Single-Ended and Balanced Flat-Group-Delay RF Low-Pass Filters With Input-Quasi-Reflectionless Behavior for Digital-Communication Systems

A class of RF low-pass filters (LPFs) simultaneously featuring flat-in-band-group-delay and input-quasi-reflectionless characteristics is reported for the first time. By means of a multilayered complementary-diplexer-based topology, a single-ended input-quasi-reflectionless LPF with in-band quasi-flat group-delay response and multiple out-of-band transmission zeros (TZs) is first conceived. It consists of the combination of single-pole LPF units with resistively terminated microstrip-to-microstrip vertical slotline transitions (MS-to-MS VSTs). Subsequently, intrinsic common-mode-(CM)-signal suppression for its balanced counterpart is realized through in-series cascading two single-ended LPF stages with a coplanar stripline (CPS) section. As a result, a balanced LPF possessing differential-mode (DM) in-band quasi-constant group-delay response, DM input-quasi-absorptive behavior, and intrinsic CM suppression is obtained. Furthermore, to increase DM selectivity and out-of-band attenuation levels along with CM suppression levels, a higher rejection balanced LPF composed of multiple cascaded single-ended LPF units and CPS sections is also devised. The RF operating principles of the engineered single-ended and balanced LPF circuits are detailed. Three demonstrative prototypes—i.e., one single-ended and two balanced LPF circuits—are manufactured and characterized for practical validation purposes. For the higher rejection balanced LPF prototype, eye-diagram tests are also carried out to verify the quality/integrity of the transmitted DM signal in the time domain.