RF-Input-Quasi-Reflectionless Filtering Dispersive-Delay Structures Based on Complementary-Diplexer Architectures

The theoretical design and practical development of a family of high-order bandpass filtering dispersive-delay structures (DDSs) with radiofrequency (RF)-input-quasi-absorptive capabilities are presented. They consist of RF complementary-diplexer-based circuits, in which the main channel is a reflective-type bandpass filtering DDS and the resistively terminated auxiliary channel exhibits a bandstop filtering response with a quasi-complementary frequency profile with regard to that of the main channel. In this manner, the out-of-band/non-transmitted RF-input-signal power echoed by the main channel is mostly dissipated by the loading resistor of the auxiliary channel. As a fundamental constituent circuit block arranged in in-series-cascaded configurations for high-order reflective-type bandpass filtering DDS realizations, a triplet coupled-resonator network with one frequency-dependent coupling (FDC) is used. Its transmission properties are nearly transferred to the total RF-input-quasi-absorptive bandpass filtering DDS, which preserves the patterned group-delay/phase-variation response of the main channel. The theoretical design method of the devised RF-input-quasi-reflectionless bandpass filtering DDSs is detailed and exemplified with fifth- and seventh-order synthesis examples. It is based on a two-step design process, as follows: 1) separate polynomial/coupling-matrix synthesis of the main and auxiliary channels for the desired bandpass filtering DDS specifications in transmission and the complementary bandstop filtering functionality, respectively, and 2) coupling-matrix optimization of the overall RF-input-quasi-absorptive bandpass filtering DDS. Furthermore, for experimental demonstration purposes, a 2-GHz microstrip prototype of the previously designed RF-input-quasi-reflectionless fifth-order filtering DDS that features a stepped-type group-delay response is manufactured and characterized.