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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]

In this article, a four-way low-loss reconfigurable power divider with arbitrary operating channels is designed with a simple circuit configuration. A reconfigurable transmission line (RTL) is proposed, which can be switched between two different characteristic impedances with constant electric length. When setting the constant electric length as 90°, the RTLs are used as switchable $K$ inverters to construct a four-way reconfigurable power divider. Then, by controlling the switchable $K$ inverters, the power divider can be fully reconfigured in one-channel (State 1), two-channel (State 2), three-channel (State 3), four-channel (State 4), and all-channel-OFF (State 5) operations. Without using extra impedance matching networks, good port matching is achieved in the states with arbitrary transmission channels. Thus, this simple circuit configuration can be applied in the feeding network of antenna systems to achieve flexibly reconfigurable coverages. For verification, a circuit is designed and fabricated. Good agreement between the simulated and measured results is observed. In comparison with reported multiway reconfigurable power dividers, the proposed one exhibits low insertion loss, wide bandwidth, and simple structure. [ABSTRACT FROM AUTHOR]

This article presents a scheme for designing single-to-balanced and balanced-to-balanced dual-channel filters using multilayer substrate integrated waveguide (SIW) cavities. By sharing the common SIW cavities, two single-to-balanced or two balanced-to-balanced filters are flexibly integrated as one dual-channel circuit, resulting in very high integration. The input and output ports are properly arranged to the SIW cavities, where the two modes of the TM110 degenerate modes can be separately excited by the ports of the two filter channels. Moreover, the resonance modes of one channel have no coupling to those of the other channel. Thus, isolation between the two channels is achieved even though the two channels are designed sharing the same SIW cavities. The desired balanced ports are achieved based on the inherent out-of-phase electric field property of the SIW cavity at TM110 degenerate modes. For verification, a six-port single-to-balanced dual-channel filter is implemented, exhibiting excellent single-to-balanced filtering responses of each channel and high isolation between the two channels. By converting the single-ended ports to balanced ports, second-order and third-order balanced-to-balanced dual-channel filters are proposed. Moreover, the proposed designs can also realize different channel bandwidth or channel frequencies to meet different wireless applications. [ABSTRACT FROM AUTHOR]

In this article, a method for designing multichannel bandpass filters (BPFs) based on trimode dielectric-loaded cavities is presented. A large number of BPFs are integrated as one multichannel BPF with multiple inputs and multiple outputs, resulting in an ultracompact size. The cubic trimode dielectric-loaded cavities are utilized with the TE101, TE011, and TM110 modes resonating at the same frequency and orthogonal to each other. Feeding probes and coupling probes are properly arranged where the three modes in one cavity can be excited for different BPF channels without interference with each other. Consequently, excellent isolation among multiple channels can be obtained. The multichannel BPF is designed based on a 3-D structure, which can be easily extended to higher filter orders with a larger number of channels to satisfy different requirements in wireless systems. For demonstration, a 12-channel BPF is fabricated and measured, which exhibits good filtering responses of each channel and high isolation among channels. Significant size reduction is achieved compared to conventional multiple single-channel filters, which is potential in high-integration base station applications. [ABSTRACT FROM AUTHOR]

In this article, we propose a circuit model for designing the switchable filtering circuit with reconfigurable functions between a single-pole quad-throw (SPQT) filtering switch (State 1) and a four-way filtering power divider (State 2) without additional switchable impedance matching network (IMN). The proposed circuit model can be utilized as the feeding network of a switchable directive and omni-directional antenna system to eliminate the omni-directional antenna in conventional solutions to reduce the size and cost. It consists of four filter channels with a common port. In State 1, it works as a SPQT filtering switch, where only one channel is turned on to realize good filtering responses and the impedances at the other three channels are adapted to infinity to realize high isolation. The four channels are turned on simultaneously in State 2 to work as a four-way filtering power divider, and the input impedance of the common port is the same as that in State 1. Thus, a switchable IMN is not required for port matching when switching between the two states. Transmission zeros are generated to enhance the skirt selectivity. For verification, the filtering circuit is constructed using high- $Q$ coaxial resonators. Experimental results show excellent performance in both states acting as a SPQT filtering switch and a four-way filtering power divider. [ABSTRACT FROM AUTHOR]

