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In this article, quarter-mode substrate-integrated waveguide (QMSIW) filters featuring ultrawide stopband and flexibly allocated transmission zeros (TZs) are proposed. Eight higher modes with resonant frequencies up to 4.7 times of the fundamental mode are highly suppressed. It is realized by the combination of three approaches without extra circuit areas. First, a slot is cut in the first and last QMSIW resonators to make their resonant frequencies of higher modes different from those of other resonators. Second, avoid exciting the specific higher modes by setting the feed point in the area where the electric field strength of the higher modes is weak. Third, mismatch the higher modes when matching the passband. It is realized by using a stripline impedance transformer with its main body not exceeding the area of QMSIW resonators. Furthermore, except for the TZs derived by the classical cross-coupling matrix, two more independently controlled TZs are generated. The packaged filter is fabricated by using a benzocyclobutene (BCB)-based microelectromechanical systems (MEMS) process, which is suitable for millimeter-wave 3-D system-on-package (SOP) applications. The fabricated thin-film filter is measured with an insertion loss of 2.96 dB at 29.4 GHz and a fractional bandwidth (BW) of 17%. Higher mode suppressions up to 138.3 GHz are achieved. [ABSTRACT FROM AUTHOR]

In this article, high selectivity filtering antennas are designed based on a mixed-mode cavity resonator, which is formed by adding a short-circuited metal cylinder at the center of a rectangular cavity. The resonant and coupling characteristics of the proposed resonator are analyzed in detail. The operating modes of the mixed-mode cavity are TE102 mode, TE012 mode, and quasi-TEM mode, which can be utilized to design filtering antennas with different transmission zero distributions. To demonstrate the concept, three slot antennas with 3rd-order filtering performance are designed and simulated, where two transmission zeros are introduced to improve the selectivity. Furthermore, a 5th-order filtering antenna array based on the mixed-mode resonator is designed and fabricated. The measured results are consistent with the simulated results, showing that a gain of greater than 13 dBi is achieved. [ABSTRACT FROM AUTHOR]

This article focuses on the synthesis equations of three-section coaxial stepped impedance resonators (SIRs) for the design of Tx bandpass filters and on the benefits of adding a third section compared with using a two-section SIR. In this topology, the three coaxial structures fit inside ones another. In terms of the electrical scheme, the model is a succession of coaxial sections in a cascade, where the ground conductor of one becomes the central core of the next and vice versa. An advantageous property of SIRs is that they allow the size and the distance from the next harmonic to be controlled. The SIR effect depends on the ratio of the characteristic impedances of two adjacent coaxial TEM sections. For a resonator based on three sections, two impedance ratios need to be considered to electrically characterize the resonator in terms of fundamental resonant frequency and transmission zeros. Thanks to the equations obtained by modeling its frequency behavior, we are able to show the added value of changing such a resonator from two to three sections. This makes it possible to enlarge the minimal gap between the cylinders, decreasing the sensitivity to technological variations in manufacture and improving immunity to multipactor phenomena under constrained conditions of height and bandwidth. It also makes the first transmission zero frequency and fundamental frequency independent, as the relative proximity of these two frequencies has an impact on the quality factor and the power handling of the structure. To demonstrate the advantages, we designed an example of a three-section fourth-order filter in the L-band. This was compared with equivalent two-section filters, considering electrical performances and power handling, particularly the multipactor effect. This filter was fabricated in aluminum and tested. The experimental results are presented and discussed. [ABSTRACT FROM AUTHOR]

In this article, we present a novel ultrawideband branch waveguide coupler (BWC) with low amplitude imbalance. This novel design is based on a new structure of branch waveguide obtained by symmetrically increasing the waveguide width. The coupling degree can be independently controlled by the height and width of the new branch waveguide, which significantly increases the design flexibility. In addition, this structure can produce a transmission zero (TZ), and a concept to improve the fractional bandwidth of the BWC with out-of-band TZ is proposed. Based on the analysis, the design guidelines for the novel BWC are provided. For a given center frequency, an n-branch novel BWC can be easily designed. Moreover, because of the larger branch waveguide height, this novel design has greatly eased the machining difficulty. Finally, to verify the design concept, a $W$ -band eight-branch prototype is processed and measured. In full $W$ -band, the amplitude imbalance is less than 0.52 dB, the phase imbalance is less than 2.5°, the return loss (RL) is greater than 21.4 dB, and the isolation is greater than 20.5 dB. The simulation and experimental results are in good agreement. [ABSTRACT FROM AUTHOR]

In this article, a new class of tunable filters and diplexers operating in $X$ -band is presented. The proposed circuits are based on an in-line dual-post resonator, which consists of a pair of antipodal partial-height posts. The resonance and coupling characteristics of the proposed resonator can be regulated by the height and height difference of the posts. The most interesting feature of using the in-line dual-post resonator is that the input–output coupling and inter-resonator coupling are inversely proportional to the frequency within the tuning range, which is necessary for maintaining the fixed absolute bandwidth of the tunable filter. A prototype tunable filter is designed and fabricated, showing a low variation ($< \pm 3.9$ %) in the absolute bandwidth over a relatively wide tuning range of about 17%. The waveguide $H$ -plane static $Y$ -junction is designed and cascaded with two channel filters to implement the tunable diplexer. Noncontiguous and contiguous frequency plans can be realized by properly arranging the frequency band of the two channel filters. Measured results show that the tunable diplexer can obtain constant frequency spacing and approximately 10% tuning range with an insertion loss of less than 1.5 dB. [ABSTRACT FROM AUTHOR]

This article presents a high-efficiency dual-band filtering Doherty power amplifier (DPA) using multi-function circuits for simplifying the structure. A dual-band filtering power combiner also plays the role of real-to-real impedance matching, in which one resonator is shared by both the power combing and filtering circuits. Thus, only one extra resonator is needed for the power combiner to provide second-order dual-band filtering responses, which contributes to high efficiency of the DPA. Transmission zeros are generated between the two passbands, realizing a sharp roll-off rate and greatly enhancing the isolation between the two passbands. Meanwhile, an impedance control network is designed to simultaneously suppress the harmonics and transform the optimized complex impedance desired by the transistor to a real one. In this way, the output matching network and extra filter in conventional design are realized by the filtering power combiner and impedance control network in the proposed design, resulting in a simpler structure. A dual-band DPA operating at 2.14 and 2.6 GHz is implemented with the measured peak efficiencies of 64.4% and 69% at the output power of 43.25 and 43.3 dBm, respectively. The 6-dB back-off efficiencies are 50% and 57% at the two passbands. Under 20-MHz long term evolution (LTE) modulated signals with a peak-to-average power ratio (PAPR) of 7.5 dB, the DPA after digital predistortion (DPD) is measured with an adjacent channel leakage ratio (ACLR) of better than −45.2 dBc and the efficiencies of 51.6% and 54.3% at interested bands. The peak efficiency under the excitation of the concurrent modulated signal is 48.6%. [ABSTRACT FROM AUTHOR]

