Wideband Differential Bandpass Filters on Multimode Slotline Resonator With Intrinsic Common-Mode Rejection

In this paper, two kinds of wideband differential filters with intrinsic common-mode (CM) rejection are proposed. Both these filters are realized based on the multimode slotline resonators under the dual- and triple-mode resonance, respectively. For Filter 1, the microstrip lines with a pair of differential ports at the two sides are directly tapped on a slotline resonator. For Filter 2, the slotline resonator is connected with two pairs of microstrip open-ended stubs both at top and bottom layers at the beginning. In these two filters, the CM signals are effectively rejected during electromagnetic field conversion. Only the differential-mode performances are left to be focused on. A systematic design method is then presented with simplified circuit models in differential mode, and frequency responses of these models are synthesized and further mapped into physical dimensions. Transformers are used in the mapping procedure and their turn ratios are calculated to find the exact final dimensions or are properly used to realize desired performance with some unattainable impedance values in synthesized simplified models. Finally, two filters are designed, fabricated, and measured to support the design method.