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This paper presents the investigation and testing of novel substrate integrated waveguide (SIW) bandpass filters, based on air-filled cavity resonators. Removing the lateral parts of the dielectric substrate in SIW rectangular cavities enables the use of the TM120 and TM210 modes to implement a dual-mode resonator. In addition, the fundamental TM110cavity mode is adopted to implement a transmission zero. This paper demonstrates the complete control of the central operation frequency, the pass bandwidth, the spurious-free bandwidth region, and the location of the transmission zeros. After a comprehensive theoretical investigation, design rules are derived and applied to the implementation of a variety of dual-mode cavities, which have been manufactured and tested. Moreover, these basic structures have been used as building blocks to design higher order filters: simulations and measurements of four-pole filters are reported for validation. [ABSTRACT FROM AUTHOR]

This paper presents a novel method to widely tune high-quality factor evanescent-mode cavity resonators and filters using contactless tuners. The proposed method resolves multiple challenges facing reliable manufacturing and operation of these filters by avoiding the attachment of the flexible tuner to the cavity and replacing it with capacitive contactless tuners. This paper discusses the design consideration and tradeoffs as well as demonstrates the performance of the proposed concept using the substrate-integrated-waveguide technology. The resonators demonstrate more than octave tuning range (TR), quality factor up to 650, and power handling exceeding 100 W. In addition, the second-order bandpass filters are demonstrated, along with the long-term measurements, showing better than 1.17% frequency error after a 15-h stability test, and less than 1.47% frequency error after a 450 cycles repeatability test over an octave TR from 1.8 to 3.6 GHz. [ABSTRACT FROM AUTHOR]

This paper presents a new miniaturized ultra-wideband (UWB) multilayer balanced bandpass filter (BPF) using the liquid-crystal polymer technology. The design is based on the use of two baluns and uses the transversal signal-interference concept for the realization of differential-mode (DM) passband and common-mode (CM) stopband. The broadside coupled stripline structure is adopted in this paper to realize UWB performances and pure TEM mode. The fabricated prototype is self-packaged, light in weight, and cost effective with a small footprint. The demonstrated balanced BPF achieves the excellent DM and CM responses within the entire bandwidth, centered at 6.85 GHz. The simulation and measurement results show good agreement. [ABSTRACT FROM AUTHOR]

This paper presents a systematic design procedure of microstrip higher order tunable bandpass filters (BPFs) that can realize a constant absolute frequency bandwidth (ABW) and transmission zeros (TZs) with the minimum number of varactors and a single controlled dc-bias voltage. To this end, intrinsic frequency dependence of interresonator couplings is employed instead of external controls, but the design has faced a difficulty of realizing ideal frequency curves of coupling coefficients at all the coupling regions, including nonadjacent couplings. To overcome such a difficulty, this paper introduces a frequency-curve shape design approach considering upward- or downward-curved convex that is dependent on the types of interresonator coupling. As illustrative examples, fourth- and sixth-order tunable BPFs are proposed and designed, starting from a coupling-matrix synthesis. Their structural parameters can be efficiently designed with the proposed design method. The frequency agility with keeping a constant ABW, acceptable low insertion loss, a good in-band return loss level, and TZs is numerically and experimentally demonstrated for both fourth- and sixth-order tunable BPFs. [ABSTRACT FROM AUTHOR]

This paper presents a new class of cavity-based triple-band filters employing resonators that support three operation modes. The proposed triple-band filters do not require junctions and can achieve an equivalent Chebyshev performance with fewer cavities, thus significantly reducing the footprint when compared to traditional approaches. The filter facilitates a systematic design approach starting from the coupling-matrix generation to the overall response optimization. The concept is potentially applicable to most triple-mode resonators and is demonstrated in this paper using waveguide and dielectric resonators. The first design is an inline structure employing elliptical waveguide cavities, whereas the second design is a folded structure using rectangular waveguide cavities. The third design is implemented by dielectric loaded cavity resonators. All the proposed triple-band filter designs offer high Q and allow for independent control of each frequency and coupling parameter. To verify the concept, three triple-band filter examples with different realizations are designed, manufactured, and tested to allow comparisons. [ABSTRACT FROM AUTHOR]

In this paper, a novel filtering 180° hybrid coupler is proposed and used to design a $2 \times 4$ filtering Butler matrix. The filtering 180° hybrid coupler can provide power division and phase shift together with a second-order bandpass transfer function, and is based only on coupled resonators. The $2 \times 4$ filtering Butler matrix is easily realized by utilizing several hybrid couplers. The analytical synthesis procedures for the hybrid coupler and Butler matrix are presented in this paper. Finally, broadside and end-fire arrays are realized by using the Butler matrix. To illustrate the concept experimentally, a filtering 180° microstrip hybrid coupler and a $2 \times 4$ filtering microstrip Butler matrix are designed, fabricated, and measured. Simulation and measured results are found to be in good agreement with each other. [ABSTRACT FROM AUTHOR]

In this paper, a design methodology for miniaturized cavity filters in a slow wave substrate-integrated waveguide technology is presented. It is applied in the context of ladder coupling scheme with a slow wave effect produced by a periodic arrangement of blind via holes. By taking into account the intrinsic periodicity of the structure during the synthesis, this paper aims at achieving a better filter response in terms of bandwidth and return loss, as well as providing a systematic approach for the via holes arrangement. In addition, a segmentation technique combined with the use of polynomial fittings is introduced to provide a fast and accurate design. As an illustration, a bandpass filter of 1-GHz bandwidth centered at 11 GHz is designed, fabricated, and measured. A good agreement between simulations and measurements is obtained. [ABSTRACT FROM AUTHOR]

This paper proposes a new coupling matrix synthesis technique called the coupling matrix compression technique, which focuses on reducing the dimension of the $N+2$ coupling matrix. The presented coupling matrices can be compressed, and this technique can be used to design dual-mode resonators for high-isolation dual-band filters. One dual-mode resonator is represented as two closely coupled resonators when using this technique. The formulae for resonant frequencies and $\omega _{m}$ in dual-mode theory are also derived in this paper. A dual-mode embedded hairpin resonator is designed and analyzed using this technique, and a dual-band filter using the proposed resonators is designed and fabricated to validate the proposed technique. [ABSTRACT FROM AUTHOR]