This article reports the design of coupled-resonator-based microwave dispersive delay structures (DDSs) with arbitrary asymmetric-type group delay response. The design process exploits a coupling matrix representation of the DDS circuit as a network of resonators with frequency-variant couplings (FVCs). The group delay response is shaped using complex transmission zeros (TZs) created by dispersive cross-couplings. We also present an optimization-based synthesis procedure for characteristic polynomials with a prescribed group delay profile. Thus, when compared with prior-art DDS approaches, the proposed DDS solutions allow a general group delay profile to be patterned while incorporating optimization-based coupling matrix design techniques for their synthesis. The design method is validated by full-wave simulations and measurements of three built proof-of-concept prototypes of DDS devices with different shapes of group delay response in waveguide and microstrip technologies. [ABSTRACT FROM AUTHOR]

A 3-D printed narrowband bandpass filter based on spherical dual-mode resonators is presented in this article. It is designed for output multiplexers (OMUXs) using high- $Q$ spherical dual-mode resonators. Realization is by laser powder bed fusion (L-PBF) technology of Invar alloy chosen for its low coefficient of thermal expansion (CTE). Using PBF circumvents the alloy’s manufacturability issues associated with its hardness in machining and free forming. Compared with polymer-based vat photopolymerization technology, PBF allows for direct metal manufacture of complex monolithic microwave components with better thermal-mechanical properties and higher power-handling capability. Using Invar can further help achieve high temperature stability of the filter in high-power operation. To demonstrate the proposed solution, detailed thermal-RF test at different temperatures was carried out. The experimental results of a 0.47% fourth-order silver-plated Invar filter with two transmission zeros verify the design and manufacturing. An insertion loss of 1 dB and an effective temperature coefficient of less than 2 ppm/K were achieved. [ABSTRACT FROM AUTHOR]

A new low-pass prototype, i.e., cascade hybrid triplet (CHT), is proposed in this article. This triplet, as a synthesis building block, offers two arbitrary-placed transmission zeros (TZs) to suppress the target frequency area or produce a complex TZ pair to flatten the group delay. The CHT can be used to accurately synthesize and design three types of real circuit configurations, i.e., double-dangled extracted-pole section (DDEPS), coupled-resonator-dangled extracted-pole section (CDEPS), and frequency-dependent triplet (FDT) with the asymmetric response. We present the detailed synthesis and the related design method for three types of circuits, while a series of typical examples is synthesized and designed to demonstrate the proposed approach. Finally, four planar four-TZ filters with asymmetric responses based on the proposed approach are designed and fabricated to exhibit the high-order filter application. A good agreement between the synthesis and experimental results is obtained. [ABSTRACT FROM AUTHOR]

In this article, a new class of high-order balanced dual-band high-temperature superconducting (HTS) bandpass filter (BPF) based on modified multimode stub-loaded resonators (SLRs) is presented. First, a detailed theoretical analysis of the multimode SLRs is provided to obtain insight into their frequency response characteristics under the differential-mode (DM) and common-mode (CM) operations. Specifically, the SLRs under the DM operation are designed to generate their second and third resonant modes in the two specified passbands, and they are sequentially cascaded by alternate $J$ and $K$ inverters for sharp rejection skirts. Following that, the working mechanism of the CM rejection of the resonator is investigated for achieving good CM suppression performance for this filter. Finally, for experimental validation, an eighth-order prototype filter is designed and fabricated on the MgO substrate with double-sided YBa2Cu3Oy (YBCO) thin films. Good agreement between measured and simulated results is obtained, and the filter is at the same time with low insertion loss, flexible frequency ratio, sharp rejection skirts, and good CM suppression level. [ABSTRACT FROM AUTHOR]

This article presents a novel waveguide loaded air slots’ resonator (WLASR) comprised of a surrounding metal box and inserted metal disk with etched air slots. The proposed WLASR exhibits a comparable unloaded quality factor ($Q_{\mathrm {u}}$) to the coaxial resonator and allows controlling the electric and magnetic couplings separately. These attractive characteristics were first deployed to design a four-pole inline electrical–magnetic mixed-coupling bandpass filter centered at 2.82 GHz with a fractional bandwidth of 6.73% and two controllable transmission zeros (TZs) located at 2.65 and 2.98 GHz, respectively. Direct metal 3-D printing technology was utilized to manufacture the prototype. The measured results were auspicious for the first demonstrated 3-D printed prototype. Hence, two six-pole filters with multiple TZs, more compact sizes, higher $Q_{\mathrm {u}}$ , and improved shape were designed. For experimental validation, one of them was printed with only two pieces. The measured results agreed well with the simulated ones. [ABSTRACT FROM AUTHOR]

This article presents a new, very compact silicon-micromachined two-port waveguide platform that features axial ports but allows to integrate waveguide devices, which is enabled by integrated broadband, back-to-back, stepped E- and H-plane bends. This approach of rotating the waveguide plane in two axes enables devices of complex geometry to be directly mounted between two waveguide flanges. A prototype transition operating at 220–320 GHz has experimentally demonstrated an insertion loss of less than 0.2 dB and a return loss of better than 18.5 dB throughout the entire WM-864 waveguide band, characterized by an integrated on-chip waveguide calibration kit. Two 5th-order waveguide filters have been designed as complex demonstrator devices and co-fabricated with the proposed axial integration platform, demonstrating a direct-coupled filter at 270 GHz and a cross-coupled filter at 300 GHz with three transmission zeros. The filters with 1.85% and 2% fractional bandwidths demonstrate a measured insertion loss of 1.92 and 1.50 dB, respectively, to which the two transitions combined add a total of 0.3 dB, and a return loss of 19 and 15 dB in the passbands, respectively. The unloaded $Q$ -factors for the resonator cavities of the two filters were extracted to 750 and 900, respectively. These results are so far unparalleled for filters of similar complexity in this frequency range. The sensitivity to fabrication tolerances is analyzed. [ABSTRACT FROM AUTHOR]

