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.

I. Introduction

Reflectionless or absorptive RF filters have been attracting considerable research interest in recent years since they allow to improve the operational performance and robustness of the entire RF front-end chain. These filters are able to protect RF active components in previous stages—e.g., power amplifiers and mixers of frequency converters—by dissipating the nontransmitted out-of-band RF-signal power inside themselves rather than reflecting it back to the source [1]. In this manner, they allow the removal of bulky isolators or attenuators initially integrated at the interstage level of the RF front-end chain [2]. Several examples of input- and two-port-reflectionless low-pass filters (LPFs) that eliminate undesired spurious signals and harmonics have been reported. In [3], two-port-reflectionless LPFs were developed using complementary even/odd-mode subcircuits associated with absorptive high-pass-type RF-signal subpaths. Moreover, twofold lumped-element-based LPFs with inverse-Chebyshev-type input-reflectionless responses were designed in [4]. In another approach, by means of the transfer function of conventional ladder-type LPFs, novel reflectionless LPF topologies with various classes of response were discussed in [5]. In addition, by exploiting resistively terminated microstrip-to-microstrip vertical transitions, a multilayered input-reflectionless LPF with extended upper stopband was proposed in [6]. However, all these previously reported single-ended reflectionless LPFs still suffer from large in-band group-delay variations especially at their 3-dB-referred passband edges, thus leading to severe phase-distortion issues for in-band-processed RF signals. In [7], a 40-GHz absorptive-type LPF with quasi-flat group-delay response was proposed. It is based on direct realization of the well-known lumped-element circuit network of the Gaussian-type filter [8], but applied to coplanar and microstrip technologies. Nevertheless, even for a tenth-order LPF design, its stopband suffers from low attenuation levels and does not have transmission zeros (TZs).