The general coupling matrix representation of bandpass filter (BPF) circuits is a widely used technique that has simplified the analysis and optimization of complex microwave filters. In this paper, we demonstrate a novel application of the general coupling matrix for modeling wireless power-transfer (WPT) systems based on the BPF model of magnetically coupled resonators. Compared to other methods of WPT analysis, our model simplifies accommodation of complex loads and provides direct expressions for impedance matching (IM) in WPT systems. Using this tool, we achieve optimal IM for two resonator systems with a complex load, thus achieving the greatest possible power-transfer efficiency (PTE). Furthermore, our model reveals additional design constraints for optimizing PTE in coupled resonator systems exhibiting low quality factor and small interresonator coupling. Overall, this paper introduces a new, versatile framework for the analysis and optimization of coupled resonator WPT systems. Experimental results are presented, verifying the optimal IM design process. [ABSTRACT FROM PUBLISHER]

This paper presents a method for designing low-loss varactor-based tunable bandpass filters (BPFs) with constant absolute bandwidth (CABW) and wide tuning range based on coaxial resonators. The whole structure consists of a metallic cavity and a piece of printed circuit board (PCB). Varactors and their biasing components are mounted on the PCB, which are connected to the coaxial resonators to enable frequency tuning. The coupling control between coaxial resonators and the nonsynchronous tuning of the resonators are utilized to realize the CABW over a wide tuning range. Moreover, transmission zeros are generated near the passband, resulting in high skirt selectivity. For demonstration, a second-order tunable coaxial BPF with a wide tuning range from 698 to 960 MHz (31.5%) is designed and fabricated. Due to the use of high- $Q$ -factor coaxial resonators, the proposed tunable filter realizes low insertion losses ranging from 0.82 to 2.03 dB under a narrowband specification. The 1-dB bandwidth is measured as 35.3 ± 2.7 MHz, showing excellent CABW responses. Moreover, a third-order tunable coaxial BPF is also designed and implemented with enhanced skirt selectivity and out-of-band rejection. Comparison with other reported tunable BPFs is given, where the proposed designs show excellent performance of wide tuning range, CABW, high selectivity, and high tuning speed. [ABSTRACT FROM AUTHOR]

In this paper, a new transversal configuration is presented for waveguide bandpass filters (BPFs) on stacked rectangular $ H$ -plane waveguide cavities. Different from the conventional ladder BPFs, cavities constituting the proposed transversal BPFs are arrayed in a stacked manner. Odd- and even-mode resonators of transversal BPF are realized by TE101 and TE102 modes of rectangular $ H$ -plane waveguides, respectively. Using the configuration proposed herein, the realization of an $N$ th-order filter could be decomposed into realizations of $N$ first-order subfilters, and each subfilter could be realized independently, which is guaranteed by the unique array of resonators of transversal filters. Therefore, the complexity of realizing BPFs could be reduced, especially for filters with multiple transmission zeros (TZs). A class of transversal filters, including BPFs and dual-band filter, are synthesized, fabricated, and measured to verify the approach. [ABSTRACT FROM AUTHOR]

This paper investigates the fabrication accuracy and repeatability of micromachined quadruplet filters designed at a center frequency of 270 GHz with a 5-GHz bandwidth using a versatile multilayer chip platform which allows for axially arranged waveguide ports. A large number of narrowband silicon-micromachined filters arranged on multiple chips are investigated for fabrication imperfections, assembly misalignment, and fabrication yield, employing fabrication-prediction and different chip-to-chip self-alignment feature strategies. A numerical technique for characterization of the entire fabrication process of the filters through extracting the error statistics for coupling coefficients of a large number of different samples from separately assembled chips is proposed. A total of 47 test filters in effectively 15 different design variants have been fabricated in two fabrication runs, evaluated, and analyzed. The most critical sources of errors are determined. The expected accuracy of the entire filters fabrication process is demonstrated through the yield analysis based on the collected error statistics. [ABSTRACT FROM AUTHOR]

In this paper, a new scheme is proposed for a dual-band Wilkinson power divider (PD) without transmission line (TL) stubs (open- or short-circuited), or additional lumped elements (inductors and capacitors). In comparison with conventional PDs, only three additional TLs are employed. One is utilized to extend the input port, while the others are symmetrically distributed at the two branches. After applying even- and odd-mode analyses to the proposed structure, the exact closed-form design equations for a frequency ratio from 2.0 to 6.5 are derived, considering the largest realizable microstrip line impedance (150 $\Omega $). In addition, a new L-type TL with simple structure, compact size, and high design flexibility is proposed to replace the conventional TL to enhance its performance by introducing a transmission zero between the two passbands, resulting in good out-of-band rejection performance. Finally, two dual-band PDs centered at 1.0 and 3.5 GHz are designed and fabricated to verify the proposed design theory. One is based on the proposed structure, while the other incorporates a couple of L-type TLs to replace the conventional TLs. Good agreement with the simulated and measured results can be observed with a low in-band insertion loss and full ports matching. Moreover, the proposed PD also features a wide isolation band with a reference of 15-dB input port return loss. Moreover, the use of the L-type TL can introduce a good stopband between the two passbands, leading to an improved out-of-band rejection performance. [ABSTRACT FROM AUTHOR]