This article proposed a codesign of a power amplifier (PA) and a bandpass filter (BPF) named filtering PA using metal integrated and substrate integrated suspended line (MI-SISL). Two BPFs are designed separately as the input and output matching network to realize bandpass filtering response and good harmonic suppression at the same time. The BPF matching circuits are implemented using SISL-based dual-layer interdigital capacitors for miniaturization and SISL-based dual-layer spiral inductors for low loss. The suspended substrate of the PA is patterned according to the shape of the PA core circuit, which helps to reduce the circuit loss and improve efficiency. Moreover, the metal integrated technique is also proposed by replacing the top and bottom substrates of the conventional SISL structure with metal plates with good electrical and thermal conductivities for heat dissipation. A filtering PA on the MI-SISL platform at 0.9 GHz is fabricated and measured as a demonstration case. An ultrasmall size of $0.12\,\,\lambda _{g} \times 0.07\,\,\lambda _{g}$ is achieved, where $\lambda _{g}$ is the guide wavelength at 0.9 GHz. To the best of our knowledge, this is the most compact filtering PA on the board level to date. In addition, a harmonic suppression level of 40 dBc in up to the fifth-harmonic frequency is achieved. [ABSTRACT FROM AUTHOR]

This article presents a new compact multiplexer structure and its design approach, based on a novel resonant manifold. Different from typical transmission-line-based manifold multiplexers, the proposed manifold junction is formed of coupled resonators, so that the manifold also contributes to the filtering poles and selectivity in the passbands. The all-resonator configuration achieves a more compact footprint than the conventional manifold. This multiport filtering network can be represented by a single coupling matrix which can be obtained by using a robust optimization-based method. Tailored channel frequency allocation schemes are presented for the proposed multiplexers with a different number of channels. To validate the proposed concept, a triplexer and a five-channel multiplexer, both at X-band, are designed and tested using rectangular waveguide technology. Good agreements have been achieved between the measurement and simulations. Comparisons have been made with conventional multiplexers. [ABSTRACT FROM AUTHOR]

In this article, a novel and simple design procedure for multibit chipless radio frequency identification (RFID) tags is introduced, which is based on the characteristics of dual-mode loop resonator (DMLR) and open-loop resonator (OLR). By removing a small patch element from the symmetry axis of the DMLR, a simple geometrical transition to an OLR to obtain different frequency codes can be achieved. Since the DMLR and OLR provide dual- and single-resonant frequencies, three different frequency codes can be observed using one resonator. Thus, $3^{N}$ codes can be performed by coupling $N$ resonators to input/output (I/O) ports. Even–odd mode analysis over the coupling mechanism of a single resonator is provided to characterize the frequency responses of the proposed resonators. To prove the proposed approach, two multiresonator circuits having four resonators were fabricated and tested in a good agreement. In the test environment, two horn antennas and a vector network analyzer have been utilized as transmitter and reader. Next, in order to achieve size reduction, meandered resonators are utilized instead of conventional DMLR and OLR. Based on these resonators, two RFID tags, including wideband monopole antennas integrated into the seven-resonator circuits on the same board, were also fabricated and tested. [ABSTRACT FROM AUTHOR]

In this article, we present a comprehensive discussion of a new class of inline microwave filters with transmission zeros in the response, namely, the path filters. The main features of this filters class are highlighted, and an original (synthesis-based) design approach is presented, relying on the derivation of suitable characteristic polynomials. In addition to the classical generalized Chebyshev characteristic, two new characteristics are introduced (namely, the bounded Chebyshev and the reduced Chebyshev), which allows improving the flexibility in the requirements assignment of path filters. A new method for the synthesis of the low-pass prototype is also introduced, which overcomes the limitation in the classical synthesis based on the manipulation of the transversal prototype (whose synthesis may fail in case of path filters). Finally, the proposed approach for designing the class of considered filters has been validated by several examples that include the evaluation of the characteristic polynomials, the prototype synthesis, and the dimensioning of the physical structures in waveguide technology. [ABSTRACT FROM AUTHOR]

This article proposed an analytical synthesis approach of generalized strongly coupled resonator triplet (GSCRT) filters, which eliminates the need for negative couplings regardless of the location of transmission zeros (TZs). The principle of removing negative couplings in GSCRTs is explained from circuit level for the first time. In contrast with conventional filter function synthesis with all in-band reflection zeros (RZs), some RZs are preassigned out of the passband, which are named redundant resonant modes (or even modes). By regulating the frequency of the redundant modes, flexible synthesis solutions can be achieved and the previously published strongly coupled resonator triplet (SCRT) becomes a special case of this new generalized version. Besides, a novel physical interpretation of GSCRT from the basis of modes is presented, which shows the modes routing clearly within the coupling matrix framework. Due to the proposed theories, additional approximation and optimization can be avoided during the synthesis procedure. Due to the redundant mode, all couplings in the GSCRT are positive, thus easing the design and manufacturing process. Also, GSCRT brings compact and in-line configuration of filters. Moreover, due to the higher flexibility provided by redundant resonant modes, reasonable synthesis solutions can be selected depending on the specifications or other physical limitations, such as bandwidth and size. A group of synthesis examples with simulations are elucidated. The simulated results show a good agreement with the synthesis theories. The simulated and measured results of an anti-interference C-band eight-pole filter with four TZs below the passband further validate the effectiveness of the proposed theories. [ABSTRACT FROM AUTHOR]

A novel compact fan-shaped folded waveguide resonator (FSFWGR) is proposed in this article. The resonator can reduce the size by 87.5% compared with the waveguide cavity while retaining a high unloaded quality ($Q_{u}$) value (>1500). Due to these advantages, a high-quality (-Q) fourth-order bandpass filter centered at 6.0 GHz with a 6.67% bandwidth is designed first. Novel slots cross-coupling is presented to produce a pair of transmission zeros (TZs) while avoiding support structures that traditional metal probes often used. Then compact deformed folded antenna is designed to integrate with the filter. Compared with published works where the last resonator of the reference filter often needs to be detuned, all parameters here do not need to change by introducing a metal probe between the feeding point of the filter and antenna. Therefore, the integrating process is more straightforward. The integrated filter/antenna achieves the same filtering function as the reference filter and a high gain of 6.7 dBi. For experimental demonstration, metal 3-D printing technology was used to fabricate models with only two pieces that significantly reduced the assembly loss. The final measured results agreed well with simulated ones. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The design of dual-band filtering power divider based on dual-mode dielectric resonator (DR) is investigated. The external quality factors ($Q_{e}$) of two modes can be controlled independently, which makes the design procedure for arbitrary power split ratio simple and efficient. Meanwhile, the phase difference between the output ports can be adjusted to be in-phase or out-of-phase by altering the quadrants where the feeding probes are located. To improve the band-to-band isolation, the generation mechanism of the transmission zero between the two bands has been analyzed. Based on the analysis, the dual-band filtering power dividers with in-phase/out-of-phase outputs and equal/unequal power split ratio are proposed. All of them can be easily built by properly changing the location or the length of the feeding probes without increasing the circuit size. To verify the design concept, the above-mentioned prototypes are simulated. Some of them are implemented and measured. The measured results match well with the simulated responses, showing good performance, such as low loss and good band selectivity. [ABSTRACT FROM AUTHOR]