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, 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]

In this paper, a method for designing substrate integrated waveguide (SIW) filtering rat-race couplers based on TE102 and TE201 orthogonal degenerate modes is proposed for the first time. Without additional circuits, the desired 0° and 180° phase differences of the rat-race coupler are realized by the inherent in-phase and out-of-phase characteristics of E-fields at TE102 and TE201 modes. E-field distributions of the SIW cavities at TE102 and TE201 modes are studied for guiding the circuit realization. By properly arranging the locations of input–output ports and coupling regions in the SIW cavities, magnitude and phase properties of the rat-race coupler topology are realized. And then the filtering responses can be integrated with the rat-race coupler. A detailed design method based on coupling matrix is presented. A filtering rat-race coupler is implemented using square SIW cavities to verify the proposed method, showing good performance of filtering responses, amplitude balance, as well as 0° and 180° phase differences. For further verification, another filtering rat-race coupler is fabricated using two circular SIW cavities to simplify the structure. Comparison with other filtering couplers indicates that the proposed designs exhibit the advantages of good filtering responses, relatively high Q factor of the entire filtering circuits, and simple structure. [ABSTRACT FROM AUTHOR]

In this paper, we propose a method for designing the narrowband filtering rat-race couplers using the quad-mode dielectric resonator (DR) for the first time. The filtering rat-race couplers are designed in single-cavity configuration with one quad-mode DR and four feeding probes, featuring compact size. Based on the investigation on resonant frequencies of a rectangular DR, a quad-mode DR is first constructed. Electromagnetic (EM) field distributions at two resonant mode pairs are studied for guiding the filtering rat-race DR coupler realization. The desired 0° and 180° phase differences of the rat-race coupler are realized by the inherent in-phase and out-of-phase EM-fields of the DR. By properly arranging the four feeding probes, the single-cavity configuration is designed to fulfill the transmission characteristics of a rat-race coupler topology, such as the magnitude and phase properties. The detailed design method is presented. For demonstration, a filtering rat-race coupler based on the rectangular DR is fabricated and measured, showing excellent performance of filtering responses, amplitude balance, as well as the 0° and 180° phase differences. Moreover, the other filtering rat-race coupler is implemented by using a cylindrical DR with an improved $Q$ -factor. Comparison with other reported filtering couplers indicates that proposed designs exhibit the advantages of compact size, high $Q$ -factor, and good filtering responses. [ABSTRACT FROM AUTHOR]

This paper presents a V‐band digitally controlled low noise variable gain amplifier (LNVGA) in 28‐nm bulk CMOS process. The amplifier includes three stages and the first stage is used to realize wideband high gain and low noise figure (NF). The gain control is implemented in the second and third stages with current steering and split cascode topology, respectively. A source degeneration inductor and a series inductor are introduced to reduce the phase variation in the second stage. The phase response function is analyzed, and the phase reduction of 1.2° is achieved. Moreover, with the combination of these two different topologies, the phase variation can be further reduced. The measurement results show the LNVGA has a peak gain of 16 dB with a 3‐dB bandwidth of 45–59 GHz. The measured gain control range is 0–16 dB in 16 different gain states and the minimum phase variation is 3.5° with power consumption of 23 mW. [ABSTRACT FROM AUTHOR]