This paper presents the design and realization of a high data rate radio front-end module for point-to-point backhaul links at E-band. The design module consists of four vertically stacked unconnected metal layers without any galvanic and electrical contact requirements among the building blocks, by using gap waveguide technology. The module components are a high-gain array antenna, diplexer, and circuitry consisting of a transmitter (Tx) and a receiver (Rx) monolithic microwave integrated circuits (MMICs) on a carrier board, which is successfully integrated into one package with a novel architecture and a compact form. The diplexer consists of two direct-coupled cavity bandpass filters with channels at 71–76 GHz and 81–86 GHz with a measured return loss of 15 dB and an isolation greater than 50 dB. A wideband $16\times 16$ slot array antenna with a measured gain of more than 31 dBi is used to provide high directivity. The measured results show that the packaged transmitter provides a conversion gain of 22 and 20 dB at 76 and 86 GHz, respectively, with an output power of 14 and 16 dBm at 1-dB gain compression point, at the same frequencies. The packaged receiver shows an average conversion gain of 20 dB at 71–76-GHz and 24 dB at 81–86-GHz bands. A real-time wireless data transmission is successfully demonstrated with a data rate of 8 Gbit/s using 32-quadrature amplitude modulated signal over 1.8-GHz channel bandwidth with spectral efficiency of 4.44 bit/s/Hz. The proposed radio front end provides the advantages of low loss, high efficiency, compact integration, and a simple mechanical assembly, which makes it a suitable solution for small-cell backhaul links. [ABSTRACT FROM AUTHOR]

This paper characterizes the performance of a miniaturized folded ridged half-mode substrate integrated waveguide (SIW) and explores its use in quarter-mode bandpass filter (BPF) design. By folding the fundamental mode of a ridged half-mode SIW (RHMSIW) around the capacitive ridge structure, an additional 50% of miniaturization can be achieved over RHMSIW and up to 87.5% compared with the standard SIW. The monomode bandwidths of both folded ridged half-mode and ridged half-mode waveguide technologies are studied, yielding 1:4.28 or greater fundamental to the second-order mode ratios for all cases and approximately twice the monomode bandwidth compared with half-mode technology. Furthermore, the study of folded ridged half-mode and quarter-mode substrate integrated cavities is presented, yielding miniaturizations compared with standard substrate integrated cavities with respect to free-space lambda equal to 96.6% and 98.0%, respectively. The design of BPFs using folded ridged quarter-mode cavity is demonstrated, with two different prototypes measured to study the choice of substrate thickness with respect to filter performance. By increasing the substrate thickness while maintaining similar cavity miniaturization, an improvement of 26.1% in unloaded quality factor between the filters is realized experimentally. [ABSTRACT FROM AUTHOR]

In this paper, a systematic research campaign on eighth-mode substrate integrated waveguide (EMSIW) filters is reported. The sizes of these EMSIW resonant cavities are only one-eighth or one-sixteenth of a conventional SIW resonant cavity. Two different coupling topologies, i.e., electric coupling and magnetic coupling, are employed between two EMSIW resonant cavities, which are analyzed theoretically via the response of the structure and the relationship between the cavities. Both coupling topologies enable structural variations that possess advantages and flexibility of the second-order EMSIW bandpass filters (BPFs) designing. By utilizing these basic coupling mechanisms, multilayer board technology, and other practical techniques, a class of triple-order SIW BPFs with the merits of compact size and high selectivity are demonstrated. Specifically, several triple-order EMSIW BPFs composed of pure EMSIW cavities or comprised by combining EMSIW and quarter-mode SIW cavities are simulated, fabricated, and measured for verification. [ABSTRACT FROM AUTHOR]

A design methodology for a compact millimeter-wave on-chip bandpass filter (BPF) is presented in this paper. Unlike the previously published works in the literature, the presented method is based on quasi-lumped elements, which consists of a resonator with enhanced self-coupling and metal–insulator–metal capacitors. Thus, this approach provides inherently compact designs comparing with the conventional distributed elements-based ones. To fully understand the insight of the approach, simplified LC-equivalent circuit models are developed. To further demonstrate the feasibility of using this approach in practice, the resonator and two compact BPFs are designed using the presented models. All three designs are fabricated in a standard 0.13- $\mu \text{m}$ (Bi)-CMOS technology. The measured results show that the resonator can generate a notch at 47 GHz with the attenuation better than 28 dB due to the enhanced self-coupling. The chip size, excluding the pads, is only $0.096 \times 0.294$ mm2. In addition, using the resonator for BPF designs, the first BPF has one transmission zero at 58 GHz with a peak attenuation of 23 dB. The center frequency of this filter is 27 GHz with an insertion loss of 2.5 dB, while the return loss is better than 10 dB from 26 to 31 GHz. The second BPF has two transmission zeros, and a minimum insertion loss of 3.5 dB is found at 29 GHz, while the return loss is better than 10 dB from 26 GHz to 34 GHz. Also, more than 20-dB stopband attenuation is achieved from dc to 20.5 GHz and from 48 to 67 GHz. The chip sizes of these two BPFs, excluding the pads, are only $0.076\times 0.296$ mm2 and $0.096\times 0.296$ mm2, respectively. [ABSTRACT FROM AUTHOR]

The demand and relevance of efficient analog fault diagnosis methods for modern RF and microwave-integrated circuits increase with the growing need and complexity of analog and mixed-signal circuitry. The well-established digital fault diagnosis methods are insufficient for analog circuitry due to the intrinsic complexity in analog faults and their corresponding identification process. In this paper, we present an artificial neural network (ANN) modeling approach to efficiently emulate the injection of analog faults in RF circuits. The resulting metamodel is used for fault identification by applying an optimization-based process using a constrained parameter extraction formulation. A generalized neural modeling formulation to include auxiliary measurements in the circuit is proposed. This generalized formulation significantly increases the uniqueness of the faults identification process. The proposed methodology is illustrated by two faulty analog circuits: a CMOS RF voltage amplifier and a reconfigurable bandpass microstrip filter. [ABSTRACT FROM AUTHOR]

In this paper, an analytic design method for a frequency- and bandwidth-tunable absorptive bandpass filter is introduced. The presented design method relies on the transformation from the normalized absorptive low-pass filter prototype to a bandpass filter structure with parallel resonators and admittance inverters. The inverter values for producing a predefined frequency response are given in closed-form expressions so that a filter design needs no heuristic approach and can be carried out in an analytic manner. This work also presents a new tunable stub resonator structure capable of changing not only its resonant frequency but also the slope parameter. As the slope parameter can be adjusted, it is allowed to use static inverters in designing a filter whose bandwidth can be tuned. For demonstrating our design method for a reconfigurable absorptive bandpass filter, a third-order filter having the target to have the frequency tuning range from 1.35 to 1.65 GHz and the bandwidth tuning range from 5% to 10% has been designed and measured. [ABSTRACT FROM AUTHOR]