Miniaturized substrate integrated waveguide (SIW) diplexers with widely separated channel passbands using common half-mode dual-mode SIW junction-cavities are presented and developed for 5G sub-6 GHz communications and wireless systems. With this kind of half-mode dual-mode junction-cavities to eliminate the conventional T-junctions and also to produce one transmission pole in each channel passband, significant circuit miniaturization can be achieved for SIW diplexers. Additionally, by exploiting the first dual modes in the half-mode SIW junction-cavities, flexibly allocated fractional bandwidths (FBWs) for the two channels can be realized through FBW design graphs, large frequency ratios can also be accommodated, which is difficult to implement using the whole-mode SIW cavity scheme due to spurious problems. Two compact SIW diplexers with large frequency ratios are synthesized, designed, fabricated, and measured for demonstration, including a third-order direct-coupled prototype centered at 3.5 and 5.0 GHz with a frequency ratio of 1.43 based on the TE101/TE301 half-mode dual-mode junction-cavity and a fourth-order cross-coupled prototype centered at 3.5 and 5.5 GHz with a frequency ratio of 1.57 based on the TE101/TE102 half-mode dual-mode junction-cavity. [ABSTRACT FROM AUTHOR]

This work presents a new negative interresonator coupling structure that can be applied to circular waveguide resonators, such as substrate-integrated waveguide (SIW) resonators and metallic cavity resonators utilizing the TM011 mode. The negative coupling is obtained by using the fact that the magnetic field in the resonator circulates in the opposite directions in the upper and lower half parts. As it can be entirely enclosed within a filter structure, it does not produce radiation loss. For verifying the proposed negative coupling structure, we have designed and fabricated a fourth-order bandpsss filter having a pair of transmission zeros using SIW resonators. In addition, the proposed negative coupling structure can be applied to the metallic waveguide cavity resonator filters. Hence, we also have designed, fabricated, and measured a fourth-order metallic cavity bandpass filter having a pair of transmission zeros by adopting the proposed negative coupling structure. [ABSTRACT FROM AUTHOR]

This article demonstrates a 10.23–15.7-GHz varactor-tuned microstrip bandpass filter (BPF) with fully controllable center frequency, bandwidth, and transmission zeros. The concept of cascading a tunable low-pass unit together with a tunable high-pass unit is adopted to achieve the reconfigurable BPF. A comprehensive loss analysis is carried out to determine the preferable circuit topologies for low-pass filter (LPF) and high-pass filter (HPF), respectively. This not only overcomes the limitation of low quality factor of varactor at high frequency, but also contributes to a highly flexible reconfigurability of the BPF. A reconfigurable LPF using defected ground structure (DGS) and a quasi-elliptic HPF using microstrip technology are then proposed to demonstrate the improved BPF performance at high frequency. The measured results show that the designed BPF is capable of operating in five working modes, with a center frequency tuning range from 10.23 to 15.7 GHz and a fractional bandwidth (FBW) tuning range from 32.48% to 95.76%. The minimum in-band insertion loss reaches 2.55 dB. A steep cutoff characteristic and ultrawide stopband with high rejection level are successfully achieved and are well preserved during the whole tuning process, demonstrating the superiority of the designed BPF. [ABSTRACT FROM AUTHOR]

Patented single-slab self-temperature-compensated air-filled substrate integrated waveguide (AFSIW) cavities and quasi-elliptic filters are proposed and demonstrated in this article. Such components are of high interest for the design of thermally stable circuits and systems on substrate meeting the size, weight, power, and cost (SWaP-C) reduction expected for future systems, especially for space applications. First, a theoretical multiphysics model is presented for the synthesis of single-slab self-temperature-compensated AFSIW cavities. It is demonstrated from theory that by properly choosing the dielectric slab material and dimensions, perfect temperature compensation can be achieved. This extended technique allows the implementation of temperature compensation in any type of cavity, filter, and multiplexer configuration. For demonstration purposes, a single-slab self-temperature-compensated in-line folded seventh-order quasi-elliptic AFSIW filter operating at 12 GHz with a 250-MHz (2.27%) −3-dB bandwidth is designed and fabricated. It achieves a measured unload $Q$ -factor of 992 at ambient temperature (23 °C) and a measured resonance frequency thermal stability as low as 0.58 ppm/°C at −20 °C and 0.89 ppm/°C at 70 °C compared with its temperature-uncompensated counterpart achieving a thermal stability of −15.5 ppm/°C at −20 °C and −17.73 ppm/°C at 70 °C. Furthermore, it is demonstrated in this article that transmission zeros (TZs) created by cross-couplings in the in-line folded configuration of the self-temperature-compensated quasi-elliptic filter achieve a thermal stability of 2.55 ppm/°C and 2.3 ppm/°C (2.34 ppm/°C and 4.37 ppm/°C) at −20 °C (70 °C), in the lower and upper bands, respectively. [ABSTRACT FROM AUTHOR]

The generalized Chebyshev (pseudoelliptical) filter functions may not be optimal for certain applications due to physical constrains. With redundancy in filter polynomials, this article breaks down the “mental barrier” in conventional synthesis and directly synthesizes the “Chebyshev-like” functions to achieve desired filter performance under physical limitations. By exploiting the redundant restrictions on exact equal-ripple response, the reflection zeros can be moved to the complex plane, thus forming the “Chebyshev-like functions.” A class of inline filters with a block of second-order dangling resonators is synthesized, which can generate a pair of symmetric transmission zeros (Tzs) but is considered unrealizable using standard Chebyshev functions. With the proposed theories, an accurate and much improved filter prototype is directly derived from the “Chebyshev-like” polynomials, avoiding additional optimization in the design process. For verification, a group of examples with synthesis results are elucidated. Eight ten-pole filters with four Tzs are synthesized and fabricated. The simulated and measured results demonstrate the effectiveness and application of the proposed theories. [ABSTRACT FROM AUTHOR]

This work presents the complete synthesis procedure and circuit transformations needed to go from the characteristic polynomials to the transversal network formed by electroacoustic resonators. This filter topology allows the possibility to achieve any filter response in spite of limited electroacoustic coupling. This is crucial for obtaining very wideband filters with arbitrary positions of transmission zeros. Additional circuit transformations reveal the flexibility of such topology to control other important parameters of the acoustic filter such as the impedance of the resonators and their resonant frequencies, which directly affect the manufacturing process. The transversal topology for electroacoustic filters has been verified by the synthesis, design, fabrication, and measurement of a very wideband filter to meet the requirements of 5G N77 frequency band, centered at 3.75 GHz with 910 MHz of bandwidth. [ABSTRACT FROM AUTHOR]