In this paper, a dual-channel dielectric resonator (DR) filter is presented for the first time. Two bandpass filters (BPFs) are integrated as one dual-channel filter with two input ports and two output ports. By using a common quad-mode DR shared by two second-order BPFs, only one metallic cavity is needed, which greatly reduces the circuit size. Frequency control of the DR is investigated and electromagnetic field properties at HEH11 and HEE11 resonant pairs are studied for guiding the dual-channel filter realization. By properly arranging the input and output feeding probes, two of the resonant modes are excited for one channel and other two for the other channel. Since the feeding probes of one channel filter do not excite the resonant modes of the other one, isolation between two filters is achieved. Three transmission zeros are generated near the passband, resulting in high selectivity. For verification, the dual-channel filter is implemented, which features good bandpass responses and offers a significant size reduction in comparison with conventional two DR filters. What is more, the dual-channel filter with the impedance transformation function is integrated with a $2 \times 2$ Doherty power amplifier for 5G massive multiple-input multiple-output system application, featuring high efficiency, good filtering responses, and high integration. [ABSTRACT FROM AUTHOR]

In this paper, a low-temperature co-fired ceramic (LTCC) diplexer is presented with compact size and high isolation. By using the common stub-loaded resonator shared by two channel filters, combining and matching networks are eliminated and only three resonators are used to design the two second-order channel filters, resulting in simple structure and miniaturized size. A novel transmission path is utilized to not only realize the higher channel filter but also prevent the signal at the lower frequency band. And a specific coupling region is selected to simultaneously realize the coupling strength required by lower channel filter and a transmission zero at higher-band frequency. In this way, each channel filter can generate a transmission zero at the passband frequency of the other channel filter, resulting in high isolation. Moreover, the bandwidths and center frequencies of the two channel filters can be individually controlled due to the design flexibility of the multilayer LTCC structure. For demonstration, a diplexer operating at 3.5 and 5.5 GHz is fabricated. The core circuit has the compact size of 1.9 mm $\times1.7$ mm \times2$ mm or 0.054~\lambda _{g} \times 0.048 \lambda _{g} \times 0.057~\lambda _{g} . Within the 3-dB passbands of the lower and higher channel filters, the measured isolation levels are better than 40 and 45 dB, respectively. [ABSTRACT FROM PUBLISHER]

In this paper, we utilize the coupling matrices to design a compact low-temperature cofired ceramic (LTCC) bandpass filter with wide stopband and high selectivity. One coupling matrix at the fundamental frequency is synthesized to obtain required passband responses. The other one at the third harmonic is manipulated to realize wide stopband. Two transmission zeros are appointed at both sides of the passband to enhance the skirt selectivity. They are generated by multiple cross-coupling paths, which are easy to realize in the multilayer LTCC structure. To reduce the circuit size, two coupled quarter-wavelength resonators are folded vertically and horizontally on different layers. An experimental filter is designed for verification. High skirt selectivity and wide stopband are observed in the measurement. Due to the LTCC techniques, the proposed circuit has a compact size of 2~\text mm\times 1.7~\text mm\times 2 mm or 0.042\lambda g\times 0.035\lambda g\times 0.042\lambda g . It is suitable for system-in-package applications. [ABSTRACT FROM PUBLISHER]

ABSTRACT In this paper, we present a compact tri-band bandpass filter (BPF). The resonator, which consists of a main transmission line, a center-loaded open stub and a side-loaded open stub, is utilized to obtain the three resonance frequencies. After selecting the impedance of each stub at this resonator, the passband frequencies can be controlled individually. Four transmission zeros are generated to enhance the selectivity. For demonstration, a compact tri-band BPF is implemented. Good agreement between the measured and simulated results is presented to verify the theoretical predication. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:573-577, 2016 [ABSTRACT FROM AUTHOR]

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 paper, we present a compact quad-band bandpass filter (BPF) using asymmetric stub-loaded resonators. The resonator consists of a main transmission line with a short stub and two pairs of open stubs with different lengths. The whole resonator can be regarded as two different half-wavelength resonators and a simple stub-loaded resonator. Therefore, the resonator consists of four controllable modes, which are utilized to realize a quad-band response. Meanwhile, by using hook shape feed lines and source-load coupling structure, transmission zeros are generated near the passband, resulting in high selectivity. For demonstration, a compact quad-band BPF with operating frequencies of 1.57/2.45/3.5/5.15 GHz is implemented. Good agreement between the measured and simulated results is presented to verify the theoretical prediction. [ABSTRACT FROM PUBLISHER]