A new synthesis for the design of a bandpass filter (BPF) with arbitrary real and/or complex termination impedances is presented in this paper. The proposed BPF can be terminated with arbitrary complex termination impedances simply by adding series transmission lines (TLs) to the first and the last coupled resonators. For design freedom, the characteristic impedances of the additional TLs can be chosen arbitrarily. Moreover, the proposed filter can be designed in either even- or odd-ordered stages using a $\lambda $ /4 stepped impedance resonator (SIR). For the validation of the proposed filter, two- and three-stage $\lambda $ /4 SIR BPFs with $Z_{S} = 30 + j10\,\,\Omega $ , $Z_{L} = 50 \,\,\Omega $ , and $Z_{S} = 50 \,\,\Omega $ , $Z_{L} = 100 + j100 \,\,\Omega $ , respectively, were designed and fabricated at the center frequency ($f_{0}$) of 2.6 GHz. The measured results are consistent with the simulation results. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate, for the first time, an isolating bandpass filter with low-loss forward transmission and high reverse isolation by modulating its constituent resonators. To understand the operating principle behind the device, we develop a spectral domain analysis method and show that the same-frequency nonreciprocity is a result of the nonreciprocal frequency conversion to the intermodulation (IM) frequencies by the time-varying resonators. With appropriate modulation frequency, modulation depth, and phase delay, the signal power at the IM frequencies is converted back to the RF frequency and adds up constructively to form a low-loss forward passband, whereas they add up destructively in the reverse direction to create the isolation. To validate the theory, a lumped-element three-pole 0.04-dB ripple isolating filter with a center frequency of 200 MHz and a ripple bandwidth of 30 MHz is designed, simulated, and measured. When modulated with a sinusoidal frequency of 30 MHz, a modulation index of 0.25, and an incremental phase difference of 45°, the filter achieves a forward insertion loss of 1.5 dB and a reverse isolation of 20 dB. The measured nonmodulated and modulated results agree very well with the simulations. Such nonreciprocal filters may find applications in wideband simultaneous transmit and receive radio front ends. [ABSTRACT FROM AUTHOR]

A new series of multifunctional reconfigurable bandpass filters (BPFs) and their analytical synthesis design are presented in this paper. They feature frequency-adaptive characteristics in both their passbands and notched bands, as well as switchable-notched-band capability. Their basic filter architecture is discussed and modeled in a progressive manner. The initial circuit consists of a second-order Chebyshev-type filter to which two stubs are added to increase filtering selectivity through transmission-zero (TZ) creation. Subsequently, to produce an embedded notched band, stubs acting as band-stop structures are also inserted at the input–output terminals. The multifunctional properties of this filter scheme are analyzed attending to the resonance frequencies of the introduced stubs. When these resonance frequencies are tuned while keeping a constant spacing between them, a center-frequency reconfiguration with constant bandwidth is achieved; when the resonance frequency of the notched band is shifted to the TZ positions, the notched band is switched-off. The described filter synthesis methodology is based on the constant and frequency-mapping technique, where the dual-mapping functions are controlled to obtain reconfigurable/switchable properties from a low-pass prototype. For experimental-validation purposes, two reconfigurable lumped-element filter prototypes using the proposed synthesis technique are designed, manufactured, and tested. One exhibits a simple reconfigurable passband, whereas the other one adds a controllable/switchable embedded notched band that results in a quasi-elliptic-type dual-passband response. When compared to some related prior-art designs, these filters have the advantages of explicit design process, quantified analysis, and multifunctional responses. [ABSTRACT FROM AUTHOR]

In this paper, a new self-packaged wideband balun with the high selective bandpass filtering response and impedance transformation characteristic is presented. By virtue of circuit transformations, corresponding two-port equivalent circuit with asymmetrical port impedances is put forward. The analysis results of this two-port network exhibit that the proposed new design is able to achieve a quasi-elliptic equal-ripple response with three transmission poles and four transmission zeros. In addition, the frequency response can be directly determined according to the desired design specifications of impedance transformation ratio k, ripple fractional bandwidth (FBW), and in-band return loss RL. For validation purposes, a demonstrated wideband balun bandpass filter (BPF) working at 2.0 GHz with 50–100- $\Omega $ impedance transformation has been implemented by utilizing the multilayer liquid-crystal polymer (LCP) bonded printed circuit board (PCB) lamination technology. Theoretical, simulated, and measured results are recorded in good agreement, well verifying the design method. [ABSTRACT FROM AUTHOR]

In this paper, a bandpass frequency selective surface (FSS) based on substrate-integrated waveguide (SIW) technology is presented for airborne radome application. The proposed FSS element consists of a tapered cross-slot on either side of the substrate surrounded by metallic vias. The structure selectively allows the impinging electromagnetic (EM) wave through it in the specified frequency band. The element shows very stable frequency response for oblique incidence and the sharp roll-off performance characteristics at the edges of operating region in the rejection band. The −10 dB relative bandwidth (BW) of the element is 12.8% from 9.5 to 10.8 GHz with a very good insertion loss (IL) of 0.1 dB. Furthermore, the spectacular advantages of the FSS element based on SIW have been used for modeling the airborne radome application. The proposed radome structure is operating at 10 GHz with a relative 10 dB BW of 25.0% (8.20–10.6 GHz) with maximum (IL) better than 0.1 dB in its passband. The behavior of the FSS radome has been analyzed at different conformal sectors and thickness is optimized for optimal performance. The key parameters of radome such as transmission performance, radiation efficiency, and radar cross section (RCS) have been studied. Experimental verifications are carried out to prove the validity of the estimated results. The results show promising performance of the proposed FSS based on SIW technology for airborne radome application. [ABSTRACT FROM AUTHOR]

