High-order common-mode and differential-mode reflectionless balanced bandpass filters with improved selectivity and out-of-band rejection

High-selectivity common-mode (CM) and differential-mode (DM) reflectionless balanced bandpass filters (BBPFs) are proposed in this article. By loading absorption networks at single/both ends of the basic ring resonator, input-/two-port wideband CM and DM reflectionless performance, wideband filtering performance and all-stop CM suppression are obtained. The absorption network composed of K-sections of coupled-lines (CLs) terminated with grounded resistors can not only extend the filtering performance to high order, but also realize wideband absorption of CM noise and out-of-band DM signals. Absorptive stubs are loaded at ports to increase the design flexibility and enhance the absorption. As for the input-reflectionless type, multiple independently controlled transmission zeros (TZs) are obtained by the TZ control network to improves the selectivity and out-of-band rejection. A set of 2 GHz micro-strip BBPFs are designed and measured, which shows simultaneous CM and DM absorption performance.