Before the emergence of ultra-wideband (UWB) radios, widely used wireless communications were based on sinusoidal carriers, and impulse technologies were employed only in specific applications (e.g. radar). In 2002, the Federal Communication Commission (FCC) allowed unlicensed operation between 3.1–10.6 GHz for UWB communication, using a wideband signal format with a low EIRP level (−41.3dBm/MHz). UWB communication systems then emerged as an alternative to narrowband systems and significant effort in this area has been invested at the regulatory, commercial, and research levels.

IEEE Transactions on Microwave Theory and Techniques and Antennas and Propagation Announce a Joint Special Issue on Ultra-Wideband (UWB) Technology

A class of frequency-reconfigurable input-reflectionless/absorptive RF/microwave filters is presented. They consist of tunable complementary-duplexer architectures that are composed of a main and an auxiliary channel with opposite filtering transfer functions. By loading the auxiliary channel with a reference-impedance resistor and by taking the output node of the main channel as the output terminal of the overall circuit, a filtering network of the same type of the main channel with theoretically perfect input-reflectionless behavior at all frequencies can be realized. This technique can be applied to design spectrally agile completely input-reflectionless filters with any kind of transfer function, such as low-pass, high-pass, and single/multiband bandpass/bandstop filters. The theoretical analysis of the first-order absorptive bandpass/bandstop filtering sections based on a coupling-matrix formulation is detailed. Furthermore, the synthesis of high-selectivity reflectionless filters either by cascading multiple first-order cells or using high-order channels in a single complementary duplexer is also described. For practical-demonstration purposes, frequency-tunable lumped-element and microstrip prototypes are manufactured and characterized. They correspond to first- and second-order bandpass/bandstop filters. In addition, their in-series cascade connection is used to implement a bandpass filter with spectrally controllable passband and out-of-band notches.

Reflectionless Adaptive RF Filters: Bandpass, Bandstop, and Cascade Designs

This paper addresses the exploitation of signal-interference concepts for the realization of single/multi-frequency Wilkinson-type filtering power dividers in planar/lumped-element technologies. By embedding transversal signal-interference filtering sections into the arms of conventional Wilkinson-type power-divider topologies, RF/microwave power-distribution actions with intrinsic mono/multi-band bandpass filtering capabilities can be obtained. Analytical equations and rules for the theoretical synthesis of this dual-function device are derived. The generalization of the approach to multi-stage schemes for enhanced-performance designs or for the shaping of frequency-asymmetrical responses is also discussed. Furthermore, for practical demonstration, three prototypes are developed and characterized. They are a microstrip quad-band circuit for the 1–5 GHz range, a dual-band lumped-element device for the band of 0.2–0.6 GHz, and a new type of two-branch channelized active bandpass filter at 3 GHz that makes use of single-band versions of this dual-behavior component as signal-division/combination blocks.

Single/multi-band Wilkinson-type power dividers with embedded transversal filtering sections and application to channelized filters

In this paper, a balanced-to-unbalanced microstrip power divider based on branch lines with several stubs and one resistor is proposed. The functions of power dividing, frequency selectivity, isolation between output ports, and common-mode suppression can be realized at the same time. The even–odd-mode equivalent circuits combining with the standard S-parameters and the mixed-mode S-parameters are adopted to derive the analytical equations at the center frequency. One or two transmission zeros can be achieved to enhance the out-of-band suppression. The center frequency, bandwidth, isolation, common-mode suppression, and the frequencies of transmission zeros can be controlled by the design procedure. To verify the theoretical prediction, two fabricated prototypes are designed and compared. One gets 7.7% 1-dB bandwidth with 0.6-dB insertion loss and one transmission zero. The other gets 1-dB bandwidth of 5% with 0.7-dB insertion loss and two transmission zeros. The isolation and common-mode suppression for both prototypes are better than 15 and 20 dB within the whole passband, respectively.

A Balanced-to-Unbalanced Microstrip Power Divider With Filtering Function

In this work, we present first-order antisymmetric (A1) mode resonators in 128° Y-cut lithium niobate (LiNbO3) thin films with electromechanical coupling coefficients ( $k^{2}$ ) as large as 46.4%, exceeding the state-of-the-art. The achievable $k^{2}$ of A1 in LiNbO3 substrates of different orientations is first explored, showing X-axis direction in 128° Y-cut LiNbO3 among the optimal combinations. Subsequently, A1 resonators with spurious mode mitigation are designed and fabricated. In addition to the large $k^{2}$ , the implemented devices show a maximum quality factor ( $Q$ ) of 598 at 3.2 GHz. Upon further optimization, the reported platform can potentially deliver a wideband acoustic-only filtering solution in 5G New Radio. [2020–0003]

