experimental measurements. The metamaterial properties of designed filter has been verified by its dispersion diagram.