Printed circuit board bandpass filters with octave bandwidth and very wide upper stopband

Abstract

Bandpass filters with broad bandwidth (up to 70%), very wide upper stopband (nearest spurious passband occurs up to five times of passband center frequency (f(0))), good stopband rejection performance (better than -30 similar to-40 dB in the whole stopband region), and matching with the conventional low cost printed circuit board process with low dielectric constant substrates are proposed in this paper. The proposed filters are designed using parallel-coupled vertically installed planar stepped-impedance resonators (VIPSIRs), which adopt the inherent nature of very tight coupling of VIP coupled line and extremely high impedance of VIP line. The extremely tightly coupled line enables the proposed filters having very wide passband and the extremely high impedance of VIP line leads to extremely large low-to-high impedance ratio that pushes the nearest spurious passband up to 5f(0). Both VIP coupled line and VIP high impedance line are analyzed and characterized by the design charts. The design procedures based on the design charts are verified by several experimental examples. The measured results agree very well with the simulated ones.