High-Order Dual-Port Quasi-Absorptive Microstrip Coupled-Line Bandpass Filters

In this article, we present the first demonstration of distributed and symmetrical all-band quasi-absorptive filters that can be designed to arbitrarily high orders. The proposed quasi-absorptive filter consists of a bandpass section (reflective-type coupled-line filter) and absorptive sections (a matched resistor in series with a shorted quarter-wavelength transmission line). Through a detailed analysis, we show that the absorptive sections not only eliminate out-of-band reflections but also determine the passband bandwidth (BW). As such, the bandpass section mainly determines the out-of-band roll-off and the order of the filter can be arbitrarily increased without affecting the filter BW by cascading more bandpass sections. A set of 2.45-GHz one-, two-, and three-pole quasi-absorptive microstrip bandpass filters are designed and measured. The filters show simultaneous input and output absorption across both the passband and the stopband. Measurement results agree very well with the simulation and validate the proposed design concept

Multilayered input-reflectionless quasi-elliptic-type wideband bandpass filtering devices on diplexer-based structures

This article is an expanded version from the 2020 Asia-Pacific Microwave Conference (APMC2020), Hong Kong, Dec. 8–11, 2020 [DOI: 10.1109/APMC47863.2020.9331419]. (Corresponding authors: Li Yang; Roberto Gómez-García.)Classes of input-reflectionless wideband bandpass filters (BPFs) and balun BPF with quasi-elliptic-type (QET) responses are reported. They consist of two signal-transmission parts in multilayered diplexer-based topologies, as follows: 1) a BPF channel shaped by a two-/three-port reflective-type BPF/balun on a microstrip-to-microstrip vertical transition and 2) an input-absorptive bandstop filter (BSF) channel built with a shunt resistively terminated lowpass filter (LPF) that is composed of hybrid microstrip sections and open-circuit-ended microstrip stubs. Two real-frequency out-of-band transmission zeros (TZs) are generated in these RF filtering devices using a cascaded short-circuit-ended two-section microstrip line and two shunt open-circuit-ended half-wavelength microstrip lines, respectively. Using a higher order LPF network, enhanced passband amplitude flatness and improved stopband power absorption ratio levels for the devised BPFs are attained. As design examples, two third-order BPFs with a shunt resistively terminated microstrip T-junction and a $\pi$ -shape structure, respectively, are first provided. Afterward, a higher order BPF based on two cascaded replicas of a third-order BPF unit is designed to show highly increased stopband power attenuation levels and enhanced power absorption ratio profile within the stopband-to-passband transitions. Subsequently, their application to an input-absorptive QET fourth-order wideband balun BPF is presented. The operational principles of these BPFs and balun are detailed by the developed design procedures, in which their associated impedance-type design parameters are synthetically determined. As practical validation, four microstrip prototypes corresponding to three 2-GHz wideband BPFs and one 1-GHz broadband balun BPF are manufactured and tested. These input-reflectionless wideband filtering components experimentally feature the desired merits in terms of QET responses, enhanced passband amplitude flatness, and improved stopband power absorption ratio levels.Agencia Estatal de InvestigaciónEuropean Comissio

Tunable quasi-reflectionless bandpass filters using substrate integrated coaxial resonators

This brief reports on the electromagnetic (EM) design and the practical development of frequency-reconfigurable quasi-reflectionless bandpass filters (BPFs) using substrate integrated coaxial (SIC) resonators. The filter concept is based on in-series-cascaded quasi-reflectionless stages that are shaped by a first-order bandpass section and two resistively-terminated first-order bandstop sections. For the first time, we explore the realization of these filters using tunable SIC resonators that exhibit high quality factor (Q) and can be widely tuned with commercially-available linear piezoelectric actuators. Synthesized examples alongside various EM design and practical integration aspects are discussed in detail. The concept is experimentally validated at S-band through the manufacturing and testing of two reconfigurable (one-stage and two-stage) prototypes