This paper reports on the RF design of fully reconfigurable quasi-elliptic-type single- and multi-band microwave bandpass filters (BPFs) and duplexers. They are based on series-cascaded resonators and multi-resonant/multi-band cells that produce frequency-reconfigurable poles and transmission zeros (TZs). By appropriately tuning the resonant frequencies of their constituent resonators, multiple levels of transfer-function adaptivity can be obtained. These capabilities include center-frequency and bandwidth tuning, as well as intrinsic RF switching-off (i.e., without the use of RF switches). As opposed to conventional BPF tuning mechanisms, the aforementioned reconfiguration characteristics avoid the use of tunable couplings. The basic operating principles of the engineered resonator-cascade BPF concept are presented through a three-pole/two-TZ example. Afterward, methods for expanding the proposed filter design approach to higher order, multi-band, and multi-port (e.g., duplexers) realizations are explained through coupling-matrix-based analysis. For practical-demonstration purposes, four L-band tunable microstrip prototypes were developed and measured. They include: 1) a five-pole/four-TZ BPF; 2) a five-pole/two-TZ BPF; 3) a dual-band BPF with two three-pole/two-TZ passbands; and 4) an RF duplexer. [ABSTRACT FROM AUTHOR]

In this paper, we propose a compact topology for high-frequency bandpass filters using coaxial cavities embedded in a bed of nails, including a complete design procedure combining equivalent circuit models and full-wave simulators. The resonance generated around a shortened cylindrical nail of the bed hosting structure is used as the basic element of the proposed filter, which is fed through groove gap waveguides. For design purposes, an equivalent circuit model of the considered resonance is first obtained, and then the coupling levels between resonators are recovered with the distance between adjacent shortened nails. In order to validate the proposed structure and its design procedure, a filter prototype with a bandpass response (centered at 30 GHz and with relative bandwidth of 1.7%) has been designed, manufactured, and measured. Good experimental results, in terms of insertion losses (with a minimum value of 1.6 dB) and return losses (greater than 16.6 dB in the whole passband), have been achieved. [ABSTRACT FROM AUTHOR]

In this paper, a concept of partial electric/magnetic walls is proposed and applied to design a class of balanced bandpass filters on (modified) doublet topology with differential-mode (DM) transmission zeros (TZs). It is first demonstrated that the partial electric/magnetic walls are equivalently attained along certain parts of these balanced filters. Using this concept, a dual-mode ring balanced bandpass filter on modified doublet topology with two DM TZs is designed, fabricated, and measured. Furthermore, another balanced bandpass filter on doublet topology with three DM TZs and wide upper stopband is designed and implemented, so as to exhibit the attractive application of the proposed concept. Good agreements between the expected response, simulated results, and measure results are observed. [ABSTRACT FROM AUTHOR]

This paper presents the proposal and design of multistate diplexer on uniform- and stepped stub-loaded resonators, that is, a three-state diplexer (TSD) and a four-state diplexer (FSD). The two proposed diplexers have an attractive application in a frequency-hopping system with miniaturization in their overall size. Next, three and four sets of second-order bandpass filters are applied to form the three and four filtering channels, respectively. These filtering channels are then used to comprise these multiple-state diplexers. In this way, compact size and high-frequency selectivity have been well achieved. For the purpose of demonstration, the prototypes of TSD and FSD are fabricated and measured. The measured results are found in good agreement with the simulated results. [ABSTRACT FROM AUTHOR]

This contribution establishes the relationship between the complementary condition and the singly terminated (ST) networks. The general theory is first presented in the form of bandpass/bandstop complementary diplexers. Furthermore, the well-known limitation of a complementary diplexer, i.e., using maximally flat prototypes, is overcome. From the first principle, it is rigorously proven that predefined complementary features can be obtained by connecting two channel filters in an ST prototype of the same filter function but exchanging the transfer and reflection part. This paper leads to a general design procedure for bandpass and bandstop complementary diplexers using arbitrary functions. A complementary diplexer design example with full electromagnetic simulated results and measured results verified the accuracy of the proposed theory. Finally, the theory is further extended to N-ports networks. A new “partially-complementary” condition is proposed. The new theory leads to an extremely simple and “direct approach” to estimate the performance of contiguous multiplexers without any optimization. [ABSTRACT FROM AUTHOR]

In this paper, the challenges of designing distributed amplifiers at frequencies close to $f_{\text {max}}$ of transistors are tackled. By using bandpass transmission lines, the maximum operation frequency of the distributed amplifier is increased compared to low-pass transmission lines. Moreover, a novel gain-boosted cascode structure with an internal feedback is employed to cancel a part of the loss in the input line and boost the bandwidth and the highest operation frequency of the amplifier. The proposed amplifier operates up to frequencies as high as 0.67 $f_{\text {max}}$ of the transistors. Two proof-of-concept prototypes are fabricated in a 0.13- $\mu \text{m}$ SiGe process with $f_{\text {max}}$ of 210 GHz. The first prototype shows an average gain of 14.4 dB from 52 to 142 GHz (bandwidth of 90 GHz at the center frequency of 97 GHz) while the second one achieves an average gain of 18.6 dB from 48 to 135 GHz (bandwidth of 87 GHz at the center frequency of 91.5 GHz). [ABSTRACT FROM AUTHOR]

In this paper, a novel triplexer using the substrate-integrated suspended line technology is proposed. Two matching circuits are designed to connect the three filter channels for the triplexer construction. The used bandpass filters are composed of a low-pass filter and a high-pass filter, and they have advantages of high controllability and low insertion loss. For the proposed triplexer, two adjacent filter channels compose a duplexer. Based on the design consideration, the triplexer can be extended to a multiplexer. The proposed design is further improved by using a patterned suspended substrate for loss reduction. The triplexer achieves good isolations among the three ports and low insertion losses of 1.4/1.3/0.8 dB for the three channels of 0.8, 2.5, and 5.8 GHz, respectively. [ABSTRACT FROM AUTHOR]

