RF-signal power reflections produced by impedance mismatches remain a critical limitation in high-frequency systems, degrading their operational performance, robustness, and RF-signal integrity. Typical RF-protection mechanisms, such as inter-RF-stage attenuators, isolators$\boldsymbol {/}$circulators, or RF-signal-cancellation loops, often impose penalties in terms of increased in-band insertion-loss levels, size, cost, DC-power consumption, and design complexity. To overcome these challenges, reflectionless or absorptive RF circuits have emerged as an alternative paradigm. They intrinsically absorb undesired stopband$\boldsymbol {/}$non-transmitted RF signals, thereby providing broad-band impedance matching. This overview paper reviews the latest advances on RF reflectionless circuits, covering their fundamental RF design principles and more-recent developments. Beyond the well-established case of RF reflectionless filters illustrated through several state-of-the-art implementations in 3-D and planar technologies, other emerging designs are also addressed. They include reflectionless beyond-transmission-band-flat-group-delay single-ended bandpass filters and balanced lowpass filters for high-data-rate digital communications in planar and integrated RF platforms, and on-chip two-port-absorptive bandstop filters. Furthermore, as key examples of reflectionless RF active devices, millimeter-wave input-absorptive RF amplifiers for beyond-5G wireless scenarios are presented. Cutting-edge applications in antennas, electromagnetic compatibility (EMC), and quantum engineering are also discussed, highlighting the versatility of the reflectionless design philosophy.

The traditional method for designing branch-line couplers involves a trial-and-error optimization process that requires multiple design iterations through electromagnetic (EM) simulations. Thus, it is extremely time consuming and labor intensive. In this paper, a novel machine-learning-based framework is proposed to tackle this issue. It integrates artificial neural networks with a self-adaptive differential evolution algorithm (ANNs-SaDE). This framework enables the self-adaptive design of various types of microwave branch-line couplers by precisely optimizing essential electrical properties, such as coupling factor, isolation, and phase difference between output ports. The effectiveness of the ANNs-SaDE framework is demonstrated by the designs of folded single-stage branch-line couplers and multi-stage wideband branch-line couplers.

This paper presents a class of on-chip passive-integrated RF bandpass filters (BPFs) in Gallium Arsenide (GaAs) technology. They exhibit two-port-RF-quasi-absorptive behavior and wideband flat-group-delay responses as main operational features, making them suitable for modern high-date-rate digital-communications systems. Firstly, by means of a reflective one-pole BPF and two shunt identical absorptive bandstop filters (BSFs) in a π-shape topology, an RF BPF with symmetrical quasi-reflectionless response and flat group delay beyond its 3-dB bandwidth (BW) is realized. Subsequently, in order to further improve the stopband attenuation levels of the previously-designed low-order BPF circuit, a higher-rejection counterpart is conceived using two modified single-pole reflective BPF units and three shunt identical absorptive BSF branches. The theoretical responses of both BPFs are obtained from their associated lumped-element-based equivalent circuits. For experimental-demonstration purposes, two GaAs-based proof-of-concept RF BPF prototypes are designed, manufactured, and tested. The two measured absorptive BPFs have 11.58-GHz center frequencies, quasi-reflectionless ranges from DC to 46.02 GHz and from DC to 46.66 GHz, 3-dB-BW ranges from 7.527 to 15.94 GHz and from 8.312 to 15.18 GHz, and maximum group-delay variation of ±0.0068 ns from 5.92 to 16.94 GHz and from 6.316 to 17.58 GHz, respectively.

This paper reports a novel class of RF dual-band bandpass filters (BPFs) with quasi-flat group delay and multiple stopband transmission zeros (TZs). These filters mainly exploit microstrip-to-microstrip (MS-to-MS) vertical transitions on stepped-impedance slotlines. A two-layer dual-band BPF with quasi-flat group-delay response and seven stopband TZs is firstly conceived by properly determining the electrical-length and line-impedance parameters of the constituent slotline sections of its devised stepped-impedance slotline resonator. In order to increase the abruptness of the filter cut-off slopes and the stopband power-rejection levels, a sharper-rejection dual-band BPF composed of several cascaded units is also realized. It includes a stepped-impedance-slotline-based vertical transition, two matching circuits, and two half-wavelength-slotline-based vertical transitions. The RF operational principles of these dual-band BPFs are illustrated through their transmission-line-based equivalent circuit models. Moreover, for validation purposes, a proof-of-concept circuit of the first dual-band BPF is designed and simulated at the electromagnetic (EM) level. The EM-simulated lower and upper passbands are centered at 5.935 and 10.025 GHz with 3-dB fractional bandwidths (FBWs) of 13.65% and 6.98% and maximum group-delay variations of ±0.05 ns from 5.152 to 6.556 GHz and 9.325 to 10.752 GHz, respectively.

