Balanced dual-band BPF design using ring resonators

This paper presents a new balanced dual-band bandpass filter (BPF) designed using ring resonators. For common-mode (CM) signals suppression, open-circuited stubs are added to one of the two resonators so that such signals can be attenuated. Meanwhile, source-load coupling is established so that cross-coupled configuration of the BPF can produce two transmission zeros near the edges of each desired differential-mode (DM) passband to sharpen the passband selectivity. The measurement agrees well with the simulation. The measured minimum DM insertion losses are 1 and 1.35 dB in the first and second passbands, respectively, in which the CM rejections are larger than 29 and 22 dB.