In this paper, the design of a cavity crossover with three intersecting channels is presented. The three fundamental modes of a cavity resonator, namely, TE011, TE101, and TM110 modes, are adopted to resonate at each of three channels, respectively. Due to the modal orthogonality of these fundamental modes, high isolation among three channels can be achieved. Two kinds of crossovers, for narrowband and broadband applications, are presented. For the narrowband case, the proposed crossover resonates at 2.91 GHz with the fractional bandwidth of 1.4%. For the broadband case, the proposed crossover resonates at 3 GHz with the fractional bandwidth of 24%. The isolations of both designs reach more than 50 dB. For a proof of concept, the broadband example of the cavity crossover structures is fabricated and measured. A good agreement between the simulated and the measured results verifies the accuracy of the proposed design methodology. [ABSTRACT FROM AUTHOR]

In this paper, we propose a method for designing a dual-channel balanced filter and a dual-channel balun filter based on the quad-mode dielectric resonator (DR) for the first time. By sharing one common quad-mode DR, two balanced filters or two balun filters are integrated as one single-cavity configuration, featuring compact size and high integration. A cylindrical DR with two short ends is investigated to construct the quad-mode DR. By properly arranging input and output feeding probes, two modes of the DR are only excited by the feeding probes of one channel and the other two modes are excited by that of the other channel. Accordingly, signals cannot be transmitted between the two channels, resulting in high isolation. Moreover, the required out-of-phase characteristics of the balanced and balun filters can be obtained by the inherent electromagnetic field properties of the DR without adding additional circuits, featuring a simple structure. For demonstration, a dual-channel balanced filter and a dual-channel balun filter are designed and fabricated, showing excellent balanced or balun filter performance of each channel as well as high isolation between the two channels. As compared to the other reported DR balanced and balun filters, the proposed designs exhibit a significant size reduction, which are attractive in wireless systems. [ABSTRACT FROM AUTHOR]

A new microstrip balanced-to-unbalanced power divider with dual bandpass filtering response is presented in this paper. The generalized mixed-mode scattering matrix for the filtering balanced-to-unbalanced power divider is derived to guide the design. Differential-mode (DM) dual-band filtering response is analyzed under the odd-mode excitation to show that the frequency ratio ($k = f_{2}/f_{1}$) and bandwidth of these two passbands could be flexibly controlled. For the even-mode excitation, two intrinsic transmission zeros generated by additional open-ended stub are introduced, aiming to achieve the enhancement of in-band common-mode (CM) suppression. In addition, good in-band isolation and port matching are realized by introducing a grounded resistor. To validate the design concept, two prototypes centering at 1 and 2.5 GHz ($k = 2.5$), as well as 0.76 and 2.75 GHz ($k = 3.62$) are implemented following the given design procedure. Tested results of the two fabricated circuits, well agreeing with the simulated ones, exhibit high performances in terms of DM response, in-band CM suppression, port-to-port isolation, and port matching. [ABSTRACT FROM AUTHOR]

In this paper, a novel tunable filter with reconfigurable single-to-balanced (SE-BAL), power dividing (PD), and single-to-single-ended (SE) filtering functionalities is proposed, for the first time, without any additional switching circuit. The proposed filter only consists of three stepped-impedance resonators, which are loaded and connected by varactors in between. With a very simple and compact configuration, the proposed filter achieves multiple reconfigurable functionalities between SE-BAL filtering, PD filtering, and conventional SE bandpass filtering. Moreover, the reconfigurable SE-BAL, PD, and SE bandpass filter can not only offer continuously frequency tuning over a wide tuning range but also have independent bandwidth tuning and maintain good phase/amplitude balance in each tuning state. The proposed filter is designed and fabricated with the microstrip line technology; the measured results show that the SE-BAL, PD, and SE filtering functions can be continuously tuned from 1.02 to 1.32, 0.96 to 1.27, and 1 to 1.28 GHz, with 1-dB bandwidth tuning from 30 to 60, 30 to 110, and 70 to 130 MHz, respectively. The amplitude and phase imbalances of the SE-BAL and PD bandpass filter are less than 0.4 dB, 1.3° and 0.5 dB, 1.5°, respectively, in the passband of each tuning state. Good agreement was obtained between simulated and measured results. [ABSTRACT FROM AUTHOR]

In this paper, a fourth-order dual-band high-temperature superconducting (HTS) differential bandpass filter (BPF) with controllable midband frequencies and bandwidths is developed by using modified multimode square ring loaded resonators (SRLRs). The differential-mode (DM) bisection and common-mode (CM) bisection of the modified SRLR are analyzed by using the even- and odd-mode method twice, and the corresponding resonant properties are obtained. With a proper design of the modified SRLR, two lowest DM resonances are chosen to construct two DM passbands with high in-band CM suppression. The DM resonances can be adjusted flexibly after adding tunable patches to the square ring. Meanwhile, the internal coupling between two modified SRLRs can be controlled separately by using two geometrical parameters. Consequently, the proposed dual-band BPF can achieve independent control of both the midband frequencies and the bandwidths of its two passbands. Moreover, a wideband CM suppression is realized by a combined use of an appropriately selected feeding structure and a frequency discrepancy technique. A fourth-order dual-band differential BPF is designed with two passbands operating at 2.325 and 4.900 GHz, respectively. The filter is fabricated using HTS YBCO thin films on a MgO substrate. Good agreement between the simulated and measured frequency responses are observed, verifying well the proposed structure and design method. [ABSTRACT FROM AUTHOR]

This paper presents a type of bandpass filter (BPF)-integrated single-pole double-throw (SPDT) switch using common shorted stepped-impedance resonator technique. The BPF-integrated ON-state with reverse-biased p-i-n diodes can be synthesized by classical coupled-resonator filter design theory, and the OFF-state with forward-biased p-i-n diodes is detuned which exhibits high OFF-state suppression (OSS). The transmission zeros due to the direct coupled feed, the cross-couplings and the cancelling effect of mixed electric and magnetic coupling are produced to improve the ON-state passband selectivity significantly. As examples, the third-order/fourth-order BPF-integrated SPDT switches at center frequency $f_{0} = 1$ GHz with ripple bandwidth of 110/100 MHz are designed, which show low insertion losses of 1.08/1.58 dB, sharp skirts, high roll-off rate of sidebands, high OSSs of 40.9/40.1 dB as well as high port-to-port isolation of 57.4/47.8 dB. Then, the switched 50- $\Omega $ matching circuits are in series with two output ports of above BPF-integrated SPDT switches, resulting in the absorptive OFF-state. The fabricated third-order/fourth-order BPF-integrated SPDT switches with absorptive OFF-state have the matched return losses of 30/28.2 dB, and their OSSs and port-to-port isolations are also improved. [ABSTRACT FROM AUTHOR]