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A1 Resonators in 128° Y-cut Lithium Niobate with Electromechanical Coupling of 46.4%

The development of 3-D reconfigurable microwave multiband bandpass filters in substrate-integrated evanescent-mode-cavity-resonator technology is presented. The proposed filter approach exploits multiband quasi-bandpass sections whose tunable resonators do not directly interact with one another. In this manner, a multiband filtering transfer function with independently frequency-controllable passbands and interband transmission zeros is realized. Moreover, the number of active passbands can be dynamically selected in this filter architecture through their spectral merging as well as their intrinsic switching-OFF through resonators’ detuning. These operational capabilities are theoretically demonstrated by means of several coupling-matrix-level synthesis examples. Furthermore, for experimental-validation purposes, a triple-band second-order prototype with three electronically tunable transmission bands in the range 2.2–2.8 GHz is designed, manufactured, and tested.

Reconfigurable Multiband Bandpass Filters in Evanescent-Mode-Cavity-Resonator Technology

This article presents a bandpass filter with independent center frequency and bandwidth control. It is based on four frequency-reconfigurable resonators arranged in a series cascade of a second-order bandpass filter and a dual-behavior resonator. A measured bandpass filter response with variable bandwidth between 440 and 0 MHz is demonstrated at 3.6 GHz. The center frequency of the filter can be continuously varied between 2.76 and 3.79 GHz (1.4:1) with continuous bandwidth variation between 300 and 100 MHz (3:1) for a minimum out-of-band isolation of −15 dB and insertion loss between 0.76 and 2.5 dB. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2745–2750, 2013

Reconfigurable bandpass filter with center frequency and bandwidth control

This paper reports on a new class of multiband band-pass-to-bandstop (BP-to-BS) tunable RF filters. The proposed $\mathbf{fi} -$ ter concept allows the simultaneous realization of multiband bandpass-and bandstop-type transfer functions that can be independently reconfigured in frequency and in number-through band merging-. Switching between the multiband bandpass-filter (BPF) and bandstop-filter (BSF) modes is achieved by changing the coupling path between the source and the load. In both modes, band reconfigurability is obtained by only tuning the resonant frequencies of the filter's resonators. The proposed spectrally-adaptive multiband BP-to-BS filter concept is presented by means of: i $)$ coupled-resonator synthesis through its coupling-matrix representation and of: ii) an experimental four-resonator microstrip $\mathbf{fil}-$ ter prototype (1-to-2 bands in the BPF mode and 1-to-4 bands in the BSF mode) that was designed and tested in the frequency range 0.9-1.6 GHz for proof-of-concept demonstration purposes.

Tunable Multiband Bandpass-to-Bandstop RF Filters

This paper focuses on the application of signal-interference techniques to develop planar multi-band Wilkinson-type power dividers. By inserting transversal signal-interference multi-band filtering cells into the branches of the classic Wilkinson-type power-divider scheme, multi-band power-splitting/combining actions with added filtering operation can be attained. Design formulas for this dual-behavior filtering/power-dividing device for a particular class of embedded multi-band filtering cell are provided. Moreover, a 1.6/3/4.4-GHz triple-band microstrip prototype is built and tested for practical validation.

A class of planar multi-band Wilkinson-type power divider with intrinsic filtering functionality

This paper reports on dual-mode-cavity-resonator-based bandpass filters (BPFs) with multiple levels of transfer-function tunability. They are based on substrate-integrated-waveguide cavity resonators that are loaded by four identical metallic posts and give rise to two degenerate modes that are exploited for the filter design. By loading the ends of the posts with variable-reactance elements, quasi-elliptic-type filtering transfer functions with multiple levels of spectral adaptivity can be obtained. These reconfiguration properties include center-frequency control, bandwidth tuning, and intrinsic RF switching-off. The proposed adaptive BPF concept can be extended to high-order transfer functions shaped by $2N$ poles and $2N$ transmission zeros (TZs) for BPFs composed of $N$ dual-mode resonators. To prove the experimental viability of the engineered tunable BPF concept, a two-pole/two-TZ prototype and a four-pole/four-TZ prototype with tunable transfer functions in the 1.5–3 GHz band were built and measured.

Dimitra Psychogiou

Papers

Reconfigurable Multiband Bandpass Filters in Evanescent-Mode-Cavity-Resonator Technology

Reconfigurable bandpass filter with center frequency and bandwidth control

Tunable Multiband Bandpass-to-Bandstop RF Filters

A class of planar multi-band Wilkinson-type power divider with intrinsic filtering functionality

Multi-Mode-Cavity-Resonator-Based Bandpass Filters With Multiple Levels of Transfer-Function Adaptivity