A reconfigurable balanced dual-band bandpass filter (BPF) with constant differential-mode (DM) absolute bandwidth (ABW) and high selectivity is designed in this article. The proposed BPF is composed of a set of asymmetric short stub-loaded resonators (A-SSLRs) and balanced microstrip-to-slotline transition structures (MSTSs). Four varactor diodes are employed to achieve the reconfigurable characteristics with small insertion losses. By changing the coupling distances and selecting the positions of the loaded varactor diodes on A-SSLRs, the DM coupling coefficients $K_{i}$ and quality factors $Q_{i}$ of the proposed BPF can be easily controlled to keep a constant DM ABW. In the overall tuning range, a superior common-mode (CM) suppression is achieved intrinsically, and the DM responses are independent of the CM ones. Furthermore, five DM transmission zeros (TZs) are generated by the mixed-coupling and source–load coupling, which can improve the selectivity obviously. Compared with the published reconfigurable balanced BPFs, the proposed one has the advantages of constant DM ABW, smaller insertion losses, and more independent control on the two DM passbands. In order to validate the design strategies, a prototype circuit is designed, fabricated, and measured. The simulated and measured results are presented with a good agreement to verify the theory and design. [ABSTRACT FROM AUTHOR]

A general design procedure for inline waveguide filters with frequency-variant couplings (FVCs) is presented. The goal of the design is the introduction of transmission zeros (TZs) in the filter response. The filter design starts with the synthesis of a low-pass prototype with FVC, adopting a method that recently appeared in the literature. The low-pass prototype is then denormalized to obtain an equivalent circuit of the filter suitable for waveguide implementation. The last step of the design consists of the dimensioning of the physical structure of the filter, including the frequency-dependent couplings, by exploiting the equivalence with the synthesized equivalent circuit. The design procedure can be applied independently on the specific structure implementing the FVCs. In the case of narrowband filters, the resulting structure may not require any further refinements by means of numerical optimization based on full-wave modeling. A design example illustrates the details of the procedure implementation. The validation of the procedure is presented through the design and fabrication of a substrate integrated waveguide (SIW) filter with two coincident TZs. [ABSTRACT FROM AUTHOR]

In this article, a novel dual-mode monoblock dielectric resonator (MDR) for compact monoblock dielectric bandpass filters is presented and put into practical use for the first time. In the dual-mode resonator, two dissimilar TEM modes are generated by forming two perpendicular metalized blind holes in a silver-coated cuboid-shapes dielectric block. The resonator can maximize the space utilization with up to 50% volume reduction while maintaining a wide spurious-free frequency band. The coupling of the two modes in such a resonator can be effectively controlled by one or more partial-height metalized posts. A convenient input/output (I/O) coupling scheme for surface-mount applications is introduced, and versatile coupling arrangements for realizing various filter characteristics are presented. To demonstrate the proposed filter miniaturization technique, a six-pole inline bandpass filter with one pure imaginary transmission zero (TZ) in a 3.5-GHz band is electromagnetic (EM) designed, and a ten-pole symmetric bandpass filter with two pure imaginary TZs in a 2.6-GHz band of fifth-generation (5G) new radio (NR) frequency band is designed, prototyped, and tuned. Experimental results show that the proposed dual-mode MDR filter can obtain an acceptable RF performance for 5G applications with a significant volume reduction compared to existing single-mode solutions. [ABSTRACT FROM AUTHOR]

Dual-mode filtering switches using hybrid microstrip-cavity structures are proposed in this article. TE101 and TE011 modes of the cavity act as the resonant modes to obtain a dual-mode filtering response. The microstrip line integrating the control circuit is employed as the feeding network. In the ON-state, the diode of the control circuit is reverse biased. Based on the electromagnetic (EM) coupling theory, the two modes are excited to attain filtering performance, and the loss of the diode is significantly alleviated. When the diode is forward biased, the two modes are suppressed and isolation in the OFF-state is realized. Benefiting from the hybrid microstrip-cavity configuration, the coupling between source/load and resonator is flexible to control. Single-band and dual-band filtering switches are constructed. The dual-band filtering switch processes high selectivity passband response in the ON-state, the insertion losses are 0.4 dB, and three transmission zeros are distributed in the two bands. In the OFF-state, the isolations of the two bands are below 37 and 30.7 dB, respectively. Thus, the dual-mode filtering switches maintain high-Q features as the cavities. Moreover, they have multifunctionality and flexibility compared to the conventional cavity structures fed by probes and waveguides. [ABSTRACT FROM AUTHOR]

This article presents two miniaturized multimode waveguide bandpass filters (BPFs) based on epsilon-near-zero (ENZ) channel concept. By connecting parallel shorting stubs on the ENZ channel, multiresonance modes and one transmission zero (TZ) are introduced, broadening the bandwidth of the passband and upper stopband, respectively. The effect of arbitrary waveguide discontinuity on this resonant mechanism is analyzed, and the effective size reduction of the third-order filter is achieved with the folded ENZ channel. The design procedure and miniaturization of the proposed BPFs are presented in detail. The proposed second-order filter shows a 6.8% 3-dB fractional bandwidth (FBW) with a center frequency of 9.6 GHz, an insertion loss of 0.42 dB at 9.6 GHz, and an out-of-band rejection of 30 dB up to 12 GHz. Especially, with the filter thickness of 2.3 mm (0.245 μg at 32 GHz), the fabricated third-order filter shows a 6.2% 3-dB FBW with a center frequency of 32 GHz, an insertion loss of 0.6 dB at 32 GHz, and an out-of-band rejection of 30 dB up to 36 GHz. Simulated and experimental results are of good agreement. [ABSTRACT FROM AUTHOR]

This article presents an analytical design for millimeter-wave monolithic microwave integrated circuit (MMIC) switches. First, the field-effect-transistor-based (FET-based) resonator is proposed, whose resonant frequency and bandwidth can be controlled flexibly. As a result, the design flexibility of MMIC switches can be highly improved by using FET-based resonators. Second, a fast and flexible impedance matching technique is presented for MMIC switches based on the coupling matrix method. By selecting appropriate coupling parameters, the MMIC switches can be easily designed without designing multistage impedance matching circuits. In this way, low insertion loss (IL) and compact chip size can be accordingly obtained. For design validation, two single-pole single-throw (SPST) switch prototypes are designed and fabricated by using two or four FET-based resonators, respectively. Implemented in 100-nm GaN-on-Silicon HEMT process, high input 1-dB compression point (IP1 dB) of over 30 dBm is realized. The return losses are better than 15 dB, the minimum IL is only 1.1 dB at 30 GHz, and the isolations are higher than 45 dB. The above features indicate the validation of the proposed design method for MMIC switches. [ABSTRACT FROM AUTHOR]

