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

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 (8.16f₀).