Balanced Flat-Group-Delay RF Low-Pass Filter With Differential-Mode Input-Quasi-Reflectionless Behavior for Digital-Communication Systems

In this paper, a novel type of balanced low-pass filter (LPF) combining for the first time differential-mode (DM) input-quasi-reflectionless behavior and flat group-delay response is proposed. Using a multilayer complementary-diplexer-based topology, a single-ended input-quasi-reflectionless LPF with flat group-delay characteristics and transmission zeros (TZs) is firstly determined through a combination of single-pole LPF units and resistively-terminated vertical microstrip-to-microstrip transitions. In order to attain a balanced LPF transfer function with the implemented input-quasi-reflectionless LPF, its balanced counterpart using the in-series cascade of two single-ended LPFs and a coplanar stripline (CPS) section is conceived. As a consequence, the common-mode (CM) signals associated with the input single-ended LPF units can be intrinsically rejected by the employed CPS structure, which results in a balanced in-band-flat-group-delay LPF with DM-input-quasi-absorptive response. The operational principles of both the single-ended and balanced LPF circuits are discussed in detail. For experimental-validation purposes, a multilayer balanced LPF prototype is manufactured and characterized. It exhibits a measured maximum in-band DM-group-delay variation of ±0.12 ns with regard to its average value, along with the expected input-quasi-absorptive and balanced operations with extended-CM-suppression capability.