In this article, a single-ended-to-balanced (SETB) broadband magic-T based on microstrip/slotline transitions and microstrip/slotline T-junctions is proposed. For the purpose of miniaturization and low interconnection loss, an SETB filtering magic-T with a sharp selectivity is also achieved by introducing a pair of microstrip stub-loaded resonators (SLRs) at the two output ports. For the first design, a U-type microstrip feed line integrated with a circular slot stub is adopted at each output port, which makes the differential-mode (DM) response independent of the common-mode (CM) one. Therefore, the design procedure is simplified significantly. Meanwhile, based on the microstrip/slotline T-junctions, in-phase and out-of-phase two-way equal power division networks can be realized, respectively. For the second design, a DM passband with two transmission zeros (TZs) is generated by introducing SLRs based on the first design. By employing the characteristic mode analysis method, it is easy to find that the center frequency and fractional bandwidth (FBW) can be controlled by changing the dimensions of SLR. To the best of the authors’ knowledge, the SETB magic-T with broadband or filtering performances may be the first ever reported. To verify the feasibility of the designs, two prototype circuits of the proposed magic-Ts are fabricated and measured. The simulation results are in good agreement with the measured ones. In these two designs, the functions of in-phase and out-of-phase power dividing, broadband or frequency selectivity, isolation between output ports, and CM suppression can be realized at the same time. [ABSTRACT FROM AUTHOR]

This article presents an analytical method for the synthesis of a low-pass prototype filter constituted by cascaded ${n}$ -tuplets, including resonant node (RN) and nonresonant node (NRN). The method extends the features of previously published solutions, limited to RNs only or that allow the inclusion of NRNs but in rigid configurations. The method begins with extracted-pole synthesis, arbitrarily defining the transmission zeros (TZs). Then, a filter topology transformation is applied by grouped node blocks to obtain the desired topology, with each ${n}$ -tuplet characterized by the assigned TZs. The procedure overcomes the ad hoc techniques currently available in the literature. Moreover, the novel filter transformation presented here (from extracted-pole to cross-coupled topology) provides an additional degree of freedom for filter synthesis, which can be further exploited by filter designers. Several examples are presented to validate the novel synthesis procedure. [ABSTRACT FROM AUTHOR]

In this article, we describe several alternative inline implementations for combline filters with transmission zeros (TZs) located both below and above the passband. A systematic design procedure is also presented using a lumped-element equivalent circuit. Following the design procedure that we propose, up to $N+1$ TZs can be implemented with an inline filter of order $N$. The generation of the TZs is explained physically in terms of mixed inductive and capacitive couplings. Several filters are also manufactured and tested. Good agreement between the measured and simulated results is demonstrated, thereby fully validating the new filter concepts. [ABSTRACT FROM AUTHOR]

This article reports on the RF design and practical development of RF co-designed bandpass filters/isolators (BPFIs). They are based on series-cascaded nonreciprocal resonant stages, microwave resonators, and multiresonant cells. The nonreciprocal stages are shaped by an in-parallel cascaded transistor-based path and a transmission line (TL) that result in a zero-phase resonance in the forward direction and high isolation (IS) in the reversed one. The operating principles of the nonreciprocal resonant stage are presented through circuit analysis and coupled-resonator diagrams (CRDs) that facilitate their design with coupled-resonator-based filter design methods. Various CRD-synthesized examples of low- and high-order transfer functions with and without transmission zeros are shown. They demonstrate the applicability of the proposed method for alternative-type of filtering responses with and without gain in the forward direction and high levels of IS in the reversed one. For proof-of-concept demonstration purposes, five prototypes were designed, manufactured, and measured at $S$ -band. [ABSTRACT FROM AUTHOR]

In this article, we present a design method of high-selectivity filtering antennas based on a dual-probe feeding structure. The first stage of the filtering circuit consists of two or three coupled resonators which are then coupled to the radiating patch acting as the last resonator of the filter. Cross-coupling is used to obtain transmission zero, and thus the selectivity of the filtering antennas is improved. A cascaded triplet (CT) filtering patch antenna, which produces a radiation zero at the high-frequency edge of the passband, and a cascaded quadruplet (CQ) filtering patch antenna, which produces radiation zeros at both edges of the passband, are designed for verification. The process of designing the filtering patch antenna is given in detail. The designed CT filtering patch antenna has a bandwidth of 1.029–1.17 GHz and a radiation zero at 1.19 GHz. The designed CQ filtering patch antenna has a bandwidth of 1.029–1.17 GHz and two radiation zeros at 1.012 and 1.19 GHz, respectively. The peak gain of the CT filtering patch antenna is 9.35 dBi and that of the CQ filtering patch antenna is 9.16 dBi. A dual-polarized CQ filtering patch antenna is fabricated, showing a good performance in measurement. [ABSTRACT FROM AUTHOR]

This article presents a millimeter-wave (mmW) ultrawideband (UWB) bandpass filter (BPF) using micromachined rectangular microcoaxial line (Rμ CL). The tightly coupled stepped-impedance stub loaded resonator (TC-SISLR) exhibiting multiple-mode resonator (MMR) property, which has nine controllable transmission poles (TPs) and two controllable transmission zeros (TZs), is proposed to achieve the design goal. Nine controllable TPs of TC-SISLR can support a 10–50-GHz UWB passband, while two controllable TZs of TC-SISLR can provide a sharp rejection passband. Step-to-step design process is given to guide the 10–50-GHz UWB BPF design, and the vertical broadside coupling configuration and the defected ground structure (DGS) are introduced to strengthen the tight coupling. Moreover, the Rμ CL-to-ground–signal–ground (GSG) probe transition is also proposed and designed for on-chip measurement. As examples, the back-to-back GSG probe transition is fabricated, which has a low loss of 0.19 dB within 10–50-GHz passband, and a low loss of 0.6 dB up to 110 GHz. A 10–50-GHz UWB BPF with GSG probe transition is also fabricated, which has a minimum insertion loss of 0.44 dB within 10–50-GHz passband. [ABSTRACT FROM AUTHOR]