Adaptive multi-band negative-group-delay RF circuits with low reflection

Two classes of frequency-reconfigurable multi-band negative-group-delay (NGD) circuit networks that feature low-input-power-reflection capabilities are reported. They consist of lossy-complementary-diplexer architectures, in which the NGD properties are obtained within the stopband regions of their lossy multi-band bandstop-filter (BSF) channel. Their complementary lossy multi-band bandpass-filter (BPF) branch absorbs in its terminating resistor the RF-input-signal energy that is not transmitted by the lossy multi-band BSF channel within its stopbands. In this manner, the input-reflectionless/absorptive behavior is realized. The theoretical foundations of the devised lossy-multi-band-BSF-based NGD structures using a coupling-routing-diagram formalism and single-to-multi-band admittance transformations are described. For the first-order case as illustration, guidelines for the synthesis in the bandpass frequency domain are provided. Furthermore, the extension of these multi-band NGD approaches to higher-order and in-series-cascade multi-stage realizations for more-general and wider-band NGD patterning, as well as to two-port/symmetrical designs, is shown. In addition, the conception of multi-functional passive components with NGD characteristics, such as wide-band BPFs and power directional couplers with embedded NGD regions, is also addressed. For experimental-demonstration purposes, an electronically-reconfigurable microstrip prototype of a two-stage-in-series-cascade dual-band NGD circuit is manufactured and measured

Planar high-order broad-band bandpass filters based on two-stage quadrature couplers and their digital modeling

2022 24th International Microwave and Radar Conference (MIKON), 12-14 September 2022, Gdansk, Poland.Planar two-stage branch-line directional power couplers are applied in this paper to the design of transversalsignal-interference high-order wide-band bandpass filters (BPFs) with very-sharp-rejection characteristics. In comparison with the previously-reported experimental BPF demonstrators of their precursors—i.e., those using transversal filtering sections (TFSs) with one-stage branch-line couplers loaded by longer dissimilar stubs—, an enlarged-bandwidth passband with a higher number of in-band reflection zeros and lower amplitude variation can be obtained. Moreover, multiple transmission zeros (TZs) can be created in the out-of-band region owing to the intrinsic signal-energy cancelation phenomena of the transversal signal-interference philosophy, which result in highselectivity filtering capabilities. As further conceptual understanding of the devised two-stage-coupler signal-interference BPFs, a digital-modeling interpretation for various illustrative synthesis examples is provided. In addition, for experimentalvalidation purposes, single- and two-TFS-based microstrip prototypes of wide-band BPFs centered at 2 GHz are manufactured and characterized. In these circuits, inter-digital-type input/output feeding sections are co-integrated. They allow to extend the lower/upper stopband bandwidths with regard to those inherent to their isolated two-stage-coupler-based TFSs.Agencia Estatal de InvestigaciónEuropean Commissio

A Survey of Differential-Fed Microstrip Bandpass Filters: Recent Techniques and Challenges

Differentially driven devices represent a highly promising research field for radio frequency (RF), microwave (MW), and millimeter-wave (mmWave) designers and engineers. Designs employing differential signals are essential elements in low-noise fourth-generation (4G) and fifth-generation (5G) communications. Apart from the conventional planar MW components, differential–fed balanced microstrip filters, as promising alternatives, have several advantages, including high common-mode rejection, low unwanted radiation levels, high noise immunity, and wideband harmonic suppression. In this paper, a comprehensive and in-depth review of the existing research on differential-fed microstrip filter designs are presented and discussed with a focus on recent advances in this research and the challenges facing the researchers. A comparison between different design techniques is presented and discussed in detail to provide the researchers with the advantages and disadvantages of each technique that could be of interest to a specific application. Challenges and future developments of balanced microstrip bandpass filters (BPFs) are also presented in this paper. Balanced filters surveyed include recent single-, dual-, tri-, and wide-band BPFs, which employ different design techniques and accomplish different performances for current and future wireless applications