In this paper, an adaptive synthesis using the particle swarm optimization (PSO) for implementations of resonator-coupled filters is proposed. The coupling matrix of in-band filtering response is achieved and optimized by the PSO-based synthesis. Meanwhile, the stopband coupling matrix of the filter is predicted based on the combination of extra resonant nodes representing stopband spurious and parasitic effect of input/output ports. In addition, an enhanced accurate prediction of filter performance is calculated by the proposed approach, considering the practical fabrication tolerance on filter design. To verify principles mentioned earlier, various resonator-coupled filters are implemented. The calculation, EM-simulation, and measurement of filters show good agreements in both passband and stopband. [ABSTRACT FROM AUTHOR]

This paper presents a novel class of multilayered ultra-broadband bandpass filters based on high-impedance slotline resonators. The equivalent circuit and electrical properties of the proposed high-impedance slotline resonator realized on a low-permittivity substrate are first discussed. To largely remove the radiation losses of the devised high-impedance slotline resonator, two identical shunt-connected folded low-impedance slotline sections are utilized to replace one short-circuited high-impedance slotline section. Through the synthesis design of an ultra-broadband microstrip-to-microstrip vertical transition, the effective impedance of the conceived shunt-connected low-impedance slotline sections is confirmed and the impedance variations of this proposed high-impedance slotline resonator are quantitatively determined by the means of the extracted turns ratios of the employed transformers. To further demonstrate the effectiveness of the engineered slotline resonator in terms of radiation reduction and verify its equivalent circuit model, a new type of multilayered high-order bandpass filters with ultra-broadband Chebyshev equiripple filtering response is theoretically presented. Furthermore, for experimental validation purposes, a 2-GHz two-layered bandpass filter prototype is fabricated and measured. It exhibits a seven-pole ultra-broadband filtering response, high in-band amplitude/group-delay flatness, highly reduced radiation losses, and sharp-rejection upper-stopband characteristics. [ABSTRACT FROM AUTHOR]

This paper presents a novel quarter-mode packaged bandpass filter with four transmission zeros (TZs) in substrate-integrated waveguide (SIW) technology. The filter has an insertion loss of 2.85 dB at 30 GHz and fractional bandwidth of 20%. The design methodology of the TZs was first demonstrated to increase the attenuations of the filter stopband. The quarter-mode SIW resonators were analyzed with high-mode suppression by properly setting their feed points. The packaged filter was consistent between the simulation and measurement results and offers the advantages of compact configuration and easy integration with other devices. Mode suppressions of up to 110 GHz were also provided. The proposed concept is attractive for designing miniaturized and high-performance-packaged filters for 3-D millimeter-wave transceiver systems. [ABSTRACT FROM AUTHOR]

A new balanced-to-balanced filtering power divider based on the square patch resonator is proposed in this paper. Originating from the field distributions of degenerate modes on the square patch resonator, a prototype of the balanced-to-balanced power divider is successfully developed with only one square patch resonator. Its working principle can be fully explained by the field distribution of the relevant resonant modes. Afterwards, to achieve the sharp skirt selectivity outside the passband, a compact second-order circuit is constructed based on the original prototype. With this new structure, both good bandpass filtering response and power division characteristics are well obtained. Moreover, to suppress the differential-mode (DM) harmonic response in the upper stopband, an improved structure is built up by modifying the feeding scheme. Ultimately, to validate the design concept, both the two types of second-order balanced-to-balanced filtering power dividers operating at 1.8 GHz are designed and fabricated. The measured results of the two fabricated circuits well agree with the simulated ones, manifest equal power division, high performances in terms of DM response, in-band common-mode (CM) suppression, port-to-port isolation, and expected harmonic suppression. [ABSTRACT FROM AUTHOR]

In this paper, we explore theoretically and experimentally broadband spatiotemporally modulated magnetless circulators realized by combining three-port nonreciprocal junctions with three identical bandpass filters. We develop a rigorous theory for the proposed circuit, which allows optimizing its design and to derive a global bound on the maximum possible bandwidth (BW). We verify our theory with simulations and measurements of a printed circuit board prototype based on a differential wye junction and second-order Chebyshev bandpass filters, resulting in a measured fractional BW of 13.9% at a center frequency of 1 GHz. [ABSTRACT FROM AUTHOR]

Four novel architectures of wideband five-port single-ended-to-balanced power dividers with broadband common-mode suppression are proposed in this paper. The bandpass filtering capability with/without close-to-passband transmission zeros are also incorporated in some of them to obtain multifunctional single-ended-to-differential-mode filtering/power division operation. Two ideal 180° phase inverters are used to replace the conventional half-wavelength transmission line-based inverters in the basic power divider structure so that wideband common-mode rejection and a smaller circuit size are achieved. Quarter-wavelength transmission lines and coupled-line sections are exploited to simultaneously realize the bandpass filtering and wideband power division functionalities. Each five-port power divider exhibits broadband 3-dB single-ended-to-balanced power division, high in-band input–output power-matching levels, high in-band power isolation levels between the two balanced outputs and wideband common-mode rejection. The theoretical analysis, parametric study, and design procedure for the conceived single-ended-to-differential-mode power dividers are provided. Furthermore, for experimental and demonstration purposes, four microstrip prototypes designed at 1 GHz are manufactured and characterized. [ABSTRACT FROM AUTHOR]