This work presents a class of micro-electro-mechanical system (MEMS)-driven radio frequency filters in the X-band. The X-band center frequencies are achieved by resorting to the third-order antisymmetric Lamb wave mode (A3) in a 650-nm-thick Z-cut lithium niobate thin film. A novel bandwidth (BW) widening technique based on using the self-inductance of the top interdigital transducers and bus lines is proposed to overcome the limitations set by the electromechanical coupling (${k} _{t}^{2}$) and satisfy the demands in miniaturization and wide BW. Four different designs of the filters are designed and fabricated to show the trade-off among BW, insertions loss (IL), out-of-band rejections, and footprint. Due to the spurious-free and high- ${Q}$ performance of the A3 lithium niobate resonators, the fabricated A3 lithium niobate filters have demonstrated small in-band ripples and sharp roll-offs. One of these fabricated has demonstrated a 3-dB BW of 190 MHz, an IL of 1.5 dB, and a compact footprint of 0.56 mm2. Another design is fabricated to demonstrate a 3-dB BW of 170 MHz, an IL of 2.5 dB, an out-of-band rejection of 28 dB, and a compact footprint of 1 mm2. [ABSTRACT FROM AUTHOR]

In this article, novel reconfigurable filtering single-pole-multithrow (SPMT) switches based on the evanescent-mode cavity (EVA) are proposed. The design incorporates varactors between two EVA resonators. By controlling the varactor capacitance value, the coupling coefficient between resonators can be set to be zero or desired value which allows the corresponding signal path to be in OFF-state with high isolation or in ON-state with bandpass filter response. Based on this technique, filtering SPMT switches can be achieved with reconfigurable frequencies. Moreover, each filtering-switch path shares two common resonators with others, which significantly reduce the filter size and make the proposed SPMT very compact. A filtering single-pole-double-throw (SPDT) switch and a filtering single-pole-5-throw (SP5T) switch were designed, fabricated, and measured to demonstrate the proposed technique. These filtering switches can be continuously tuned from 1.953–2.782 and 1.866–2.257 GHz, respectively, with isolation better than 47.2 and 36.1 dB, respectively. Good agreement is obtained between simulated and measured results. [ABSTRACT FROM AUTHOR]

This article demonstrates the design of microstrip line symmetrical all-band absorptive bandpass filters (BPFs) with arbitrarily prescribed wideband flat group delay (GD) characteristics. The proposed BPFs consist of coupled line sections with series resistor connected stubs (short-circuited quarter-wavelength or open-circuited half-wavelength stubs). Analytical design equations are derived to obtain circuit parameters for arbitrarily prescribed flat GD characteristics. A detailed analysis shows that the absorptive sections (series resistor connected stubs) not only eliminate out-of-band reflections, but also determine the passband GD flatness characteristics. The proposed absorptive BPFs with arbitrarily prescribed flat GD can be easily extended to a higher order by cascading the first-order symmetrical quasi-absorptive BPF. For experimental validation, first- and second-order quasi-absorptive BPFs were designed with arbitrarily prescribed GD of 1.75 and 3.40 ns, respectively, and measurements were taken at a center frequency of 3.5 GHz. The results of the measurement are in good agreement with the predictions obtained through theoretical methods and simulation. [ABSTRACT FROM AUTHOR]

An approach to design RF all-port-reflectionless quasi-elliptic-type single-/dual-band bandpass filters (BPFs) and diplexers with frequency-adaptive constant-absolute-bandwidth (ABW) response is reported. The tunable reflectionless BPF case is first addressed, where a balanced-circuit scheme composed of two identical wideband 3-dB quadrature couplers and two equal reflective-type BPF units with synchronous center-frequency agility is exploited. The input signal is split into the two BPF units with equal amplitude and 90° phase shift by the first coupler and then recombined at the output node of the second coupler after being processed by the balanced-circuit branches. In this manner, the signals reflected at the stopband regions of the BPF units are mutually canceled at the coupler input port and absorbed at the isolated port by its loading resistor. This allows a symmetrical low-reflection behavior to be attained in the overall circuit, whose filtering transfer function is the one determined by the BPF units. Furthermore, by using dual-band BPFs in the balanced-circuit branches, this concept is readily extended to frequency-tunable constant-ABW dual-band BPFs whose passbands can be separately switched OFF (i.e., absorptive switched-OFF states) as an added reconfiguration capability. For practical-demonstration purposes, starting from theoretical design guidelines followed by an optimization step, the third- and second-order microstrip prototypes of reflectionless frequency-controllable single- and dual-band BPFs with predefined constant ABW, respectively, are developed and tested. Afterward, inspired by the devised tunable absorptive BPF structure, an all-port-reflectionless diplexer with frequency-adjustable constant-ABW channels is shown. This controllable multiport-absorptive filtering device is also verified with the fabrication and testing of a microstrip prototype. All the constructed circuits employ varactor diodes for electronic tuning, showing close agreement between simulation and measurements. [ABSTRACT FROM AUTHOR]

This article presents a method to design new diplexer-integrated single-pole double-throw (SPDT) switches for composite time division duplexing (TDD) and frequency division duplexing (FDD) requirements. The common fractal stub-loaded resonator (F-SLR) is employed to provide four magnetic coupling paths to introduce the mixed electric and magnetic coupling (MEMC) for every channel. Together with the cross-couplings, the ON-state channel with high selectivity bandpass response can be achieved. As examples, a third-order diplexer-integrated SPDT switch and a fourth-order diplexer-integrated SPDT switch operating at 0.9/1.8 GHz are designed and fabricated. These two diplexer-integrated SPDT switches have three major operating modes, i.e., the 0.9-GHz bandpass filter (BPF)-integrated SPDT switch operating mode, the 1.8-GHz BPF-integrated SPDT switch operating mode, and the 0.9-/1.8-GHz diplexer-integrated SPDT switch operating mode. In the design, the loaded positions of p-i-n diodes on the uniform impedance resonators of 0.9-GHz channel are optimized to improve the OFF-state rejection bandwidth. Measured results also show that these two diplexer-integrated SPDT switches have low ON-state passband insertion loss, high port-to-port isolation, and compact circuit size. [ABSTRACT FROM AUTHOR]

This article discusses the synthesis of a new class of cross-coupled blocks incorporating frequency-variant couplings, that is, the frequency-variant blocks (FVBs), and their planar implementations based on an analytical approach. The proposed FVBs require fewer coupling elements with regard to traditional blocks, while can realize a quasi-canonical characteristic (i.e., realizing $k$ -1 transmission zeros for a block of $k$ th order). Derivation of the FVBs relies on a uniform synthesis process, where each transformation is mathematically simplified as a non-reciprocal matrix. An $N$ th-order quasi-canonical filter (possessing either symmetric or asymmetric TZs) can thereafter be synthesized by properly cascading the FVBs. Moreover, a unique planar technique is introduced to construct the quasi-canonical filters (with TZs above passband) using parallel-coupled microstrips, where a complete design procedure from synthesis to circuit parameters will be given. Validity of this work is eventually demonstrated by three illustrative examples, whose synthesis, simulation, and test results perform desired responses and are well-matched with each other. [ABSTRACT FROM AUTHOR]