A family of on-chip passive-integrated absorptive flat-group-delay RF bandpass filters (BPFs) in gallium arsenide (GaAs) technology is presented. These wideband BPFs feature broadband quasi-reflectionless behavior along with quasi-constant group-delay responses beyond their associated 3-dB-bandwidth (BW) ranges. Firstly. by means of a$\pi $-shape network composed of a reflective first-order BPF and two shunt identical lossy bandstop filters (BSFs), a two-port-absorptive RF BPF is engineered. To further increase the stopband attenuation levels, the extension of this filter concept to higher-rejection BPFs usingncascaded reflective single-pole BPF units and (${n} +1$) replicas of a shunt lossy BSF is then approached. Subsequently, in order to equip such BPFs with higher-selectivity filtering responses, the development of input- and two-port-reflectionless BPFs with multiple transmission zeros (TZs) is addressed. Moreover, a multi-TZ flat-group-delay BPF with input-absorptive behavior is devised. It exploits a reflective BPF channel, which is shaped by a high-selectivity BPF unit with two close-to-passband TZs and a shunt series-LCresonator that produces an additional TZ, along with a shunt absorptive BSF in a complementary-diplexer-based topology. Finally, by cascading two duplicated high-selectivity BPFs and the associated absorptive BSFs with a modified shunt series-LCresonator in a back-to-back connection, a type of two-port-reflectionless BPF with multiple TZs is further engineered. Following this approach, a modified shunt lossy BSF instead of the previous shunt series-LCresonator is employed in the overall reflectionless BPF to obtain a sharper-rejection passband and flatter group delay versus the corresponding beyond-3-dB BW. The RF operational foundations of these absorptive BPFs are detailed with analyses of their relevant lumped-element-based equivalent circuits. Furthermore, proof-of-concept prototypes for the five suggested RF BPFs are simulated, built, and measured to experimentally validate their design concepts for application in power-efficient high-data-rate digital-communication systems.

An input-reflectionless RF amplifier operated at millimeter-wave (mm-wave) frequencies is reported in this paper. Its design methodology allowing it to attain a broadband RF-input-power-absorption behavior is detailed through the circuit analysis of the designed amplifier. For practical-demonstration purposes, a single-stage cascode input-reflectionless amplifier chip is fabricated and tested using 45-nm silicon-on-insulator (SOI) CMOS technology. Under a 1.2-V power supply, the DC-power consumption of the designed amplifier is 5.3 mW. It has a peak gain of 5.3 dB at 31 GHz. Moreover, the measured input 1-dB compression point (P1dB) is around -2.6 dBm. The measured input-power-matching levels of the designed amplifier are higher than 10 dB from DC to 60 GHz. In addition, a two-stage amplifier is also designed in simulation by cascading a conventional cascode amplifier with the proposed single-stage reflectionless amplifier. Simulation results show that a good improvement in terms of signal-to-interference ratio is achievable. The core size of the designed input-reflectionless amplifier is only 0.25 × 0.7 mm2.

This paper presents a high-order multilayer RF filter with wideband quasi-flat group-delay response and sharp multi-transmission-zero (TZ) stopbands. It is composed of three in-series-cascaded stages of low-order/single-unit input-absorptive wideband bandpass filters (BPFs). Its constituent BPF unit, which is based on a modified two-layer microstrip-to-microstrip vertical transition and a shunt resistivelyterminated microstrip line and exhibits quasi-flat group-delay response and broadband input-absorptive capabilities, is firstly discussed. Due to the employed short-circuit-ended two-section microstrip stub in the modified vertical transition, two close-to-passband TZs are realized to obtain a sharper-rejection BPF response. To achieve highly-attenuated multi-TZ stopbands, a high-order wideband quasi-flat-group-delay BPF with three in-series-cascaded single-unit BPFs using different impedanceparameter values for the short-circuit-ended microstrip stubs is then studied. For experimental-validation purposes, a 2-GHz proof-of-concept microstrip prototype in a two-layer substrate is simulated, built, and tested. The measured input-absorptive BPF is centered at 1.956 GHz with 3-dB absolute bandwidth of 0.828 GHz and maximum group-delay variation of ± 0.05 ns from 1.387 to 2.618 GHz (i.e., 1.231-GHz absolute bandwidth).

