Compact dual-passband microstrip bandpass filters with larger design flexibility and multi-spurious suppression

Abstract

Compact folded stepped impedance resonators (SIRs) with a novel asymmetric structure are used to design dual-passband filters with multi-spurious suppression. A complete design procedure enhancing both overall design flexibility and suppressing spurious responses is detailed. The new asymmetric SIR structure can effectively increase the degree of freedom (DOF) in the dual-passband bandwidth design. Extra DOF is simultaneously incorporated so that transmission zeros of the coupling stages may be adjusted to suppress unwanted spurious responses. Owing to circuit symmetry and a short coupling length, the circuit can be easily folded and the size is decreased by 62% compared with the previous work. The measured results agree well with the simulation and show that the attenuation level of the upper stopband is greater than 30 dB up to more than 20 GHz (8.16f(0)).