A Reconfigurable Pseudohairpin Filter Based on MEMS Switches

This work presents a bandpass-reconfigurable planar pseudohairpin filter based on RF-MEMS switches. Hairpin-line structures are preferred to design microstrip filters because this class of filters offers a more compact size, and, in general, hairpin filters do not need ground connections for resonators. In this work, the U-shape resonators are arranged to obtain an interdigit capacitor to improve the coupling between the resonators. RF-MEMS switches modify the lengths of coupled resonators by adding microstrip segments to control the filter bandwidth, moving the center frequency and the return loss. An experimental hairpin tunable filter prototype based on RF-MEMS has been designed, fabricated, numerically and experimentally assessed, and compared concerning its tunability, quality factor, and capability with standard tunable filters based on PIN diodes. In conclusion, the tunable hairpin filter based on RF-MEMS switches offers the best performance in center frequency tuning range, compactness, and power consumption regarding reconfigurable filters based on standard PIN diodes switches. The obtained results are appealing and demonstrate the capabilities and potentialities of RF-MEMS to operate with the new communication standards that work at high microwave frequency bands

High-Selectivity Reflectionless Unbalanced-to-Balanced Filtering Power Combiner

A high-selectivity reflectionless unbalanced-to-balanced (UTB) filtering power combiner is proposed. The proposed power combiner composes of two absorbing branches, two filtering sections, two transmission lines, a grounded resistor, and a phase inverter. Two transmission zeros respectively located at the lower and upper sides of the passband are achieved by the filtering sections, resulting in high selectivity. The input-port reflectionless response in the bandstop region is obtained by the absorbing branch. A 1.0-GHz UTB filtering power combiner is designed and fabricated. Finally, the measurement performances are given in this paper. The input-reflection absorptive bandwidth and transmission bandwidth are measured as 545 MHz and 132 MHz, respectively. The absorptive bandwidth is 4.12 times the transmission bandwidth

Balanced quasi-elliptic-type combline diplexer with multiextracted-pole junction/output sections

A class of coupled-multiline quasi-elliptic-type balanced diplexer device with closely spaced channels and its balanced coupling-routing-diagram formalism to simultaneously model its differential- and common-mode operation are reported. It employs third-order combline-type bandpass filters (BPFs) in its filtering channels, whose resonating lines are connected at one of their extremes to the symmetry plane of the balanced-circuit structure (i.e., virtual grounds for the differential mode), along with a multiextracted-pole/coupled-multiline dual-band BPF junction and single-band BPF output cells. In differential-mode operation, the dual-band BPF junction adds one in-band pole to each diplexer channel and three transmission zeros (TZs) to both channels, whereas the single-band BPF output cells incorporate one in-band pole and two TZs in their corresponding channels. In this manner, increased-selectivity differential-mode BPF transfer functions for the diplexer channels when compared to those of its third-order combline-type BPFs are obtained. Grounded resistors are connected at the symmetry plane of the balanced diplexer for all the resonating lines of the combline-type BPFs, which become detuned with regard to the poles associated with the dual-band BPF junction and the single-band BPF output cells under common-mode excitation. This allows for high common-mode suppression levels in wide spectral ranges to be realized for both channels while maintaining the same TZs as in differential-mode operation. For practical-validation purposes, a 1.6-GHz/1.8-GHz microstrip prototype is manufactured and tested.Ministerio de Economía, Industria y CompetitividadEuropean Commissio

Non-reciprocal balanced bandpass filters with quasi-elliptic response

This paper reports on the RF design and practical development of a non-reciprocal balanced bandpass filter (BPF) that exhibits a highly-selective quasi-elliptic response in the forward direction of propagation that is shaped by four transmission poles and two transmission zeros (TZs). By modulating some of the filterâ s resonators with phase-progressed AC signals, a non-reciprocal response is obtained in the differential mode. Its common-mode is also highly suppressed due to the incorporation of a balanced network that results in two additional TZs and resistive loss that are unique to the common-mode. The filter order can be increased by cascading additional resonators. For validation purposes, a microstrip prototype centered at 725 MHz was designed, manufactured, and measured. It showed a high isolation in the differential-mode reverse transmission of up to 62.1 dB. Moreover, the common-mode was suppressed by over 45 dB in a bandwidth greater than one octave