Moreover, they are particularly grateful to the authors of all the submitted papers for their efforts and contributions to this Special Issue, and to the reviewers for devoting their valuable time and expertise to review the submitted manuscripts. Authors of the accepted and presented papers during the iWEM 2021 were invited to submit expanded versions of their conference papers in this special issue. This special issue of Microwave and Optical Technology Letters (MOTL) includes nine expanded papers from the 2021 IEEE International Workshop on Electromagnetic: Applications and Student Innovation Competition (IEEE iWEM2021). [Extracted from the article]

This article presents a novel structure to design miniaturized dual-band filtering power dividers. It is realized by combining the feeding lines and coupled stub-loaded quarter-wavelength resonators. The operating frequencies can be adjusted easily by controlling the stub length of the resonator. Source–load coupling is employed to generate transmission zeros to enhance the selectivity. A resistor is added to guarantee the isolation and port matching. For demonstration, a dual-band filtering power divider is implemented. Good agreement between the prediction and measurement validates the proposed method. [ABSTRACT FROM AUTHOR]

This Letter presents a novel tunable balanced bandpass filter with high common‐mode (CM) suppression by using discriminating coupling. Four half‐wavelength resonators terminated with varactors are utilised to obtain the tunable responses. The high CM suppression is realised by discriminating coupling between feeding structures and resonators, which blocks the CM signals. An experimental tunable balanced bandpass filter is implemented which achieve the continuous tuning range from 0.84 to 1.15 GHz and more than 50 dB CM rejection levels. [ABSTRACT FROM AUTHOR]

This paper presents a dual-polarized patch antenna with quasi-elliptic bandpass responses. The proposed antenna is mainly composed of a feeding network, a driven patch, and a stacked patch, with its entire height being $0.09\lambda $ . The feeding network consists of two orthogonal H-shaped lines that coupled to the driven patch, each for one polarization. The elaborately-designed feeding lines not only ensure a sharp roll-off rate at the lower band edge, but also help to achieve low cross polarization and high isolation between two feeding ports. On the other hand, the upper stacked patch provides improved suppression levels at the upper stopband and also an enhanced gain within passband. Consequently, a compact dual-polarized antenna with satisfying filtering performance is obtained, without using extra filtering circuits. For demonstration, an antenna is designed to fit the specification of LTE band (2.49–2.69 GHz). The implemented antenna achieves an average a gain of 9 dBi, a cross-polarization ratio of 29 dB, an isolation of 35 dB within LTE band. The out-of-band suppression level is more than 40 dB within the 2G and 3G frequency bands from 1.71–2.17 GHz. It can be used as the antenna elements in multiband base station antenna arrays to reduce the mutual coupling. [ABSTRACT FROM PUBLISHER]

This paper presents a new method of using electric and magnetic coupling cancellation to extend the stopband of bandpass filters (BPFs). The mechanism is theoretically analyzed based on the lumped-element equivalent circuit. Due to the cancellation of electric and magnetic coupling between two resonators, three transmission zeros (TZs) can be generated and controlled by manipulating the coupling strength. Using the proposed method together with source–load coupling, a low temperature cofire ceramic (LTCC) BPF is designed with four TZs at the passband edges and within the stopband, resulting in high skirt selectivity and harmonic suppression. Due to the LTCC multilayer structure, the filter size is 0.054 \(\lambda _{g}* 0.045 \lambda _{g}*0.013\lambda _{g}\) , or \(2.48~{\rm mm}\times 2.02~ {\rm mm}\times 0.6\) mm. The theoretical, simulated and measured results of the LTCC BPF are presented to validate the proposed design. [ABSTRACT FROM AUTHOR]