A concurrent dual-band self-oscillating mixer (SOM), based on a ring-shaped stepped-impedance resonator, is proposed and analyzed in detail. Taking advantage of the ring even and odd resonances, the circuit can operate in concurrent dual quasi-periodic mode and injection-locked mode. In the second case, it behaves as a dual-band zero-intermediate-frequency (IF) mixer. Initially, an analytical study of the SOM behavior in the two modes is presented. Then a variety of accurate numerical methods are used for an in-depth investigation of the main aspects of its performance, including stability, conversion gain, linearity, and phase noise. The recently proposed contour-intersection technique and the outer-tier perturbation analysis are suitably adapted to the SOM case. A method is also presented to distinguish the parameter intervals leading to heterodyne and to zero-IF operation at both the lower and upper frequency bands. In the zero-IF SOM, the possible instantaneous unlocking in the presence of modulated input signals is investigated and avoided. The methods have been applied to a dual mixer at the frequencies 2.4 and 4.1 GHz. [ABSTRACT FROM AUTHOR]

This article proposes a new class of box-like substrate integrated waveguide (SIW) filters with wide stopband. First, a two-pole doublet bandpass filter with one finite-transmission zero (FTZ) is proposed using a new planar dual-mode rectangular SIW cavity, which is realized by TE201 and TE103 modes. TE102, TE202, and TE104 modes are suppressed intrinsically by arranging feeding lines at the horizontal centerline of the dual-mode SIW cavity. Then, a four-pole box-section filter with one FTZ is proposed using one dual-mode and two single-mode rectangular SIW cavities, and the two single-mode cavities have different width-length ratios to further improve the stopband performance. To further improve the cutoff skirt, a new design method for five-pole quasi-elliptic SIW filter is proposed, which is achieved by the four-pole filter loaded by one single-mode SIW cavity. Some design examples with the center frequency of 10 GHz are presented to verify the performance of the proposed dual-mode SIW filters and the design method. [ABSTRACT FROM AUTHOR]

In this article, two types of substrate-integrated defected ground structure (SIDGS) resonant cells with wide upper stopband and low radiation loss are presented for filter implementation. Such SIDGS resonant cells are composed of two dissimilar DGSs surrounded by the bottom ground and metal-vias, which cannot only introduce wide stopband with low radiation loss but also be flexible for integration. Based on the aforementioned SIDGS resonant cells, single- and dual-band bandpass filters (BPFs) are designed and fabricated. The single-band BPF centered at 2.40 GHz exhibits an ultrawide upper stopband up to 19.7 GHz with a rejection level of 31 dB, whereas the measured stopband total loss (i.e., including radiation, metal, and substrate loss) remains about 30% up to 19.3 GHz. The dual-band BPF operated at 2.10 and 3.78 GHz exhibits an ultrawide upper stopband up to 17.8 GHz with a rejection level of 23 dB, whereas the measured stopband total loss is less than 16% up to 11.4 GHz. [ABSTRACT FROM AUTHOR]

This article solved the inverse problem for trisection and quadruple bandpass filters (BPFs) with mixed cross coupling. It allows for a given placement of transmission zeros and known main coupling coefficients to determine mixed cross coupling $K = K_{m} + K_{e}$ , containing magnetic and electrical components. Based on the obtained solution, it was established that trisection BPF has ten different options for placing two transmission zeros on the complex plane $S = \sigma + j\Omega $. It is shown that the considered trisection and quadruplet BPFs can have a second-order transmission zero on the $j\Omega $ axis, which provides a deeper attenuation pole at insertion loss curve. With the help of the obtained inverse problem solution, some restrictions are established on the possible options for the placement of three transmission zeros of quadruplet BPF with mixed cross coupling $K_{14}$ : transmission zeros cannot be placed on the $\sigma $ -axis and two of the three transmission zeros on the $j\Omega $ -axis cannot be equidistantly relative to $S =0$. It is found that to obtain a flat group delay, the transmission zeros must be located in the S-plane at the corners of an isosceles triangle, the vertex of which lies on the $j\Omega $ axis, and the sides intersect the $\sigma $ -axis. In this case, in addition to the flat group delay, the insertion loss curve has an attenuation pole. Theoretical results are validated with two microstrip quasi-inline trisection BPFs and one stripline quadruplet BPF. [ABSTRACT FROM AUTHOR]

Compact ultra-wideband (UWB) bandpass filters based on the half-mode substrate integrated waveguide (SIW) with fully reconfigurable single and dual notched bands are proposed in this article. The lower stopband of the UWB bandpass filter is formed by the intrinsic cutoff frequency of the half-mode SIW, and transmission zeros in the upper stopband are created by utilizing three U-shaped slot resonators. Coupling microstrip resonators loaded by varactor diodes are placed along the open side of the half-mode SIW and microstrip feeding line to construct the in-band notches. By controlling the bias voltages and the size of resonators, fully reconfigurable single and dual notched bands can be obtained. Moreover, the frequency and bandwidth of each notched band can be controlled independently. To demonstrate the proposed technique, five and seven distributed coupling microstrip resonators are first applied to obtain the fully reconfigurable single notched band with the suppression level of 30 and 36 dB, respectively. Then, reconfigurable dual notched bands are demonstrated by changing the bias voltages using the same prototype with seven resonators. At last, nine resonant cells with two sets of different sizes are shown to design reconfigurable dual notched bands with frequency tuning range covering almost the full UWB passband. [ABSTRACT FROM AUTHOR]

This article presents different physical arrangements of filters using conical posts as the basic element of design. The proposed resonator is similar to that of the classical combline filter, however, in this case, a conical post (truncated cone) is exploited. A hollowed enclosure is positioned on top of a rectangular metallic cavity for housing the upper part of the conical resonating post. Considering that the post upper radius is smaller than the post lower radius, an enclosure having the same dimensions as the resonating post can be taken. This results in a housing that can also work as an additional resonator for another cavity. As a result, cavities can be piled in a vertical arrangement and the conical post of each cavity is also used for housing the post of the cavity below it. Mixed vertically and/or horizontally arrangements are also feasible. More compact structures can be achieved with the possibility to obtain different footprints while maintaining similar unloaded $Q$ -factors than conventional combline filters. The structure also has the advantage of having a wider spurious-free range than the classical combline filter configuration (cylindrical post inside a rectangular cavity). Enhanced responses with higher rejections are also possible thanks to the introduction of transmission zeros. Several different filter configurations are presented in this article: 3-order filter and 4-order filter. The design procedure is explained for both of them. A 4-order filter has been manufactured using additive manufacturing techniques and measured in order to validate the proposed structure. [ABSTRACT FROM AUTHOR]