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%

This paper focuses on a new family of narrow-band bandpass filters, along with its coupling-matrix-based design approach which feature quasi-elliptic frequency response, effective quality factors (Qs eff ) of the order of 1,000 and small form factors. Hybrid acoustic-wave-lumped-element resonator (AWLR) architectures are proposed as fundamental elements of these filters leading to low-loss passbands with fractional bandwidth (FBW) that is much broader (3.8–7.5 times) than in all acoustic wave (AW) filters and Qs eff that are 10–20 times larger than in traditional lumped-element filters. Experimental prototypes based on commercially-available surface acoustic wave (SAW) resonators were built and tested at 418 MHz. Measured Qs eff between 750–1550 and bandwidths ranging from 0.65 to 0.94 MHz (i.e., 2–3 times the electromechanical coupling coefficient of the AW resonator) are presented.

High-Q bandpass filters using hybrid acoustic-wave-lumped-element resonators (AWLRs) for UHF applications

This paper presents a review of multi-functional RF passive components. Special emphasis is given on planar passive devices that co-integrate the filtering functionality with other RF-analog-processing actions in order to reduce the size and the loss of conventional transmitter and receiver chains based on series-cascaded RF mono-functional passive blocks. Examples of multi-operational RF circuits that are covered in this overview article include: i) frequency-static and reconfigurable Wilkinson-type power dividers with added single/multi-band filtering behavior, ii) frequency-adaptive single/multi-band filtering impedance transformers, and iii) frequency-tunable broad-band bandpass filters with controllable notches inserted in the transmission band. For these multi-functional circuits, experimental results of several proof-of-concept microstrip prototypes are shown.

Single/multi-band multi-functional passive components with reconfiguration capabilities

Modern trends towards multi-frequency and multi-standard communication services have created the need for highly versatile RF transceivers which in turn call for RF filters with tunable transfer function. In this paper, recent advances in reconfigurable waveguide-resonator based filter architectures are reported. Alternative tuning concepts that are primarily based on radio frequency micro-electro-mechanical systems and facilitate the actualization of RF filters with wide tuning range (>2:1) and high quality factor (Q > 300–1000) are discussed. Three-dimensional (3-D) integration concepts and novel fabrication technologies that allow the realization of these filters at frequency bands as low as UHF-band (300–3000 MHz) to as high as W-band (75–110 GHz) are thoroughly reviewed.

Advances in high- Q tunable filter technologies

This paper presents four continuously variable W-band phase shifters in terms of design, fabrication, and radiofrequency (RF) characterization. They are based on low-loss ridge waveguide resonators tuned by electrostatically actuated highly conductive rigid fingers with measured variable deflection between 0.3° and 8.25° (at a control voltage of 0–27.5 V). A transmission-type phase shifter based on a tunable highly coupled resonator has been manufactured and measured. It shows a maximum figure of merit (FOM) of 19.5°/dB and a transmission phase variation of 70° at 98.4 GHz. The FOM and the transmission phase shift are increased to 55°/dB and 134°, respectively, by the effective coupling of two tunable resonances at the same device with a single tuning element. The FOM can be further improved for a tunable reflective-type phase shifter, consisting of a transmission-type phase shifter in series with a passive resonator and a waveguide short. Such a reflective-type phase shifter has been built and tested. It shows a maximum FOM of 101°/dB at 107.4 GHz. Here, the maximum phase shift varied between 0° and 377° for fingers deflections between 0.3° and 8.25°.

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Continuously variable W-band phase shifters based on MEMS-actuated conductive fingers

The disclosed invention relates to a device to efficiently realize (that is, with low dissipative loss) a large variable phase shift in a millimeter wavelength electromagnetic wave. Accordingly, a phase shifter device (1) is disclosed which comprises an axially extended waveguide (2) with a cavity (20) for conducting the electromagnetic wave, a MEMS-actuation mechanism (3), and a plurality of conductive fingers (4, 4a, 4b) which - due to their mechanical movement by means of the MEMS-actuator (3) - adjust a distributed electrical capacitance in the waveguide (2) and thus cause the adjustable phase shift to the electromagnetic wave. Specifically, at least two conductive fingers (4a, 4b) are arranged at an axial distance in a side wall (22) of the waveguide (2). By rotating opposing ends of these fingers into the cavity (20) of the waveguide (2) towards the opposing surface, which can have a predefined elevation profile, a large adjustable phase shift is achieved due to the distributed interaction between the fingers and the electromagnetic wave. Furthermore, by integrating the MEMS device directly into an air-filled metallic waveguide, a low-loss, high-power handling system is realized.

Dimitra Psychogiou

Papers

High-Q bandpass filters using hybrid acoustic-wave-lumped-element resonators (AWLRs) for UHF applications

Single/multi-band multi-functional passive components with reconfiguration capabilities

Advances in high- Q tunable filter technologies

Continuously variable W-band phase shifters based on MEMS-actuated conductive fingers

Waveguide-mems phase shifter