The theoretical design and practical development of RF analog filtering devices with co-integrated lowpass, single$\bm{/}$multi-band bandpass, and highpass transfer functions is reported. For this purpose, two different classes of generalized frequency transformation that convert the equivalent normalized lowpass filter prototype into the desired RF filter with several co-designed filtering actions are proposed. They realize a frequency mapping of the reactance of a normalized unitary capacitor into that of a one-port single$/$multi-resonance cell with added lowpass and highpass filtering capabilities. For these devised frequency transformations, the theoretical foundations and various illustrative filter examples designed at the ideal-circuit-model level are presented. In addition, higher-selectivity filter architectures based on the generation of additional out-of-band transmission zeros (TZs) by means of cross-coupling techniques are shown. Afterwards, the extension of this design methodology to RF multi-functional filtering components, such as input-reflectionless$\bm{/}$absorptive filters based on complementary-diplexer circuit networks and two-way filtering power-distribution circuits, is also demonstrated. Furthermore, two design examples of distributed-element and inverterless lumped-element RF filters are provided. Besides, for the distributed-element circuit, a proof-of-concept microstrip prototype is manufactured and measured as experimental validation.

A class of reflectionless RF bandpass filters (BPFs) with wideband quasi-constant in-band group-delay response in transmission for high-data-rate digital-communication systems is presented. These BPFs are developed by exploiting multilayer wideband microstrip-to-microstrip vertical transitions and lossy multi-section microstrip lines in complementary-diplexer-based architectures. A two-port-reflectionless wideband BPF with low-in-band-variation equi-ripple-type group-delay characteristics is firstly realized by using two resistively-terminated one-section microstrip lines. To further flatten its passband group delay, its counterpart based on lossy two-section microstrip lines is then conceived to attain extremely-flat wideband group-delay pattern. Subsequently, by means of a modified wideband microstrip transition with a short-circuit-ended two-section microstrip line and a resistively-terminated two-section microstrip line, an input-reflectionless BPF with two close-to-passband transmission zeros (TZs) and wideband in-band flat group-delay profile is reported. In order to obtain a higher-order flat-group-delay wideband BPF response with more TZs, its structure shaped by two in-series-cascaded units of the previous input-absorptive BPF section with two TZs is engineered, in which different impedance values for their constituent short-circuit-ended microstrip lines are utilized. The operational foundations of all the proposed wideband flat-group-delay RF BPFs are described in detail. Furthermore, for practical-validation purposes, three 2-GHz microstrip prototypes of these reflectionless wideband RF BPFs with quasi-constant group-delay responses beyond their 3-dB absolute bandwidths (BWs) are designed, simulated, manufactured, and characterized.

Various families of flat-in-band-group-delay RF planar bandpass filters (BPFs) with multiple out-of-band transmission zeros (TZs) in their filtering transfer function are presented. As the first approach, a type of BPFs with theoretically-constant group-delay profile and perfect two-port-reflectionless behavior is proposed. The theoretical sufficient conditions to obtain a frequency-constant group delay in a generalized form of transfer function for these BPFs are derived. They are then particularized in transmission-line-based transversal-circuit realizations with flat-in-band-group-delay and quasi-reflectionless characteristics. Afterwards, a class of reflective-type BPFs based on directional power couplers that are arranged in transversal-mode topologies are addressed toward its practical demonstration for the first time. These BPFs, which exhibit flat in-band group delay, are compared with their classic Bessel-type BPF counterparts without TZs. Moreover, discrete-time models of such BPFs are derived, analyzed, and discussed. For experimental-validation purposes, proof-of-concept microstrip prototypes centered at 2.5-GHz of the engineered quasi-absorptive/reflective-type flat-in-band-group-delay BPFs are designed, manufactured, and characterized.

An approach to design quasi-elliptic-type planar filters with single- and dual-band bandpass transfer functions and input-absorptive capabilities is presented. Two-branch channelized passive circuit configurations are exploited for this purpose, in which the low-order reflective-type filtering profile of their branches is converted into the desired sharp-rejection input-reflectionless filtering action in the overall circuit. This is achieved by means of fully-destructive and frequency-selective transversal signal-interference effects at the total input and output accesses of the channelized filter, respectively. The theoretical operational principles of the proposed concept of single/dual-passband input-reflectionless two-branch channelized filter are detailed, along with design considerations for their RF transmission-line-based implementation. Besides, its generalization to$\boldsymbol {N}$-channel architectures is also analyzed. Furthermore, as the fundamental elements of the devised channelized filtering philosophy to increase selectivity, alternative solutions to realize their output phase-delay sections for a more-flexible control of the transmission zeros in the overall transfer function are discussed. For experimental-demonstration purposes, two microstrip proof-of-concept prototypes are developed and measured. They consist of 3-GHz single-band and$2.58/3.43$-GHz dual-band designs with input-quasi-reflectionless spectral ratio above 2.5:1 and 3:1, respectively.