This paper presents a complete design procedure for in-line evanescent mode filters using an innovative pseudoelliptic structure, namely, the strongly coupled resonator triplet (SCRT). The SCRT can be seen as the combination of a strongly coupled resonator pair (SCRP) and a third resonator next to the pair. Compared to the SCRP, which at the passband frequencies generates a pole and a transmission zero, the new SCRT is capable of generating an additional passband pole, thus leading to the design of more compact in-line filters. Similar to the SCRP, the basic structure for filter design purposes is obtained by embedding the SCRT between two outer resonators. The coupling mechanisms and functional properties of this basic structure are explained. A suitable circuit model for the SCRT is introduced, thus allowing the synthesis of a low-pass prototype as well as the evaluation of the coupling coefficients and resonating frequencies to assign to the physical filtering structure. The proposed design procedure is validated by means of several examples, one of which including the detailed dimensions and experimental results of a manufactured prototype. [ABSTRACT FROM AUTHOR]

Quasi-optical (QO) filter design is commonly based on the direct coupled bandpass filter topology with distributed resonator elements. Conventionally, prototypes with symmetrical impedance inverters are employed during synthesis, and as a result, this leads to resonators with custom lengths across the filter. This can be impractical from a QO filter implementation perspective, since spacers of arbitrary thickness may not be readily available. In this paper, we propose a novel impedance inverter equivalent network, which allows the development of prototypes where transmission lines across the filter structure to have predefined fixed lengths. Based on this technique, we developed a fourth-order QO filter prototype that composed of 2-D periodic arrays with subwavelength features. Its performance is verified with free-space measurement, and the measured results show good agreement with both full-wave and circuit simulation. Thus, it is shown that the proposed impedance inverter circuit can simplify the implementation of QO filters of higher order. [ABSTRACT FROM AUTHOR]

A new approach for the design of a balanced bandpass filter (BPF) is presented in this paper. Stemming from the mode degeneration properties of a square patch resonator, a complete set of resonant modes in a right-angled isosceles triangular patch resonator (RAITPR) is thoroughly investigated so as to construct a modified RAITPR with one electric wall and two magnetic walls along its three sides for filter design. This proposed resonator can support the half-TM10-mode of the RAITPR, and thus, it is named as the half-mode RAITPR here. By properly adopting the relationship between one RAITPR and two coupled half-mode RAITPRs, a second-order coupling topology is presented. Its working principle is then explained by using the field distribution of the desired resonant modes involved. A few high-order balanced filters are in final implemented and fabricated to evidently demonstrate their good differential-mode filtering selectivity and high common-mode suppression. Good agreement is achieved between the simulated and measured results for all the designed three balanced BPFs. To further improve the concerned out-of-band rejection performance, the feeding scheme of the modified structure has been discussed. [ABSTRACT FROM AUTHOR]

When two filters are cascaded together, unwanted spikes appear in the rejection region. These spikes can have a strong impact on the rejection performance of the resulting filter. Therefore, it is desired to suppress or move them far away from the operational band, including part of the rejection band as well. This paper addresses a systematic evaluation of such phenomenon and proposes a simple yet effective method, called spike envelope curve, to have, in a single run and for any frequency, the worst possible case, without the need of repeating onerous analyses. It is also shown that a judicious choice of the length and losses of the line separating the filters can be exploited to mitigate considerably such a negative effect. The method is assessed by full-wave simulations and measurements. [ABSTRACT FROM AUTHOR]

This paper presents a design guide for the implementation of thickness customizable high-order ($N \ge 2$) bandpass frequency-selective surface (FSS). Admittance inverters are used to synthesize the filter response given desired characteristics such as filter type, center frequency, and bandwidth. The spatial inverter layers are essentially electromagnetic coupling interlayers that can be adjusted to customize the thickness of multilayered FSS without degrading the desired filter performance. A generalized equivalent circuit is used to provide physical insights of the proposed design. This synthesized technique can be adopted to deliver a versatile implementation capability of high-order FSS filters using various dielectric spacers with arbitrary thicknesses. Such a technique enables the realization of spatial filters with variable weight and size, while maintaining the desired filter response. To highlight its thickness controllability, a third-order bandpass FSS operating at the X-band (center frequency at $f_{0} = 10$ GHz) with $\approx 10$ % fractional bandwidth is synthesized in four different thickness configurations (electrical size varying from $\lambda _{0}$ /12 to $\lambda _{0}$ /6). The simulation results of all the four thickness customized configurations deliver the same filter response. To validate the proposed technique, two prototypes among the four design configurations are fabricated and measured in a free-space environment. Simulated and measured results show good agreement. [ABSTRACT FROM AUTHOR]

This paper presents a fusion design approach of multiport (1-to- $N$) in-phase/antiphase power dividing network with a bandpass response, which can be applied in constructing either the balanced or single-ended filtering power divider. The relationships of external $Q$ -factors ($Q_{e}\text{s}$) between the in-phase/antiphase ports are constructed, and it is found that their $Q_{e}$ ratio is determined by power allocation regardless of the phase difference. Meanwhile, the two functions of power allocation and bandpass response can be designed independently. Based on the theoretical analysis, two kinds of filtering power dividers using the dielectric resonator (DR) are proposed. One is balanced and the other is single ended. Both of them can be easily built by properly adding feeding probes for the DR without increasing the circuit size, while the phase difference between the ports can be adjusted to be either in-phase or antiphase by altering the direction of the feeding probes. To verify the proposed design concept, the filtering power dividers mentioned earlier are simulated. Some of them are implemented and measured, and their measured results match well with the simulated data, showing good performance, such as low loss and good selectivity. [ABSTRACT FROM AUTHOR]

This paper presents an airflow analysis and a shielding effectiveness (SE) analysis for a multilayered waveguide-below-cutoff array (WBCA) used to prevent electromagnetic penetration. The performance of the conventional unit square WBCA and the multilayered WBCA is compared. For the airflow analysis, the simulations are performed at the entrance and exit of the shielding structure when an airflow velocity of 1 m/s is injected at the inlet. The velocity loss and pressure difference of the multilayered WBCA are improved compared with the unit square WBCA. In terms of the shielding, the SE of the multilayered WBCA is compensated by increasing the number of overlapping layers. Furthermore, a new SE equation is derived by modifying that of the unit square WBCA. [ABSTRACT FROM AUTHOR]